author: [Michael Hladky]
created: 2020-02-02
status: [draft | in review | approved | implemented | obsolete]
doc url: https://hackmd.io/8_3rp0A7RweSYJiulsifbQ?both
feedback url: https://github.com/ngrx/platform/issues/2441
design doc template version: 1.0.0
The subject of this design doc is to create a structural directive, similar to *ngIf
directive but without the display/show functionality.
It should just bind observable values to the view.
It takes asynchronous primitives and binds and renders their value to the template.
The main goal is to have such a directive also work zone-less.
In this document use cases and scenarios are defined to add a structural directive to the new ngrx/component package. This directive is one of many reactive primitives to make Angular more reactive.
We can look at it as a derivative of the ngrxPush
pipe.
The directive solves following primitive reactive problems:
As this document uses specific terminology find detailed explanations in the gist here: Reactive Angular Terminology
Especially rendering should be done only over the ngrxPush
or *ngrxLet
as the render process is different from template expressions to template bindings.
Handling of change detection in the component would make it impossible to interact with either only the EnbeddedView or the ComponentView and would lead to unwanted performance drawbacks. This topic is explained in depth in another document listed in the caveats section.
With the *ngrxLet
directive, the user will be able to bind the emitted notifications of an observable to an EmbeddedView
.
This is done by using the structural directive in the template, in the same way, he would use the *ngIf
to bind values but without the display/hide logic of the *ngIf
.
The user can use any observable that can be referenced over the component class.
File: any.component.ts
import { Component, Input } from '@angular/core';
import { Subject, interval } from 'rxjs';
@Component({
selector: 'any',
templateUrl: 'any.component.html'
// Implementation works independent of the ChangeDetectionStrategy
})
export class AnyComponent {
inputDecoratorSubject$ = new Subject<any>();
@Input()
set value(value: any) {
this.inputDecorator$.next(value);
};
classInternalObservable$ = interval(1000);
routerParams$ = this.activatedRoute.params;
constructor(public activatedRoute: ActivatedRoute,
public store$: Store<any>) {
}
}
and bind it's valued to the part of the template the pipe is used.
File: any.component.html
<ng-container *ngrxLet="inputDecorator$ as o">
{{o}}
</ng-container>
<div *ngrxLet="classInternalObservable$ as o">
{{o}}
</div>
<ng-container *ngrxLet="routerParams$ as o">
{{o}}
</ng-container>
<div *ngrxLet="store$ as o">
{{o}}
</div>
If there is already existing code that uses the *ngIf
directive used only to bind observable values it the template, and not relying on the hide/show functionality of it, the *ngrxLet
pipe can be used as a drop-in replacement and also drastically reduce the complexity of that code as the if approach sometimes requires dirty hacks to work with boolean values.
<!-- before: -->
<ng-container *ngIf="observable$ | async as o">
{{o}}
</ng-container>
<!-- after: -->
<ng-container *ngrxLet="observable$ as o">
{{o}}
</ng-container>
The current approach to bind observable values to an EmbededView looks like that:
<ng-container *ngIf="observableNumber$ | async as n">
<app-number [number]="n">
</app-number>
<app-number-special [number]="n">
</app-number-special>
</ng-container>
The problem is *ngIf
is also interfering with rendering and in case of a 0
the container would be hidden.
At the moment it is not possible to bind observable values to an EmbeddedView without additional behavior based on the emitted values-
*ngIf
<ng-container *ngIf="observableNumber$ | async as n">
{{n}}
</ng-container>
*ngFor
<ng-container *ngFor="let n of [observableNumber$ | async]>
{{n}}
</ng-container>
ngrxLet.directive.ts
// LetContext defined ths context structure
export class LetContext {
constructor(
// to enable let we have to use $implicit
public $implicit?: any,
// to enable as we have to assign this
public ngrxLet?: any,
// value of error of undefined
public $error?: Error | undefined,
// true or undefined
public $complete?: true | undefined
) {}
}
@Directive({selector: '[ngrxLet]'})
export class LetDirective implements OnInit, OnDestroy {
// Initates the viewContext with an empty LetContext instance
private viewContext = new LetContext();
// subscription to the renderer process
private subscription = new Subscription();
// Enables to receive input binding changes push based
private observablesSubject = new Subject<Observable<any>>();
// Input binding for the observable to bind to the EnbeddedView
@Input()
set ngrxLet(obs: Observable<any>) {
this.observablesSubject.next(obs);
}
constructor(
private cdRef: ChangeDetectorRef,
private readonly templateRef: TemplateRef<LetContext>,
private readonly viewContainerRef: ViewContainerRef
) {
// Retreive values from passed argument
this.subscription = this.observablesSubject.pipe(
tap({
// Assign value that will get returned from the transform function
// on the next change detection
next: renderedValue => {
// to enable `let` syntax we have to use $implicit (var; let v = var)
this.viewContext.$implicit = renderedValue;
// to enable `as` syntax we have to assign the directives selector (var as v)
this.viewContext.ngrxLet = renderedValue;
this.cdRef.detectChanges();
}
})
)
// Start to render passed values
.subscribe();
}
ngOnInit() {
// Create and embadded view with the created viewContext and bind it to the templateRef.
this.viewContainerRef.createEmbeddedView(
this.templateRef,
this.viewContext
);
}
ngOnDestroy() {
// Stop to render values
this.subscription.unsubscribe();
// Clear the viewContainerRef
this.viewContainerRef.clear();
}
}
As these are a lot of equal code blocks also used in the push pipe to ensure the type of passed values or the flattening and update behavior of emitted notifications.
