ViewChild返回未定义

Question

I am using material design and Angular 5. I was trying to use the material loader, so when navigation start show loader and when ends remove loader as per the answer in this question . I tried viewchild by

<mat-progress-bar #spinnerElement [mode]="'indeterminate'" [color]="'primary'"></mat-progress-bar>

and called in my component constructor as below:

@ViewChild('spinnerElement')
spinnerElement: ElementRef;

constructor(
.....
    private ngZone: NgZone,
    private renderer: Renderer) {
        console.log(this.spinnerElement, 'spinnerElement');
    }

But console always returns undefined . Bit new to Angular. Any idea why guys?

Answer 1

Take a look at angular life cycle .

You can't access your DOM elements in your constructor because they still not rendered.

Try to access your 'spinnerElement' in your ngOnInit().

ngOnInit():

Initialize the directive/component after Angular first displays the data-bound properties and sets the directive/component's input properties. Called once, after the first ngOnChanges().

Answer 2

在ngOnInit()访问@ViewChild在初始化之前不能使用它。