问题
Question
What is the most elegant way to get @ViewChild after corresponding element in template was shown?
Below is an example. Also Plunker available.
Template:
<div id=\"layout\" *ngIf=\"display\">
<div #contentPlaceholder></div>
</div>
Component:
export class AppComponent {
display = false;
@ViewChild(\'contentPlaceholder\', {read: ViewContainerRef}) viewContainerRef;
show() {
this.display = true;
console.log(this.viewContainerRef); // undefined
setTimeout(()=> {
console.log(this.viewContainerRef); // OK
}, 1);
}
}
I have a component with its contents hidden by default. When someone calls show() method it becomes visible. However, before Angular 2 change detection completes, I can not reference to viewContainerRef. I usually wrap all required actions into setTimeout(()=>{},1) as shown above. Is there a more correct way?
I know there is an option with ngAfterViewChecked, but it causes too much useless calls.
ANSWER (Plunker)
回答1:
The accepted answer using a QueryList did not work for me. However what did work was using a setter for the ViewChild:
private contentPlaceholder: ElementRef;
@ViewChild('contentPlaceholder') set content(content: ElementRef) {
this.contentPlaceholder = content;
}
The setter is called once *ngIf becomes true.
回答2:
An alternative to overcome this is running the change detector manually.
You first inject the ChangeDetectorRef:
constructor(private changeDetector : ChangeDetectorRef) {}
Then you call it after updating the variable that controls the *ngIf
show() {
this.display = true;
this.changeDetector.detectChanges();
}
回答3:
The answers above did not work for me because in my project, the ngIf is on an input element. I needed access to the nativeElement attribute in order to focus on the input when ngIf is true. There seems to be no nativeElement attribute on ViewContainerRef. Here is what I did (following @ViewChild documentation):
<button (click)='showAsset()'>Add Asset</button>
<div *ngIf='showAssetInput'>
<input #assetInput />
</div>
...
private assetInputElRef:ElementRef;
@ViewChild('assetInput') set assetInput(elRef: ElementRef) {
this.assetInputElRef = elRef;
}
...
showAsset() {
this.showAssetInput = true;
setTimeout(() => { this.assetInputElRef.nativeElement.focus(); });
}
I used setTimeout before focusing because the ViewChild takes a sec to be assigned. Otherwise it would be undefined.
回答4:
Angular 8
You should add { static: false } as a second option for @ViewChild
Example:
export class AppComponent {
@ViewChild('contentPlaceholder', { static: false }) contentPlaceholder: ElementRef;
display = false;
constructor(private changeDetectorRef: ChangeDetectorRef) {
}
show() {
this.display = true;
this.changeDetectorRef.detectChanges();
console.log(this.contentPlaceholder);
}
}
Stackblitz example: https://stackblitz.com/edit/angular-d8ezsn
回答5:
As was mention by other, the fastest and quickest solution is to use [hidden] instead of *ngIf, this way the component will be created but not visible, there for you can have access to it, though it might not be the most efficient way.
回答6:
This could work but I don't know if it's convenient for your case:
@ViewChildren('contentPlaceholder', {read: ViewContainerRef}) viewContainerRefs: QueryList;
ngAfterViewInit() {
this.viewContainerRefs.changes.subscribe(item => {
if(this.viewContainerRefs.toArray().length) {
// shown
}
})
}
回答7:
Another quick "trick" (easy solution) is just to use [hidden] tag instead of *ngIf, just important to know that in that case Angular build the object and paint it under class:hidden this is why the ViewChild work without a problem. So it's important to keep in mind that you should not use hidden on heavy or expensive items that can cause performance issue
<div class="addTable" [hidden]="CONDITION">
回答8:
My goal was to avoid any hacky methods that assume something (e.g. setTimeout) and I ended up implementing the accepted solution with a bit of RxJS flavour on top:
private ngUnsubscribe = new Subject();
private tabSetInitialized = new Subject();
public tabSet: TabsetComponent;
@ViewChild('tabSet') set setTabSet(tabset: TabsetComponent) {
if (!!tabSet) {
this.tabSet = tabSet;
this.tabSetInitialized.next();
}
}
ngOnInit() {
combineLatest(
this.route.queryParams,
this.tabSetInitialized
).pipe(
takeUntil(this.ngUnsubscribe)
).subscribe(([queryParams, isTabSetInitialized]) => {
let tab = [undefined, 'translate', 'versions'].indexOf(queryParams['view']);
this.tabSet.tabs[tab > -1 ? tab : 0].active = true;
});
}
My scenario: I wanted to fire an action on a @ViewChild element depending on the router queryParams. Due to a wrapping *ngIf being false until the HTTP request returns the data, the initialization of the @ViewChild element happens with a delay.
