Micro syntax `as` not working with my own directive

Question

I want to support the following micro syntax:

foo$ | async as xyz

Example:

<p *appVar="foo$ | async as xyz">
   Value is {{ xyz }}!
</p>

That stream is defined as

$foo = of('some value')

Here is my directive:

@Directive({
   selector: '[appVar]'
})
export class VarDirective<T> {
   @Input() appVar: T;

   constructor(
      private templateRef: TemplateRef<T>,
      private viewContainer: ViewContainerRef
   ) {}

   ngOnInit(): void {
      this.viewContainer.createEmbeddedView(this.templateRef);
   }
}

DEMO

Now, whatever I do, that xyz variable is always undefined.

If I create a context (which is suggested in this post)

 const context = { $implicit: this.appVar };
 this.viewContainer.createEmbeddedView(this.templateRef, context);

DEMO

I get the same result. Any suggestions what I do wrong here?

Answer 1

The expression has to be available in the context under the same name as the directive itself, which is appVar in this case:

this.viewContainer.createEmbeddedView(this.templateRef, {
  $implicit: this.appVar,
  appVar: this.appVar
});

Demo

Compare it to the built-in ngIf directive:

@Input()
set ngIf(condition: any) {
  this._context.$implicit = this._context.ngIf = condition;
  this._updateView();
}

and to the TuiLetContext:

get $implicit(): T {
  return this.internalDirectiveInstance.tuiLet;
}

get tuiLet(): T {
  return this.internalDirectiveInstance.tuiLet;
}