在ngAfterViewInit中调用时，ViewContainerRef是未定义的

Question

I want to dynamically create a child component when the parent component is initialised, but when I tried to create it in ngAgterViewInit(), it throws the error that the ViewContainerRef is undefined.

component.ts

  @ViewChild('container', {read: ViewContainerRef}) container: ViewContainerRef;

  constructor(private resolver: ComponentFactoryResolver) {
  }

  ngAfterViewInit(){
    const factory = this.resolver.resolveComponentFactory(ChildComponent);
    this.container.createComponent(factory); //container is undefined here

  }

component.html

...
<div class="row" #container ></div>
...

Answer 1

Since the div is inside an ngIf conditional block, it may not be available in ngAfterViewInit . You can protect the code against that possibility by monitoring the presence of the element with ViewChildren and the QueryList.changes event:

@ViewChildren('container', { read: ViewContainerRef }) containers: QueryList<ViewContainerRef>;

ngAfterViewInit() {
  if (this.containers.length > 0) {
    // The container already exists
    this.addComponent();
  };

  this.containers.changes.subscribe(() => {
    // The container has been added to the DOM
    this.addComponent();
  });
}

private addComponent() {
  const container = this.containers.first;
  const factory = this.resolver.resolveComponentFactory(ChildComponent);
  container.createComponent(factory);
}

See this stackblitz for a demo.