Therefore we pull out some parts here to have these snippets available across exposed parts of the package.
The first repetitive code is the type checking of passed values
toObservableValue.ts
export function toObservableValue<T>(
potentialObservableValue$: potentialObservableValue<T>
): Observable<T | undefined | null> {
if (isUndefinedOrNullGuard(potentialObservableValue$)) {
return of(potentialObservableValue$);
}
if (
isPromiseGuard(potentialObservableValue$) ||
isObservableGuard(potentialObservableValue$)
) {
return from(potentialObservableValue$);
}
throw new ArgumentNotObservableError();
}
processCdAwareObservables.ts
export function processCdAwareObservables<T>(
resetContextBehaviour: (
o$$: Observable<Observable<T>>
) => Observable<Observable<T>>,
updateContextBehaviour: (
o$$: Observable<Observable<T>>
) => Observable<Observable<T>>
) {
return (o$: Observable<potentialObservableValue<T>>): Observable<T> => {
return o$.pipe(
toObservableValue(),
// Ignore observables of the same instances
distinctUntilChanged(),
resetContextBehaviour,
// Add apply changes to context behaviour
updateContextBehaviour,
// @NOTICE Configure observable here with config
// Add cd optimization behaviour
// ----
// unsubscribe from previous observables
// then flatten the latest internal observables into the output
switchAll(),
// reduce number of emissions to distinct values compared to teh previous one
distinctUntilChanged()
);
};
}
ngrxLet.directive.ts
@Directive({selector: '[ngrxLet]'})
export class LetDirective implements OnInit, OnDestroy {
// Initates the viewContext with an empty LetContext instance
private viewContext = new LetContext();
// subscription to the renderer process
private subscription = new Subscription();
// Enables to receive input binding changes push based
protected observablesSubject = new Subject<Observable<unknown> | Promise<unknown> | null | undefined>();
protected observables$ = this.observablesSubject.pipe(
processCdAwareObservables(
// In case we dont have a value set yet we will receive undefined
// In some cases people try to stop rendering by appliing null
// Also null is a legitiment value for and value not assigned yet
tap({
next: (obs: Observable<unknown>) => {
// Apply values that should get rendered
this.renderedValue = undefined;
// Render new values to the template
this.cdRef.detectChanges();
}
}),
// Update renderedValue and render it to the template
tap({
// Assign value that will get returned from the transform function
// on the next change detection
next: renderedValue => {
// to enable `let` syntax we have to use $implicit (var; let v = var)
this.viewContext.$implicit = renderedValue;
// to enable `as` syntax we have to assign the directives selector (var as v)
this.viewContext.ngrxLet = renderedValue;
// Render new values to the template
this.cdRef.detectChanges();
},
// Get error object here and apply needed error logic
error: (e: unknown) => {
// Logic to deal with error object
// e. g. this.renderedValue = e.message; this.cdRef.detectCahnges();
// @Notice:
// This is not catching the error
}
})
)
);
// Input binding for the observable to bind to the EnbeddedView
@Input()
set ngrxLet(obs: Observable<any>) {
this.observablesSubject.next(obs);
}
constructor(
private cdRef: ChangeDetectorRef,
private readonly templateRef: TemplateRef<LetContext>,
private readonly viewContainerRef: ViewContainerRef
) {
// Retreive values from passed argument
this.subscription = this.observables$.
// Start to render passed values
.subscribe();
}
ngOnInit() {
// Create and embadded view with the created viewContext and bind it to the templateRef.
this.viewContainerRef.createEmbeddedView(
this.templateRef,
this.viewContext
);
}
ngOnDestroy() {
// Stop to render values
this.subscription.unsubscribe();
// Clear the viewContainerRef
this.viewContainerRef.clear();
}
}
ViewEngine / Ivy interoperability
With the requirement to support both, we increase the complexity of the implementation.
We have to do this because in ViewEngine applications it's recommended to keep supporting View Engine in Angular versions 9 and 10
Usage of Angulars internal ɵ API:
With the current situation, we rely on Angulars internal ɵmarkDirty
and ɵdetectChanges
function.
This may have critical effects as these APIs can change in any release.
We need to ensure we can use ChangeDetectorRef
instead.
It would make a lot of sense if they expose it Expose the isPromise
utility function to the public API.
Immutability:
The second distinctUntilChanged
operator for emitted values of the passed observables forces the user to work immutable. Even if this is in most cases given as people that
work more reactively use normally also ChangeDetectionStrategy.OnPush
.
Still, we have to consider clear communication.
