Angular for Architects – preview/beta class, Jan 2021

First we worked for years on numerous enterprise Angular projects. Then we spent months organizing the knowledge of our experts. The result: Angular for Architects, now available from Oasis Digital.

This class is about both the coding aspects of Angular architecture, and also the decisions, patterns, costs, and constraints that drive architectural decisions.

Early access / Beta test class

To prepare and validate the curriculum for Angular for Architects, we are running a special beta-test version of the class. To show appreciation for attendees willing to jump in at this stage, the price is cut by half.

The first such Beta test is in January. Depending on how it goes we run a second “beta”, or proceed to standard pricing.

Who Should Attend?

  • Decision makers responsible for large or complex Angular projects
  • Angular tech leads and project managers
  • Application and system architects
  • Teams looking for a shared understanding of Angular architecture and practices

NgRx is 40x faster than your code – find out why

When I started using @ngrx/store to hold collections of information, I usually put the data into the store as a JavaScript array. It seemed to be the simplest and most appropriate data structure for the information. However, when @ngrx/entity came out, I saw that it used a different pattern – instead of using the array directly, it converts the array to two data structures; an array of ids and an object map keyed by those ids. Why did they do this? And is there a lesson we can learn for our own code?

Continue reading NgRx is 40x faster than your code – find out why

Advanced, Angular-related training and mentoring

About five years ago (it feels like forever) Oasis Digital started training on Angular. Our flagship course Angular Boot Camp has become quite popular, we’ve taught it many hundreds of times to many thousands of students. For the first few years, this offering was a perfect fit for almost every company that contacted us, as software teams were initially adopting Angular. Over the last few years though, Angular has become mature and robust, and Angular has achieved broad adoption across organizations large and small. Aggregate needs of Angular teams inevitably shift toward bigger scale, more difficult and important uses of the technology.

As a result, our training efforts have substantially pivoted toward more advanced topics.

Continue reading Advanced, Angular-related training and mentoring

Transposing Rows and Columns in ag-grid

Real-world Angular applications often need to present tabular/grid data, and most grids make the most sense when presented with each column representing a certain type of data. For example, on a spreadsheet showing a pay schedule for a loan, the first column could be a date, the second column could be the interest accrued, the next could be the size of the payment, etc.

However, we sometimes need to show data in a transposed format, where the rows instead of the columns need to show a consistent data type. This is a rare case, which is why some major grid libraries like ag-grid don’t provide native support for the feature, but it’s still necessary.

Fortunately, ag-grid gives enough power to developers to be able to transpose data for display, and even to have features like renderers and editors apply by row instead of by column.

I’ve shared a small Angular app on GitHub that I’ll use to step through the process of transposing data. Think of it as a prototype for an app that shows names as they are translated in various languages (Matthew, Mateo, Matthäus, Матфей, etc). You can see the live-running app on StackBlitz, which shows the evolving grid on separate tabs.

Continue reading Transposing Rows and Columns in ag-grid

Querying without OR in Firestore


Here at Oasis Digital we have successfully used the Firebase Realtime Database, and more recently the (beta as of July 2018) Firebase Firestore. These similarly branded offerings have important feature differences, and the latter appears likely to be the recommended choice in the future.

Firestore is a globally scalable, fully managed, document oriented NoSQL database. It is suitable for a very small team to build an application which could then scale to a vast user base with very little system administration work; of course, there are feature trade-offs which enable these amazing properties. Notably, Firestore has important structural limits on the types of queries that can be performed. For example, it has no joins, no “OR” criteria, and limited range (inequality) queries. As I understand, these limitations are what make it possible to engineer Firestore operations to “cost” (and therefore be priced!) in proportion to the amount of data returned.

We recently implemented a Firestore application (with Angular, Angularfire2, and Firebase Functions) in which the query limitations initially were an obstacle; but we found solutions capable of producing great results nonetheless.

Querying workflow state

There are countless scenarios for querying a data store, of course. A simple, common such scenario is an application in which each “document” represents an entity that moves through a workflow over its lifespan. The problem domain doesn’t matter; but one common concrete example is order workflow in a e-commerce system. Such a workflow could look something like this:

New -> Verified -> Scheduled -> In progress -> Preparing -> Shipped -> Delivered -> Closed

Or more generally, think of a workflow feature as the movement of an entity through a series of states:

S1 -> S2 -> S3 -> S4

(In both examples I have drawn simple, linear flows – most of our production/customer software has workflows with looping, branching, and other complex considerations.)

In a system with workflow features, it is very common to need to query a group of entities which are in a certain state – and also very common to query the entities within a set of states. For example, a feature might tally or otherwise view “all orders that have not yet shipped”, which comprises 5 states in the example workflow above.

Firestore “schema” design

Given the absence of OR queries in Firestore, and the frequent need to query entities that are in one state OR another, how should we represent workflow state in a Firestore implementation?

With a traditional, relational database, the main driver of data modeling is to concisely represent the underlying data and ideally “make illegal states unrepresentable”. A representation oriented for this kind of database should follow one of the normal forms, mathematically justified decades ago.

With Firestore (as with most NoSQL data stores), data modeling has a different main driver. An application instead stores data in a way to enable whatever kinds of queries are needed, given the query capabilities. This potentially implies a significantly different data layout, although we often start with something similar to a traditional RDBMS schema then diverge as needed.

Keeping that in mind, show should we store “what state is this entity in?” in a document in Firestore?

Approach 1: Single state field

The simplest solution is one field to represent the state of the entity represented by a Firestore document. The data storage looks something like this:

state: ‘Scheduled’

Querying entities in a single state is trivial with such representation:

.where(‘state’, ‘==’, ‘Scheduled’)

Querying entities in several states requires running a separate query per state, then combining the results together in client code.