How does it work: combineLatest emits a value for the first time only when each of the provided Observables emit the first value since the moment combineLatest was subscribed to. My Subject tabSetInitialized emits a value when the @ViewChild element is being set. Therewith, I delay the execution of the code under subscribe until the *ngIf turns positive and the @ViewChild gets initialized.
Of course don't forget to unsubscribe on ngOnDestroy, I do it using the ngUnsubscribe Subject:
ngOnDestroy() {
this.ngUnsubscribe.next();
this.ngUnsubscribe.complete();
}
回答9:
A simplified version, I had a similar issue to this when using the Google Maps JS SDK.
My solution was to extract the divand ViewChild into it's own child component which when used in the parent component was able to be hid/displayed using an *ngIf.
Before
HomePageComponent Template
<div *ngIf="showMap">
<div #map id="map" class="map-container"></div>
</div>
HomePageComponent Component
@ViewChild('map') public mapElement: ElementRef;
public ionViewDidLoad() {
this.loadMap();
});
private loadMap() {
const latLng = new google.maps.LatLng(-1234, 4567);
const mapOptions = {
center: latLng,
zoom: 15,
mapTypeId: google.maps.MapTypeId.ROADMAP,
};
this.map = new google.maps.Map(this.mapElement.nativeElement, mapOptions);
}
public toggleMap() {
this.showMap = !this.showMap;
}
After
MapComponent Template
<div>
<div #map id="map" class="map-container"></div>
</div>
MapComponent Component
@ViewChild('map') public mapElement: ElementRef;
public ngOnInit() {
this.loadMap();
});
private loadMap() {
const latLng = new google.maps.LatLng(-1234, 4567);
const mapOptions = {
center: latLng,
zoom: 15,
mapTypeId: google.maps.MapTypeId.ROADMAP,
};
this.map = new google.maps.Map(this.mapElement.nativeElement, mapOptions);
}
HomePageComponent Template
<map *ngIf="showMap"></map>
HomePageComponent Component
public toggleMap() {
this.showMap = !this.showMap;
}
回答10:
In my case I needed to load a whole module only when the div existed in the template, meaning the outlet was inside an ngif. This way everytime angular detected the element #geolocalisationOutlet it created the component inside of it. The module only loads once as well.
constructor(
public wlService: WhitelabelService,
public lmService: LeftMenuService,
private loader: NgModuleFactoryLoader,
private injector: Injector
) {
}
@ViewChild('geolocalisationOutlet', {read: ViewContainerRef}) set geolocalisation(geolocalisationOutlet: ViewContainerRef) {
const path = 'src/app/components/engine/sections/geolocalisation/geolocalisation.module#GeolocalisationModule';
this.loader.load(path).then((moduleFactory: NgModuleFactory<any>) => {
const moduleRef = moduleFactory.create(this.injector);
const compFactory = moduleRef.componentFactoryResolver
.resolveComponentFactory(GeolocalisationComponent);
if (geolocalisationOutlet && geolocalisationOutlet.length === 0) {
geolocalisationOutlet.createComponent(compFactory);
}
});
}
<div *ngIf="section === 'geolocalisation'" id="geolocalisation">
<div #geolocalisationOutlet></div>
</div>
回答11:
I think using defer from lodash makes a lot of sense especially in my case where my @ViewChild() was inside async pipe
回答12:
Working on Angular 8 No need to import ChangeDector
ngIf allows you not to load the element and avoid adding more stress to your application. Here's how I got it running without ChangeDetector
elem: ElementRef;
@ViewChild('elemOnHTML', {static: false}) set elemOnHTML(elemOnHTML: ElementRef) {
if (!!elemOnHTML) {
this.elem = elemOnHTML;
}
}
Then when I change my ngIf value to be truthy I would use setTimeout like this for it to wait only for the next change cycle:
this.showElem = true;
console.log(this.elem); // undefined here
setTimeout(() => {
console.log(this.elem); // back here through ViewChild set
this.elem.do();
});
This also allowed me to avoid using any additional libraries or imports.
来源：https://stackoverflow.com/questions/39366981/viewchild-in-ngif