可观察到的角度* ngif和来自异步调用的数据

Question

I'm using a few REST services to gather data and then change the value of an Observable in my component code. I'm trying to use an *ngIf to show/hide div tags based on the result. However, it isn't working, the *ngIf is not updating based on the change to the Observable.

Do I have to trigger change detection? What's the best way to achieve this in Angular 7? Here's one example I tried:

HTML:

<div *ngIf="isAnimal$">
     <p>animal</p>
</div>

Component:

public isAnimal$: Observable<boolean> = of(false);
...
ngOnInit() {
  checkAnimal();
}

private checkAnimal() {
  this._dataService.getData()
      .subscribe((result) => {
         if (result === 'animal') {
           this.isAnimal$ = of(true);
         }
      });
}

I also tried creating a simple service and that didn't work either. Like this: Angular *ngIf binding to function

Answer 1

I suggest the following:

HTML:

<div *ngIf="isAnimal">
     <p>animal</p>
</div>

Component:

public isAnimal: boolean = false;

ngOnInit() {
  checkAnimal();
}

private checkAnimal() {
  this._dataService.getData().subscribe(
    (result) => {
       if (result === 'animal') {
         this.isAnimal = true;
       }
    },
    (error) => {
      console.log(error);
    }
 );
}

I am not sure, but I think you could use the async pipe instead using your implementation:

<div *ngIf="isAnimal$ | async">
     <p>animal</p>
</div>

The reason your solution is not working is because isAnimal$ is an observable that has to be resolved. The async pipe could do this, but I prefer the first solution to be honest. It does not seem natural to me to create another observable of(true) in your subscription.