Unfortunately, this “combine results in client code” approach, though mentioned in the documentation, has unpleasant consequences. Consider a case where there are 500 entities in state S1, and 500 more entities and state S2, along with some other fields (perhaps “due date”) that ranks all of the entities. Then try to write a query (or pair of queries) to retrieve the 500 “soonest” entities that are in either of these two states:

.where(‘state’, ‘==’, ‘S1’).orderBy(‘dueDate’).limit(500);

.where(‘state’, ‘==’, ‘S2’).orderBy(‘dueDate’).limit(500);

(Your client code would combine the results, sort by due date, and discard all but the first 500 of the combined list.)

Unfortunately, this query now potentially “costs” twice as much, in both time and money, as it should; it may query up to 1000 documents only to discard 500 of them. Still, with this extra cost, time, and client-side query implementation, the single state field approach does work.

Approach 2: One field per state

With this next approach, entity state is represented by a set of flags, one for each state. Typical data could look something like this:

state: {
    New: false
    Verified: false
    Scheduled: true
    InProgress: false
    Preparing: false
    Shipped: false
    Delivered: false
    Closed: false

This is more verbose, but easy to understand and implement. Querying for a single state is as easy as before:

.where(‘state.Scheduled’, ‘==’, true)

At first glance, the limitation on OR queries appears to stymie a multiple-state query with this schema. However, with a bit of Boolean logic these OR operations can be swapped out for ANDs and NOTs in the right combination. Transform the desired OR of all the states you want to exclude – into a query which instead excludes all the states you want to exclude.

Continuing with the order-management example, imagine we want all of the orders in the first three states. The query looks something like this:

.where(‘state.InProgress', '==', false)

.where(‘state.Preparing', '==', false)

.where(‘state.Shipped', '==', false)

.where(‘state.Delivered', '==', false)

.where(‘state.Closed', '==', false)

This is simple to implement, mechanically. A bit of utility code could perform the correct set of WHERE operations, leaving application code straightforward.

How might this scale? This is an unknown – these are a lot of ANDs (especially for an entity with many states), which might need more Firestore indexes, or might stress Firestore query mechanism in unexpected ways, or might hit a limit on the number of allowable (or advisable) ANDs.

This is probably the best approach for a small number of states.

Approach 3: Combinatorial state fields

Given how well mathematical logic worked in the previous approach, what if we took it further? When writing an updated state of an entity/document, application code (utility code) could emit all of the combinations of states that include the current state. Concretely, consider the S1/S2/S3/S4 example. If an entity is in state S2, that could be represented like so:

state: {
    S1: false, // or omit the ‘false’ entries
    S2: true,
    S3: false,
    S4: false,
    S1_S2: true,
    S1_S3: false,
    S1_S4: false,
    S2_S3: true,
    S2_S4: true,
    S3_S4: false,
    S1_S2_S3: true,
    S1_S2_S4: true,
    S1_S3_S4: false,
    S2_S3_S4: false,
    // S1_S2_S3_S4 not necessary, would always be true

This approach pre-computes the answers to all possible queries of sets of states. Such a representation could be generated easily and consistently by utility code. Queries, again with the bit of utility code to generate them, can find all documents (entities) in any set of states with a single WHERE. For example, to look for all documents that are in state S2 or S3:

.where(‘state.S2_S3’, '==', true)

This will have efficient query characteristics, but could run into limitations around the number of allowable fields in a single Firestore document. Also, with Firestore there is an index for each of these fields, and each index increases the Firestore storage costs and makes document updates a bit slower.

Approach 4: Partial combinatorial state fields

This approach is like the previous approach, with one optimization: rather than pre-generate all of the possible combinations, instead only generate those combinations which enable queries the application actually performs. Typically an application will only use a a few handfuls of combinations of states that an application ever queries for – vastly fewer than the number of combinations of states – which as you may recall from math courses, involves factorials.

The obvious downside: if an application later needs to query a different combination of states, it must first update every document to add the new bit of data, or fall back to another approach.

Approach 5: State range with a numeric field

Up to this point, every approach would work with any kind of workflow, regardless of its linearity. But many applications have a primarily or entirely linear workflow – and that can be used to ease the query challenge. Consider a simple numerical indicator of the state:

state: 3 // entity is in state S3

This enables state “range” queries, like so:

.where(‘state’, '<=', 2) // Entities in state S1 or S2

.where(‘state’, '>=', 3) // Entities in state S3 or S4.

It’s also possible to query any contiguous range of states:

.where(‘state’, '>=', 2)

.where(‘state’, '<=', 3)

Unfortunately, this approach has a serious downside: a Firestore query can only do range queries on a single field. Therefore with this approach, it is impossible to query questions like “entities in state S2 or S3, ordered by due date, first 50”, because the single range query “slot” is used up by the state part of the query. Because of this obstacle, it’s hard to recommend this approach.

Approach 6: State range with boolean fields

Fortunately, there is another variation of the range idea that is quite workable. Use a flag for each state, marked as true if the entity is at or after that state. So for example, an entity at state S3 would be like so:

state: {
    S1: true,
    S2: true,
    S3: true,
    S4: false,

Each flag represents a range of states; as before, utility code could implement this representation generically and reliably. Conceptually, a single WHERE can find all entities before or after a point in the workflow. For example, these queries split the state space before/after the S2/S3 boundary:

.where(‘state.S3’, '==', false) // Entities in state S1 or S2

.where(‘state.S3’, '==', true) // Entities in state S3 or S4.

A pair of WHERE clauses (implicitly ANDed) can find all the entities in any contiguous range of states. For example, to find all entities in S2 or S3:

.where(‘state.S2’, '==', true)

.where(‘state.S4’, '==', false)

This approach both avoids an factorial expansion in the number of state variables, avoids major query complexity, and keeps the inequality-query “slot” available for other uses, and is therefore likely an excellent choice if states are mostly linear.

Conclusions and the future

For an application being implemented today on Firestore, which has workflow scenarios where the lack of OR queries is an obstacle, likely one of the above approaches will do a sufficient job. Looking forward to the future, it seems likely the Firestore team will find ways to expand the query capabilities without loss of performance – perhaps rendering these approaches obsolete.


