How to check whether ng-content exists

问题

Suppose i have simple Bootstrap panel component with multiple transclusion slots. Template example:

<div class="panel panel-default">
  <div class="panel-heading">
    <ng-content select="my-panel-heading"></ng-content>
  </div>
  <div class="panel-body">
    <ng-content select="my-panel-content"></ng-content>
  </div>
</div>

I want to make panel-heading optional. How do i hide <div class="panel-heading"> element, if there is no content provided for <ng-content select="my-panel-heading"></ng-content>

回答1:

If you have a parent element of <ng-content> with a template variable (#panelHeading)

<div class="panel panel-default">
  <div class="panel-heading" #panelHeading [hidden]="!showHeading">
    <ng-content select="my-panel-heading"></ng-content>
  </div>
  <div class="panel-body">
    <ng-content select="my-panel-content"></ng-content>
  </div>
</div>

then you can query for it like

@ViewChild('panelHeading') panelHeading;

and set a property depending on whether there are children or not

constructor(private cdRef:ChangeDetectorRef) {}

showHeading:boolean = false;

ngAfterViewInit() {
  this.showHeading = this.panelHeading.nativeElement && this.panelHeading.nativeElement.children.length > 0;
  this.cdRef.detectChanges();
}

If <my-panel-heading> is an Angular2 component, then you can also use

@ContentChild(MyPanelHeading) panelHeading:MyPanelHeading;

constructor(private cdRef:ChangeDetectorRef) {}

showHeading:boolean = false;

ngAfterViewInit() {
  this.showHeading = this.panelHeading != null;
  this.cdRef.detectChanges();
}

回答2:

You'd have to remove all your spaces but you could do this with CSS if you really cared about it (ng-content doesn't take up space):

.panel-heading:empty { display: none }

<div class="panel-heading"><ng-content select="my-panel-heading"></ng-content></div>

来源：https://stackoverflow.com/questions/38692881/how-to-check-whether-ng-content-exists

标签

angular

transclusion