How to create custom directive with enabled “as syntax” in angular 5?

Question

In angular 5 there is built in feature for ngIf directive - as syntax. You can find the example here https://toddmotto.com/angular-ngif-async-pipe.

In the example below user$ is an observable.

<div *ngIf="(user$ | async) as user; else loading">
  <user-profile
    [user]="user.profile">
  </user-profile>
  <user-messages
    [user]="user.messages">
  </user-messages>
</div>

<ng-template #loading>
  Loading stuff...
</ng-template>

I've created my own directive that replaces content of element with loading spinner and I want to enable passing observable as input, see code below.

@Directive({ selector: '[appLoadingData]' })
export class LoadingDataDirective {
  constructor(
    private templateRef: TemplateRef<any>,
    private viewContainerRef: ViewContainerRef,
    private componentFactoryResolver: ComponentFactoryResolver
  ) { }
  @Input()
  set appLoadingData(data: any) {
    if (data != null) {
      if (data.subscribe) {
        this.addLoadingPlaceholder();
        data.subscribe(() => {
          setTimeout(() => this.addView(), 0);
        });
      } else {
        this.addView();
      }
    } else {
      this.addLoadingPlaceholder();
    }
  }

  private addView() {
    this.viewContainerRef.clear();
    this.viewContainerRef.createEmbeddedView(this.templateRef);
  }

  private addLoadingPlaceholder() {
    this.viewContainerRef.clear();
    const loadingComponentFactory = this.componentFactoryResolver.resolveComponentFactory(LoadingDataComponent);
    this.viewContainerRef.createComponent(loadingComponentFactory);
  }
}

And I want to use in the next way

    <ng-container *ngFor="let chartConfig of chartsConfigs">
      <div class="chart-wrapper"
        *appLoadingData="(chartConfig.chartGroups$ | async) as groups">
        <chart-summary [chartData]="groups"
          [chartOptionsName]="chartConfig.chartType">
        </chart-summary>
      </div>
    </ng-container>

or ideally *appLoadingData="chartConfig.chartGroups$ as groups"

Without as syntax(*appLoadingData="chartConfig.chartGroups$ | async" and [chartData]="chartConfig.chartGroups$ | async"), after my loader dismissed there is a delay, I think it happens because on the addView() in directive, async pipe called second time. I want to implement as syntax like for ngIf directive. I've tried to use it like this: *appLoadingData="(chartConfig.chartGroups$ | async) as groups" , but unfortunately in this case groups is always undefined.

How to implement as syntax in custom directive?

Nikita Nikita · Accepted Answer · 2018-08-31T14:21:17

You can see that they are using _context variable. https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_if.ts

Also see there @yurzui answer. How to declare a variable in a template in Angular2

My final result:

export class LoadingDataDirective {
  context: any = {};
  constructor(
    private templateRef: TemplateRef<any>,
    private viewContainerRef: ViewContainerRef,
    private componentFactoryResolver: ComponentFactoryResolver
  ) { }
  @Input()
  set appLoadingData(data: any) {
    if (data != null) {
      if (this.isObservable(data)) {
        this.addLoadingPlaceholder();
        const subscription = data.subscribe((observableResult: any) => {
          this.addView(observableResult);
          subscription.unsubscribe();
        });
      } else {
        this.addView(data);
      }
    } else {
      this.addLoadingPlaceholder();
    }
  }

  private addView(contextData: any) {
    this.context.$implicit = this.context.appLoadingData = contextData;
    this.viewContainerRef.clear();
    this.viewContainerRef.createEmbeddedView(this.templateRef, this.context);
  }

  private addLoadingPlaceholder() {
    this.viewContainerRef.clear();
    const loadingComponentFactory = this.componentFactoryResolver.resolveComponentFactory(LoadingDataComponent);
    this.viewContainerRef.createComponent(loadingComponentFactory);
  }

  private isObservable(data: any): boolean {
    return !!data.subscribe;
  }
}

chartConfig.charGroup$ - is the observable

   <ng-container *ngFor="let chartConfig of chartsConfigs">
      <div class="chart-wrapper"
        *appLoadingData="chartConfig.chartGroups$ as groups">
        <chart-summary [chartData]="groups"
          [chartOptionsName]="chartConfig.chartType">
        </chart-summary>
      </div>
    </ng-container>

How to create custom directive with enabled “as syntax” in angular 5?

1 Answers