Writing a Generic Type-Safe ng-bootstrap NgbModal Launcher

For an Angular project for one of our clients, I’ve recently started using ng-bootstrap to implement standard modal dialogs in a ModalService. This service has methods to launch confirmation dialogs, input dialogs, message dialogs, etc; you can see a simplified version of this service on StackBlitz.

An addition to the reusable standard dialogs, we also needed to support custom one-off dialogs, and we wanted to use the same general approach, without adding a bunch of duplicate code. Most of the implementation was straight-forward, but adding type safety to the generic launcher was more interesting. Read below to see how interfaces from TypeScript and Angular made it easy.

Continue reading Writing a Generic Type-Safe ng-bootstrap NgbModal Launcher

ng-conf 2018

ng-conf 2018, one of the two “main events” of the Angular calendar, just wrapped up. As usual Oasis Digital was a sponsor, among more and more companies each year.

Perhaps the most interesting news from the conference came in the first few minutes: the main website (containing the documentation and other overall information about the framework) now receives 1.25 million unique visitors per month. Of course not all of these are Angular developers, on the other hand there are no doubt many Angular developers who visit the documentation quite rarely. Therefore it seems safe to assume there are at least 1.25 million Angular developers out there – the Angular user community is large and growing fast.

On a closer-to-home note, it is always rewarding to have numerous past students visit our booth, and to see numerous past private class customers visible at the conference as major Angular users.

For more on the technical content, please join us for our Angular Lunch user group meeting this week.



Angular Boot Camp Unleashed

Oasis Digital is pleased to announce that…

we are publishing extensive example code that we use in Angular Boot Camp. This example code is available under an open source license (in case you want to grab a bit to use in a project), and is hosted on GitHub for easy browsing and instant editing on StackBlitz:

We’ve published 49 examples so far, with more coming. Why are we publishing this?

  • For students to peruse before class, to better understand what we teach.
  • For students to review after class, as a reminder of what they learned, and to grab code snippets.
  • To provide working, up to date, concise examples of Angular concepts for anyone in the community who needs them.

Here’s a one minute video showing just how easy it is to browse the examples, run them, and view/edit the code:

We have some FAQs also. If you are interested in learning Angular deeply, please consider our class, Angular Boot Camp.

CSS grid with Angular and CLI – the time is now

Today, early December 2017, is the time to begin using CSS grid for layout in Angular applications, even if they must support Internet Explorer. We can stop enduring the costs and delays of old “float” based CSS layout, and get better results with less work, using CSS Grid – even with Internet Explorer support requirements – with caveats described below.

Take a look at a running example on the browser of your choice, including both modern browsers and IE11.


If you’re not familiar with CSS grid, the best source is Rachel Andrew, the global guru of CSS grid. Either read and all of her Grid content, or peruse the links below (thanks mostly to Bill Odom, our early CSS Grid cheerleader, for gathering these). Now is a good time to read and watch, I’ll wait.

Welcome back, CSS Grid fan. Of course the big problem with Grid today is that while support is excellent among current browsers, many users (especially paying, enterprise users) are still wallowing in Internet Explorer. IE has basic support for CSS Grid, but the support is for an older spec which has both fewer features and different syntax. The syntax is irritatingly different enough that manually maintaining both is prone to error.

Fortunately, the incredible Autoprefixer does a very good job, in version 7, of papering over the syntactic differences. In many cases the benefits of Grid can be obtained even without the newer semantics.

Yak shave

Unfortunately, Angular CLI (as of version 1.6.1, as I write this) uses Autoprefixer 6, and exposes no way to adjust Autoprefixer settings. This CLI issue tracker has many open issues, and it appears the team is closely focused on core application bundling and ergonomic considerations, so it’s hard to predict when CLI team attention could turn to issues like this.

Yet here at Oasis Digital, we are ready to use Grid today, and our customers are ready to deploy software today. Therefore, a series of workarounds is in order. To see them in action and in detail, visit this demo repository:

…and for an explanation, read on.

Upgrading Autoprefixer

To use version 7 in an Angular CLI app today, a way must be found to override the Autoprefixer dependency. The traditional answer to override dependencies and settings in CLI is to “eject” – but that is a big leap, not easily reversed, and not recommended. An application based on ejected CLI presents a greater maintenance burden for developers. Instead, we generally recommend sticking with CLI but applying whatever patches are needed at run time to get the right behavior.

Unfortunately as of late 2017, it is still unduly difficult to override dependencies with NPM; searches looking for the way to do it, lead in circles toward old NPM versions. Happily, Yarn can do it quite easily, as a first-class feature. Switch to Yarn, then add a section like this to the package.json file:

"resolutions": {
  "autoprefixer": "^7.2.3"

Turn on grid support

Next, Angular CLI does not yet provide a way to pass options to Autoprefixer, and using Grid requires turning the support on. To work past this, the venerable approach of “monkey patch in a postinstall script” solves the problem easily. The script content is essentially just:

sed -i.bak -e 's/autoprefixer()/autoprefixer({grid:true})/' \

This reaches into the relevant file inside the installed CLI code, and edits it in place. I think of this as a rough but necessary hack, to deliver value today, reaching ahead to the future when the tools will make the hack unnecessary.

Fortunately, between these two workarounds there are just a few lines of edit needed in a project. Study the repository above (especially the second commit in the commit history) to see the exact changes.

Conclusion and caveats

Of course there are caveats here, explained in depth by the Rachel Andrew page I linked above. The situation is not quite as severe as that page suggests though, because of what Autoprefixer does. With this setup, you can use today’s Grid syntax, but the subset of Grid semantics supported by IE. This means:

  • Use modern grid-column definitions etc., no need for the older “span” concept.
  • No “flow” in to grid cells – assign grid locations manually. Fortunately, for application layout Flow manual assignment is common anyway.
  • No “gaps” – leave an empty track instead. Easily done.
  • No grid-template-areas.
  • As always, remember to test on IE.