Coalescing and Scoped Coalescing:
This problem is explained in the document Design Doc - Coalescing of Change Detection
Security Considerations:
Errors happening in the observables are caught and swallowed without any thrown error.
The Observable completes instead. The error object is handled in another part of the code.
Sanitization:
Sanitization of the emitted values to get rendered in the template is done by default by Angular.
It could be possible we run into similar problems as with the Design Doc - Push Pipe
If so, details on the change detection coalescing feature can be found in Design Doc - Coalescing of Change Detection.
As the scope of this package is quite small I consider the documentation in text and some examples in the demo app and as code snippets
is everything we need.
Here a first draft of the documentation:
let.md
The needed documentation has no impact on the current way the documentation is maintained.
The new directive does not affect the existing API interface. It will get introduced under a new package @ngrx/component
.
It exposes only the pipes class and its usage in the template as a pipe reference.
Following dependencies are needed:
The new directive is prefixed with ngrx
and named let
.
It is used the same way as the *ngIf
, *ngSwitch
or *ngFor
structural directives are used
<ng-container *ngrxLet="interval$ as n">{{n}}</ng-container>
.
The term 'let' refers to ECMAScript's let statement, which declares a block scope local variable whose initial value is optional.
This tries to give the user an intuitive understanding of its behavior out of the pipe's name itself.
The new directive will not introduce any breaking changes.
The directive and especially its features can be rolled out in the following way:
Alpha releases of the @ngrx/component package and the directive include all features the async pipe has regarding the processing of passed values, to have the directive ready for a drop-in replacement of the situations where
<ng-container *ngIf="observableNumber$ | async as n">
{{n}}
</ng-container>
is used. Also, the detection depending if zone is present or not.
Shipped Features:
zone.js
is present or not (detectChanges
or markForCheck
)Beta releases of the @ngrx/component package and the directive include a configuration argument to opt-in the coalescing of change detection calls.
Shipped Features:
$error
and $completed
Rc and official releases of the @ngrx/component package and the directive include all features and the change detection coalescing is opt-out by default.
Alos internal operators and utils related to zone checks and event coalescing can be exposed in this releases.
Shipped Features:
In the late alpha versions, we introduce the coalescing of change detection calls.
If any critical problems occur in the alpha version we can just don't ship further versions.
Until the first beta, we should have to clarify this. This is the moment we introduce the configuration option for the pipe.
The coalescing can be rolled back by removing the option from the config object.
If a major issue is found in RC or even official release we can remove the push pipe from the package as it is no other part relies on the pipe.
There are no implementation details that could delay the release of other packages. Also, the bundle size of other features will not increase by introducing this feature.
If we can get rid of the usage of Angulars internal API we have pretty low maintenance costs.
Depending on how long we want to support interrupt ViewEngine and Ivy we have a year to 1,5 to remove the ViewEngine related code.
The rest of the code relies on some pretty common Angular parts from the core package and some RxJS operators creation functions and utils naming: Observable, pipe, isObservable, from, of, filter, tap, switchAll, distinctUntilChanged, shareReplay
shareReplay
will have a breaking change for our current usage as the configuration object will get removed.
A minor refactoring.
From all the other used operator we only use one where a breaking change can happen which is tap
.
If Observabls make it in the standard the most probably will have callbacks as arguments instead of an observer object.
But this is also a minor refactoring.
Imperative Code style
An alternative implementation could be done without RxJS as Angulars async
pipe is built. I went the Reactive way because
Drawbacks:
The use of RxJS could force the user to include operators in their bundle which they don't want.
An increased bundle size is expected through the following operators (rare used operators at the top):
Change Detection over ApplicationRef.tick()
An alternative way of triggering the changeDetection would be to call ApplicationRef.tick()
.
I did not go that way or even tried it out as It would result in a full application render for every change detection.
Change Detection in the Component
In the component, not all Observables values need to get rendered. Examples can be any background process lite polling or refresh clicks that dispatch an action or so.
Mention multi async
example?
The example looks like that:
*ngIf = {
a: a$ | async,
b: b$ | async
} as vm
<ng-container *ngIf="{
o1: observable1$ | async,
o2: observable2$ | async
} as viewContext">
{{viewContext.o1}}
{{viewContext.o2}}
</ng-container>
I did not include it on purpose as composition should be placed in the Typescript section.
More flexible, less noisy template.
Providing template slots for error and complete
Here a solution that provides template slots. The document is not focusing on this feature as it trys to start with the very minimum and add features after a solid base.
In the planed release it is easily possible to implement it manually.
Already existing Packages
The suggested version is the most primitive implementation.
In the RFC for this package is listed alternative implementations.
Those implementations either didn't respect the zone-less mode or had additional logic implemented to add templates for the context or other things that would go over the features of a reactive primitives.
As the pipes scope is small enough to get maintained by one person at a time we don't need to describe the breakdown of work.
Research Paper/Design Docs/StackBlitz/Video/Podcast/Blog/Tweet/Graphic:
Github Pull Request/Gists/Issue/Doc/Source Link: