Can't get to @ViewChildren in parent component

问题

I am trying to get control over the children instances of a component and can't go past an error. I am trying to follow along the answers from this issue.

The parent component Sequence contains children components of SequenceStep. The parent contains the following declaration:

@ViewChildren(SequenceStep) steps: QueryList<SequenceStep>;

then I am trying to do something about the children:

ngAfterViewInit() {
    this.steps.changes.subscribe(steps => {
        console.log(steps);
    });
}

The error I'm getting is:

metadata_resolver.js:639 Uncaught Error: Can't construct a query for the property "steps" of "Sequence" since the query selector wasn't defined.

whereas both Sequence and SequenceStep components do have their selectors defined in their @Component decorators (sequence and sequence-step respectively).

What am I doing wrong here?

回答1:

There are a few things

If you pass children from the outside, they are content not children. @ViewChild() only works for children added to the template of a component directly.

@ContentChildren() works on transcluded content:
```
@ContentChildren(TheChild) kids: QueryList<TheChild>;
```
this.kids.changes() only notifies about changes after the initialization. Therefore just access this.kids directly in ngAfterViewInit() and then subsribe to get notified about later changes
```
ngAfterViewInit() {
  this.myKidsCount = this.kids.length;
  this.cdRef.detectChanges();

  this.kids.changes.subscribe(kids => {
      this.myKidsCount = kids.length;
  });
}
```
Angular doesn't like when change detection causes changes. ngAfterViewInit() is called by change detection, this is why this.myKidsCount = this.kids.length causes an exception.

To work around I invoke change detection explicitly after changing the property myKidsCount:

constructor(private cdRef:ChangeDetectorRef){}

ngAfterViewInit() {
  this.myKidsCount = this.kids.length;
  this.cdRef.detectChanges();
}

Plunker example

回答2:

Have you tried adding quotes to your @ViewChildren arg?

@ViewChildren('SequenceStep') steps: QueryList<SequenceStep>;

回答3:

This issue can be related to the import of SequenceStep, check the class name of the related import row.

回答4:

@ViewChildren('SequenceStep') steps: QueryList<SequenceStep>;

Worked for me. Angular 4.X.X Angular CLI 1.X.X

来源：https://stackoverflow.com/questions/39958199/cant-get-to-viewchildren-in-parent-component

标签

angular

typescript

angular-decorator