While these caveats are a bit frustrating (especially the lack of grid-template-area), this use of Grid is still an enormous improvement over legacy CSS approaches for many (or most) application screen layouts. With this approach, I see no further reason to wait to start using Grid broadly in Angular applications.

Future work

If the lack of grid-template-areas proves too frustrating, I may look at a similar approach to squeeze in support for postcss-grid-kiss; it provides syntax far beyond that offered by grid-template-areas, and also provides more semantics on IE through use of greater CSS contortions.


Angular routing – advice for real applications

There are plenty of examples and documentation about the Angular router, but these things sometimes leave important questions unaddressed.  Documentation often intentionally demures from questions like “what is the best way to use this?”. Even my own previous post briefly reintroducing the router does the same.

Here are our recommendations from extensive use (at Oasis Digital, in classes and complex customer applications), with my specific take on contentious points, on that category of Routing question. How can the built-in capabilities of Angular, including the router, be used with maximum leverage? How can an application be written “with the grain” of Angular to produce the greatest value with the least code? How can the router be used to provide a good user experience and functionality?

URL/route for navigational state

The standard use of intra-application URLs is to represent and control navigational state. Navigational state means “where” the user is in the application. Which screen; which entity; what they are working on; what they are looking at. This type of state so strongly belongs in the URL that (in a polished, important application) it should always be managed via the router -even if some other state mechanism is being used to manage other aspects of application state.

Pop-ups and auxiliary routes

The Angular router has an auxiliary route feature, uncommon among other routers for other frameworks. This feature has various uses, particularly for (unusual) applications with more than one section of the screen that might be navigated separately. But it also has a common use: if an application has a pop-up/popover/dialogue of some kind (for example, a list of users in which editing a user happens on the same screen), the state of whether a pop-up is currently visible should be represented as an auxiliary route.

Resist the temptation to have a pop-up work separately from the route, because that would mean that bookmarking or sharing a URL would not capture this aspect of the user’s navigational state.

Router state and form state

Sometimes a form is used for data entry; for these cases the state of the form (particularly if you’re using model driven/reactive forms) is a fine place to keep that interim data entry state.

But in other cases, a form is used for something like a faceted search. Search parameters can easily stray into navigational state. For example, if the user is currently searching a list of orders for a certain date range that mention a certain product, they could very reasonably want to navigate forward and back to that state, they could want to bookmark and share the URL, and have those search parameters come along.

In these cases, it is reasonable to mutually interconnect the router state and the state of a form. That sounds difficult, but requires just a few lines of code. The result can easily provide a near ideal user experience around searching, URLs, the back button, bookmarks, and so on.

Router state and ngrx/Store

Ngrx/Store users have some extra tools at their disposal around the router state. There is an optional add-on package which integrates router state into Store state, so that it can be managed via the same mechanisms (actions, reducers, effects, etc. An application of significant complexity, so much so that it needs Store, almost certainly also has significant navigational state, and should strongly consider integrating them together.

Don’t fear ugly URL parameters

In simple cases, a URL contains a flat name-value pair list of optional parameters, in which the contents of such strings are most typically just a single value, it is also acceptable to pack in many values in such a single parameter by encoding a broader swath of state as JSON. For example, consider a simple search of orders in the system for order management. It might have single search parameter, perhaps which matches a product description. The URL for the state of searching for such a description could look like:


But for a more complex search (for example think of a faceted search with 15 different fields by which the user could search for old orders), you may need more (bug-hiding) code to shuffle search parameters into and out of URL parameters. It is also acceptable, and sometimes more advisable, to encode all of the complex search parameters like so:


where … Represents a URL-encoded JSON object describing the search parameters

Such a URL is less straightforward to inspect by hand, but also less work to manipulate programmatically and easier to expand to encompass more parameters. Make the trade-off at the application level, as to whether this yields a better overall system.

Router security concerns

I’ve seen suggestions of route guards is a security mechanism; but it’s important to remember that the entire browser is a user agent, it is literally an agent of the user, not the agent of the developer or of the backend system. At best a browser application can avoid making security worse, but it doesn’t actually provide security. Never assume that route guards or other client-side mechanisms are providing any real security, rather think of these mechanisms as advisory security. Advisory security is UX/UI which makes it easier for the user to avoid wandering into a screen which will break because server-side security rules interfere with its operation.

But there is a new and interesting way that browser-based applications can get things wrong with security where the router is concerned. The entire route URL, which means all route segments, parameters, outlets, etc. is untrusted user input. It could accidentally or intentionally contain errant or malicious data. Make sure to treat route data as such, sanitizing it etc. as one would any other user input.

Matrix parameters

Although not used very widely, there is a URL pattern called a matrix parameter, in which each “segment” of the URL has its own parameters rather than just one single bucket of parameters for the entire URL. The Angular router supports this nicely, by using it you can sometimes conserve application code quite significantly while still providing a more ideal user experience around navigational state captured in the URL.

Route guards for data loading

Longtime Angular users who started with AngularJS often point back to the “route resolve” feature is a critical capability they’re looking for an Angular. The Resolve feature makes it possible to delay (or cancel/fail) loading of a route until the data needed to populate the screen for the route is ready.

I recommend using this feature with caution and sparingly. Often a better user experience can be achieved by proceeding directly to a route (for example, a customer history detail display), and then asynchronously loading various parts of the data which appear on that screen. While the screen painting can be a bit messier this way, the user will perceive that the screen started loading much more quickly than if loading is delayed until all data is available. Even if the difference is only a few hundred milliseconds, showing the user partial results is typically a better default.

Angular routing, a basic Q&A

At Angular Boot Camp, we thoroughly introduce and teach the Angular router – over the course of 3 days, spread out into relevant bits and pieces of other learning. Outside of class though, customers ask a straightforward question: What is the Angular Router, and why should I care?

