如何在angular 5中启用“as syntax”创建自定义指令？

Question

在角度5中，有ngIf指令的内置功能 - as syntax。你可以在这里找到https://toddmotto.com/angular-ngif-async-pipe的例子。

在下面的例子中，user$是一个可观察的。

<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>

我创建了自己的指令，用加载微调器替换元素的内容，我想启用传递observable作为输入，请参阅下面的代码。

@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);
  }
}

我希望以下一种方式使用

    <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>

或者理想的*appLoadingData="chartConfig.chartGroups$ as groups"

没有作为语法（*appLoadingData="chartConfig.chartGroups$ | async"和[chartData]="chartConfig.chartGroups$ | async"），在我的加载器被解雇后有一个延迟，我认为它发生是因为在addView()指令中，异步管道第二次调用。我想为ngIf指令实现as syntax。我试图像这样使用它：*appLoadingData="(chartConfig.chartGroups$ | async) as groups"，但不幸的是在这种情况下groups总是undefined。

如何在自定义指令中实现as syntax？

Answer 1

你可以看到他们正在使用_context变量。 https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_if.ts

另见@yurzui的回答。 How to declare a variable in a template in Angular2

我的最终结果：

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$ - 是可观察的

   <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>