指令中的异步主机绑定

Question

I'm looking for the best way to handle HostBinding with async value.

Before Angular v2.1.2 I could use the host property in the @Directive decorator like that :

@Directive({
    selector: 'img[my-directive]',
    host    : {
        '[alt]'  : "alt | async"
    }
})
export class MyDirective {
    alt: Observable<string>;
}

But it looks like this was not the intended behavior, since version 2.1.2 fixes it. See don't access view local variables nor pipes in host expressions .

Now, when compiling with AoT compilation, I get Parser Error: Host binding expression cannot contain pipes in Directive .

Answer 1

Tobias Bosch (member of the Angular team) writes:

Host bindings of a component ("child") are executed in the component that uses that component ("parent"). And the parent component can belong to a different NgModule. So if you use a pipe, the pipe is resolved against the NgModule of the parent component. However, if that NgModule does not declare the pipe that you are using, your component is broken.

This is the reason why we never wanted to have pipes in host bindings. After one of the bigger compiler refactorings before 2.0 final, we accidentally reintroduced it, but this was a bug, not a feature, as the semantics are wrong.

Source:

Async Host Binding No Longer Works #12671

There also is an open ticket to use Observables with HostBindings:

https://github.com/angular/angular/issues/19483

Answer 2

I'm sure there is no special way to do it. You need to explicitly assign to the property

@Directive({
    selector: 'img[my-directive]',
    host    : {
        '[alt]'  : "alt"
    }
})
export class MyDirective {
    altObservable: Observable<string>;
    alt: string;

    subscription:Subscription;    

    ngOnInit() {
      this.subscription = this.altObservable.subscribe(val => this.alt = val)
    }

    ngOnDestroy(){
      this.subscription && this.subscription.unsubscribe();
    }
}

Ensure you unsubscribe subscriptions you create imperatively.