To answer that, this post is a tidy re-introduction to routing in Angular. It is presented in Q&A form – there is little reason to reproduce the router documentation, so this is more like the average of many conversations.

What is routing?

Most concisely, in a web application routing means the relationship between the URL and the state of the application. State can mean a lot of things, but in this context it means “what screen the user is looking at” and “what specific entity/data the user is looking at on that screen”. For example, an application might have “/orders”in the URL when they are looking at a list of orders, or “/orders/12345” when they are looking at order number 12345.

Why use a router?

Routing is about translating between this concise string in a URL, and the rest of the machinery of an application, without coding that translation “by hand”. Developers sometimes ask why they need a thing called a router to do that, whether they might just instead inspect the “window.location” variable and make the application show the right thing. In a sense, the answer is yes – you could certainly do that. But it tends to get complex as an application grows, and if you do it ad hoc, your code won’t have as much in common with other application code. By using a router, you can write less code, and have a standard off-the-shelf solution to a problem that most applications need to solve.

Why care about the URL?

As you learn Angular, you can see how to use variables and ngIfs to make different data appear on the screen in response to user clicks. For example, you could have a variable “orderScreen” and some section of your template using ngIf to display only if orderScreen==true; then have a button which sets orderScreen=true. So you can easily see how to display different data based on what the user clicks, without caring about the URL.

But URLs (and by this I mean the part after the domain name) are the standard, well proven Web way of expressing “where” the user is. Users understand URLs, and users can copy and paste URLs in email, users can bookmark URLs. If your application has a specific URL to mean “order list screen”, a user could bookmark that and navigate directly to it when they like. Fundamentally, URLs are user-friendly.

Would it be easier to just write a separate “application” for my orders list screen? Could I avoid having to understand the router by making each screen a separate application?

I’ve seen applications, especially those adapted to fit inside an server-side system, which eschew the notion of “routing” between different screens and instead have an entirely separate application for each screen. This is possible, but inefficient. The browser ends up needlessly reloading much of the same JavaScript as the user navigates from one screen to another. With the router, the user will only need to load the new, different code for the next screen as they navigate.

Similarly, the router implements “lazy loading”, so just like the idea of totally separate applications, with the router, a user doesn’t have to wait for their browser to load the “order screen” JavaScript until they are ready to use that screen.

The router provides the ideal mix of efficiency in development, efficiency and deployment.

How do I get started with the Angular router?

As you create a new application with Angular CLI, there is a routing option which sets up the basic structure of routing for you. Unfortunately as of late 2017, you still need to manually code up specific routes, which you can do by following the Angular router documentation or various tutorials online (or of course, learn in our class). I expect a future evolution of the CLI will automate more of the router configuration process.

When should I get started with the Angular router?

A few years ago, I used to recommend waiting for routing until you really need it, until your application has more than one “screen”. But now it seems more advisable to simply follow the standard patterns for routing from the very beginning. As you create your first screen in an Angular application, go ahead and implement that screen in a module, and use router lazy loading the load that one module. This seems like extra structure, but will save you from having to rearrange your application code when a second “screen” is inevitably needed down the road. This is also exactly the path we teach in Angular Boot Camp.

What about that idea of routing to a specific (for example) order, rather than to the list of orders?

The idea of routing to a specific individual entity in your application problem domain, use a route parameter. A route parameter is simply a section of a route which can be filled in at runtime with a string. For example, “/orders/12345” suggests a routing set up where the second segment of the route (12345” is a parameter. This is easily configured in your Angular routing configuration, you can see the documentation for the exact syntax.

The more interesting part of a route parameter is consuming that parameters, being aware of it from inside application code. These route parameters arrive at your application component as an observable value. You’ll need to use a small amount of RxJS code to trigger loading of the appropriate data based on that route parameter. This sounds confusing and complex, but you can find examples online would show it is often just a few lines of code.

How do I link to a route?

You can link to a Angular route in an ordinary anchor element (“<A….”) in a component template. You do this using the router link directive (attribute). The documentation shows the exact syntax, but the important thing here is simply that you link to a route within the same application, with the syntax only mildly different from linking to any other page on the Internet.

Things get slightly more complex when you want to link to a specific entity (going back to our example, “/orders/12345”). To do this you use something called a route parameter array, in which the application code snippet has an array with these two parts of the route (orders, and 12345), which then get assembled automatically by angular routing into a working route link.

Of course in real application, users often click a button to do something rather than follow an ordinary web link; you can accommodate this with routing either by styling the link to look like a button (quite easy with bootstrap, for example) or with a line of code in a click handler to ask the router to navigate to a link.

So my links could be navigation in a sidebar or top bar, right?

Yes, the most common use for router links is in a navigation bar of some kind.

In this context, it also makes sense to visually mark which route link is currently “active”. To make it obvious to the user which part of the application they have already navigated to. The Angular router also makes this quite easy with an attribute “routerLinkActive”.

Is there anything else to know about routing?

There is an abundance of important capabilities in the router beyond this quick Q&A introduction. Past this introduction though, it starts to get a little bit more philosophical, and make sense to study after you are already experienced with basic use of the Angular router.  I will follow up with another post on some of these other routing thoughts.



Angular Runtime Performance Guide

Co-authored by Paul Spears and Andrew Wiens

1.0 Introduction

Smooth, highly-responsive interfaces increase users’ confidence in an application and create an overall positive experience. Whereas small applications with simple interactions are built without a focus on runtime performance, standard approaches sometimes do not scale well as the data size or feature complexity increases. A common scenario that may be familiar to the reader is a table that works well with small quantities of data but begins stuttering and lagging when the amount of data is increased. This guide will show how to increase performance in these kinds of applications.

Additionally, high framerates enable developers to build entirely new types of applications with Angular. Introducing animations and interactive graphics create new and exciting ways to engage with users. Here at Oasis Digital, we used the techniques in this guide to build an interactive visualization for issue tracking [1], multiple customer projects and a demo application that showcases the kind of performance that is possible within an Angular application [2].

