Angular：访问位于 `async` pipe 元素内的模板引用变量

Question

I want to access a template reference variable using @ViewChild while the element is inside an element with async pipe present as follows:

<ng-container *ngIf="productTemplates$ | async as templates; else loading">
  <div class="row">
    <mat-checkbox #selectAll (change)="toggleAllTemplates($event)">Select All</mat-checkbox>
  </div>
</ng-container>

How can I safely access the #selectAll reference early in the lifecycle of the component (like in ngAfterViewInit ) while it is not present until the API call for products is returned?

Answer 1

I would replace the @ViewChild with @ViewChildren instead. It might seem not that intuitive at first, but the reasoning behind it is that @ViewChildren always returns you a QueryList that is not null inside the ngAfterViewInit lifecycle method.

You can then subscribe to the changes observable to be notified each time this query list changes

@ViewChildren('selectAll', { read: MatCheckbox }) selectAllCheckboxes!: QueryList<MatCheckbox>;

ngAfterViewInit() {
  this.selectAllCheckboxes.changes.pipe(
    filter(() => this.selectAllCheckboxes.length > 0)
  ).subscribe(() => this.doSomethingWithTheCheckbox());
}

private doSomethingWithTheCheckbox() {
  const selectAllCheckbox = this.selectAllCheckboxes.first;
  // here you should be able to safely use your selectAllCheckbox
}