Although we typically write Angular applications with relatively little concern for what Angular does behind the scenes, in performance-sensitive applications we achieve the desired responsiveness by knowing more about how Angular works. In this regard, an app’s implementation can have a large effect on performance: while Angular’s change detection system can complete hundreds of thousands of cycles in a few milliseconds for simple changes, application logic takes the overwhelming majority of time to execute. In this guide, we will describe how to meet the expectations of performance-sensitive applications, explain the relevant parts of Angular change detection, and highlight potential pitfalls along the way.

2.0 Toward 60 Frames Per Second

Fig. 1. Top level overview of execution control during change detection. Angular (red) calls application code (A; dark blue) during change detection (B) and updates the DOM. The browser (light blue) then updates the view, completing the change detection cycle (C). This cycle must complete in less than 17ms to achieve 60 FPS.

In the industry, 60 frames per second is the gold standard for application responsiveness, and any application that achieves it must render updates in less than a mere 17 milliseconds. Performance most often suffers in Angular applications when responding poorly to user input or other regularly-occurring events. The total time to re-render a view in response to any changes can be split into three parts: First, as shown in Figure 1A, application-specific callbacks are executed. Second, Angular’s change detection system runs as shown in Figure 1B. This system is responsible for delegating control to the application callbacks and using the results to notify the browser of any necessary DOM updates. The third piece in this process, the browser, paints the required changes. The application then waits for additional input before repeating this cycle (Figure 1C).

Since we generally only have control of our own code and how it interacts with Angular, improving runtime performance tends to involve optimizing three main aspects of our app:

  1. Executing application event handlers quickly
  2. Reducing the number of callback executions needed to complete a change detection cycle
  3. Reducing the execution duration of Angular’s change detection cycle

As the last two of these three aspects may imply, Angular’s change detection system has a substantial effect on runtime performance. Thus, it is important to gain a basic understanding of how the change detection system operates.

3.0 Angular Change Detection System

Once an Angular application is loaded, Angular listens for user events and other asynchronous events. Angular understands the context for these events and calls the appropriate handlers. After these handlers return, control is given back to Angular to perform change detection. Although Angular knows the data bindings between components, changes in other values may affect the template as well. For example, a template element may depend on a property of a shared object. Therefore, by default, the change detection system responds to updates by re-evaluating the template expressions of all components. If the change detection system determines that the value of a template expression has changed, it interacts with the browser to modify the corresponding portion of the DOM.

Fig. 2. Stepwise explanation of an Angular change detection cycle.

For example, a tree of components is shown in Figure 2. In this diagram, child components reside within their parents, and events can occur within any of the components. When a DOM event occurs, Angular will call the associated application event handler. Depending on how the application is structured, this may result in a component event firing rather than a DOM event. If a component event does fire, the associated event handler in the parent component is called, and this process is repeated. Once all events have been handled, Angular begins checking components and their templates for updates. This process starts from the root and works its way down to the leaves in a breadth-first manner.

Although this view of the change detection system is sufficient for our purposes, there are additional resources that explain the inner workings of this system. For a deeper explanation of Angular’s change detection system, see the blog posts from Victor Savkin and [3-4].

4.0 Executing event handlers quickly

Event handlers can exist in numerous locations within an Angular application. The most obvious examples are DOM and component event bindings. An application responds to events such as mouse clicks or key presses by providing Angular a callback to execute as shown in Figure 3.

Fig. 3. A button executes a callback when clicked, effectively blocking change detection until the callback completes.

When such a callback is executed, Angular must wait for the callback to finish before change detection can continue. Once all events are processed, the change detection process evaluates template data bindings to determine which DOM properties to update. This process includes checking and updating component inputs. Angular provides developers control over how a component should respond to changes to its input bindings in the form of callbacks – OnChanges and input setters – which affect the execution time in a similar manner as event handlers.

The callbacks of event bindings, OnChanges, and input setters are the primary mechanisms for passing data between services and components in an Angular application, and it can be difficult to keep these slim. However, it is not always obvious how much code is executed during these callbacks.

4.1 Event Bindings

It is common practice to use event bindings for communicating user updates to shared locations such as services or components at a higher level of the hierarchy. Figure 4 shows a trivial example.

Fig. 4 A DOM event handler results in the execution of a service method.

As control moves between location,s additional processing is often required. For example, a search term combines with an array to produce a filtered list. The following code in Figures 5 and 6 demonstrates how a button click hands control to a long running service from the component. The service, in turn produces the filtered list.

Fig. 5 A component and service cooperate to produce a filtered list of instructors.

Fig. 6 The results of the calculation of Figure 5 are displayed on the screen with an ngFor.

Thus, a single event can percolate through multiple layers. By default, this computation will occur as part of the change detection cycle started by the original event binding. Figure 7 shows a stack trace of the click handler, change detection and the multiple layers of application code. Notice the “Long process” near the bottom of the image. This was inserted on line 42 of Figure 5 to emulate a calculation that could take longer than normal to run. The trace visually demonstrates that the change detection process cannot complete until all callbacks and their subsequent method calls have finished executing

Fig. 7 A stack trace demonstrating control flow during a click event.

Though not always obvious it is important to remember that function calls usually executes as part of change detection regardless of where they reside. A key to performance is being cognizant of this fact and writing code that respects it.

The pattern of calculating a new application state from user and system events is often used with great success in many enterprise scale applications. In particular the use of a library such as ngrx/store or redux strongly encourages it. In these situations, it is important to ensure that any reducers execute as efficiently as possible. Also, as we will see in the later section on RxJS Observables, it is also possible that event handlers may update an Observable. If the Observable pipeline executes synchronously, as in Figure 8, the cost of this computation is added to the total cost of the change detection cycle.

Fig. 8 The anonymous function defined on line 27 is executed as part of any change detection cycle in which the search value is updated.

4.2 Component Input Setters and OnChanges

Event handlers are not the only application code that executes during a change detection cycle. After event propagation completes, Angular continues the change detection cycle by updating the component hierarchy and template data bindings. As mentioned above, this process starts at the root component and works down towards the templates of the leaf components. Along the way, Angular will execute any setter methods associated with component inputs. Similarly, the ngOnChanges methods, similar to those in Figure 9 will be executed in components that implement OnChanges.

Fig. 9 Line 18 demonstrates the syntax for a basic ngOnChanges method.

Generally, problematic situations are created in the callbacks of the input setters and ngOnChanges relatively infrequently. It is often easier to spot problems when they do occur as issues are usually isolated to a single component. However, there are still a couple hazardous scenarios to point out. It is usually recommended to compute any state or UI changes needed as part of the event propagation phase of the change detection cycle. However, some situations may still occur that encourage the use of OnChanges to compute additional state needed locally within a component. Consider the filtered list example: For the sake of argument, assume that the current filter criteria and the unfiltered list are only available as inputs, and the filtered results must be computed immediately prior to display as shown in Figure 10.

Fig. 10 Demonstration of recalculating a filtered list as Input values update

This could be achieved by utilizing OnChanges. However, doing so would cause every input change to trigger a recalculation of the filtered list. If another input were added to the component (see Figure 11), there would be a wasted calculation every time the new input value is changed.

Fig. 11 The ngOnChanges method defined on lines 19 – 27 demonstrate a extraneous calculations that occur when the selectedInstructor is updated

Input setters serve a similar purpose as OnChanges, however they only fire in response to updates to a corresponding input. Generally speaking, the use of input setters will lead to more performant change handlers as there is no need for identifying which input changed, nor will it be called more often than is necessary. Although the granularity of input setters make for a better default choice, it is still possible to populate the callbacks with expensive operations, and they should be treated with the same level of care as OnChanges.

5.0 Reducing the quantity of call back executions needed

Executing application event handlers during change detection has the potential to hand execution control to multiple services and components. Being mindful of how the change detection cycle hands control to the various callbacks can help reduce its overall run time. For example, the updated values of any reactive form controls are passed to their subscribers, and the associated callbacks are then executed. This can be particularly costly if the application is undergoing a rapid succession of user input. If a debounce (.debounceTime) operator is applied to the value changes, then any processing is deferred until the input has settled. Figure 12 demonstrates the use of debounce by reducing the number of subscription callbacks that are executed. In this example the only values that are operated on are changes that occur after 350 milliseconds of stability to the search term.

Fig. 12 A list filtering example that debounces the user’s input

Similarly, when choosing to emit values to event emitters, any duplicate events whose processing does not provide value should not be emitted. Figure 13 demonstrates this by emitting search terms instead of acting on them immediately. However, it only emits values that have a semantic difference to the previous value.

Fig. 13 A demonstration of selectively emitting values based on context

Also, when working with data-bound objects, Angular calculates equality by reference. This means that OnChanges will fire each time a bound object’s reference changes even if its content has not. Being intentional about changing such backing data can reduce the number of unneeded OnChange and input setter executions.

5.1 Controlling change detection

The effects of carefully controlling which callbacks are executed are magnified when taking direct control of change detection. The description provided earlier concerning change detection was based on Angular’s default behavior. However, Angular has an API that provides additional methods for controlling how and when change detection runs. The first of these APIs, ChangeDetectionStrategy.OnPush, will change the behavior of change detection for a given component. When applied, the change detection process will skip the component unless one of its inputs change or an Observable connected to an async pipe in its template receives an update. Consequently, any child components, located within the component’s template, will also be skipped. The change detection process can thus be reduced to only checking exactly what is needed to render changes by structuring the application to take advantage of this API. Figure 14 illustrates what this looks like by showing the step-wise checks that take place in one such scenario.

Fig. 14 Demonstration of change detection with OnPush in play

Utilizing this new strategy the filtered list code above can easily be rearranged to meet such a requirement as demonstrated in Figures 15 – 17.

Fig. 15 The instructor-list component written to utilize OnPush. Notice that Inputs are the only source of change

Fig. 16 The template for the instructor-list component is also free of data mutation.

Fig. 17 The app-component html indicates that the filtered list is computed before providing it to app-instructor-list.

Alternatively, it is also possible to request that change detection be stopped entirely for a component. Figure 18 demonstrates some of the options available when controlling change detection manually. How to use this properly and effectively is highly dependent on the situation. It is rare that performance issues need this level of control to be resolved and its use should be reserved for exceptional cases.

Fig. 18 A brief highlight of the API available for manual change detection

Another way to control change detection is to execute long-running code outside of change detection entirely. If a particular block of code can be executed asynchronously, Angular provides an API to mark a callback to run outside of change detection. Using this API will allow the current change detection process to complete and the browser to rerender. The callback will then execute; when it finishes, a new change detection cycle will begin to display the results.

Example Coming Soon!

For particularly expensive calculations, a web worker can be used in conjunction with manual change detection. The following repository contains an example that runs d3 force calculation – a particularly expensive operation – inside a web worker [2]. The results are returned after completion, and Angular is informed that change detection is needed using a component change detector reference.

6.0 Reducing the duration of change detection

During change detection, Angular checks which data bindings need to be updated to apply the most recent changes. Features built into Angular can be leveraged to speed up this process; similarly, there are pitfalls that can make this process slower.

6.1 Template Methods

Angular has a very convenient feature that allows binding data directly to the result of a method call. By using Angular’s template binding syntax to assign an attribute to a method, the results will be recalculated with every change detection cycle. While this can be convenient, it also adds the results of these calculations to the cost of every change detection cycle. This cost has the potential to greatly impact an application’s responsiveness, for example, when binding to a method is combined with an ngFor. There are generally two approaches for improving performance when this happens: pre-computing the results or implementing the method as a pure pipe.

The most common situation in which an ngFor is combined with a method call is to perform a calculation based on each entry that is displayed. Rather than recomputing the display value on every change detection, there is often opportunity to calculate the additional properties as needed. For example consider the following code:

Fig. 19 (Before) A simple template binding that executes numClasses for each entry in instructorList on every change detection cycle

Fig. 20 (Before) The backing component class for the template sources its data with no upfront processing. Line 37 defines the method to call from the template

Fig. 21 (After) After some changes in how the instructorList is obtained, there is now a numClasses property that contains the desired value

Fig. 22 (After) The backing component class demonstrates how the desired property could be computed upon retrieval and added to the objects.

In this example, object properties are only recalculated if the list changes. This occurs significantly less often than each change detection cycle, possibly never again. This is the most performant way to handle such situations, but it can sometimes be difficult to achieve.

Creating and using a custom pure pipe is generally far more convenient than restructuring the application’s data flow, but it is slightly less performant. A pure pipe is a pipe that behaves much like a pure function: The results of executing it are based solely on its input, and the input is left unchanged. When using a pure pipe in place of method bindings, the pipe is still executed each change detection cycle. However, the execution will benefit from the fact that Angular caches the results of previous executions: If a pipe is executed more than once with the same parameters, the results of the first execution are returned. As a result, although the pipe will still be invoked each change detection cycle in place of the method call, performance will benefit from the caching provided by Angular. Figures 23 and 24 demonstrate the previous example once more with the utilization of a pure pipe.

Fig. 23 The template now executes a pipe to produce the desired class count

Fig. 24 A custom pipe is introduced, removing the need to precompute the additional data property.

6.2 ngFor

ngFor can also cause excessive DOM manipulation. By default, when iterating over a list of objects, Angular will use object identity to determine if items have been added, removed, or rearranged. This works well for most situations. However, if immutable practices are utilized when updating the data within the list, the identities will be updated and ngFor will generate a new collection of DOM elements to be rendered. If the list is long or complex enough, this will increase the time it takes the browser to render the change. To mitigate this issue, it is possible to use trackBy to tell Angular how to identify the entries as seen in figures 25 and 26.

Fig. 25 Expanding a basic ngFor to utilize a trackBy method

Fig. 26 Line 23 shows the method structure for a trackBy method.

This will reduce the amount of DOM regeneration needed to render any changes even in the case of rapid changes to an immutable data set.

6.3 AOT

The goal of change detection is to translate data changes into a newly-rendered view by updating DOM attributes. Angular runs in just-in-time (JIT) mode by default where its interpretation of component templates is executed as part of the digest cycle. This mode of operation is great when building and debugging an application, but it adds significant overhead in the browser at run time. Compiling Angular using the command line interface (CLI) with both prod mode and ahead-of-time (AOT) compiling reduces this overhead by precompiling the application’s component templates and removing the need for JIT processing.

7.0 Observable Pipelines

Observables are a powerful abstraction for dealing with asynchronous events. Proper usage can result in drastically reduced line counts in an application. However, as a source of change in Angular applications, they should be subject to the same performance scrutiny as component event handlers and change handlers. Observables are closely related to all three of the primary points listed in section 2.0. As such, it is crucial to select the right operators and understand how they are used to ensure that an application’s performance is not degraded by their use.

7.1 distinctUntilChange

When using Observables it is not uncommon that an Observable may emit consecutive duplicates. Depending on the situation it may not be of any benefit to reprocess the same data twice. Rxjs provides an operation, distinctUntilChanged, that will filter any duplicate, consecutive updates from flowing downstream [6]. This operation is shown in Figure 27 as a marble diagram by RxMarbles [7].

Fig. 27 Marble diagram showing how values pass into and out of distinctUntilChanged

7.2 share

It is quite common in an Angular application to use the data that flows out of an Observable in more than one location. When this happens all of the upstream processing needed to produce the data executes once for each subscription and usage of async pipe. If the callback in the Observable pipeline contain any sufficiently lengthy calculations the cost will add up quickly. Ideally the computation would be executed once for each unique update and the result would be made available to all subscribers. This can be achieved with the observable operator share [8]. Figure 28 utilizes share by sending the result of the Http call into a list display as well as a method used to calculate the total number of classes. In the absence of share, any processing that may be added between lines 12 and 16 of Figure 29 (as well as the Http request!) will be executed for each reference to “results”.

Fig. 28 Lines 16 and 31 demonstrate separate uses of the same Observable data.

Fig. 29 share is introduced on line 16 to prevent extraneous calculations

7.3 withLatestFrom

Another common use case with Observables is the need to combine multiple streams of data together to calculate some results. This is most often achieved with the usage of the combineLatest operator. As the name implies it is used to combine the latest results from each Observable that is passed in. The callback is passed the results and executed each time any of the supplied observables receive an update. There are situations however, where the calculations need only run when one specific observable changes, but the most recent values of other others are still needed for calculation. In these scenarios it is possible to reduce the number of executions by switching to withLatestFrom [9]. As described above, withLatestFrom will rerun the desired calculation only when the observable it is applied to changes, but makes available the most recent values of all other observables passed as parameters. This operation is shown in Figure 30 as a marble diagram.

Fig. 30 Marble diagram showing how values pass into and out of withLatestFrom

7.4 throttleTime

Some forms of streaming data occur at a very high frequency. Though it may not be necessary to display each update in the UI. Some use cases only require notifying the user of updates once every n milliseconds. In these situations it may be possible to utilize an operator called throttleTime [10]. This operation is shown in Figure 31 as a marble diagram.

Fig. 31 Marble diagram showing how values pass into and out of throttleTime

8.0 Conclusion

Angular’s change detection system is incredibly quick. However, the ease that Angular affords developers to synchronize custom application functionality to UI updates makes it possible to create unintended performance bottlenecks. Knowing where to look to eliminate these bottlenecks can be difficult. Armed with the performance improvements outlined in this guide, a motivated Angular developer can meet runtime performance needs by designing the application to use Angular’s resources optimally or moving code blocks outside of the Angular layer.

9.0 References