I have look at several topics related to ChangeDetectionStrategy and I am confused with the different usages of it. I simply want to update data in the child component whenever the data is changed on parent component. For this aim, I see that changeDetection: ChangeDetectionStrategy.OnPush is added to the Component field of the parent, but I am not sure if I should use ChangeDetectorRef parameter or some default methods e.g. ngDoCheck() method. So, how can I perform this between parent and child components? And should I explicitly add the parameter that come from parent, or it is automatically update the #Input variables?
import { Component,
Input,
ChangeDetectionStrategy,
ChangeDetectorRef } from '#angular/core';
#Component({
selector: 'app-child',
templateUrl: './child.component.html',
changeDetection: ChangeDetectionStrategy.OnPush
})
export class ChildComponent {
#Input() data: string[];
constructor(private cd: ChangeDetectorRef) {}
refresh() {
this.cd.detectChanges();
}
}
The difference for the ChangeDetectionStrategy.OnPush, your component will only will detect the changes when something like below happens :
The Input reference changes
An event originated from the component or one of its children
Use the async pipe in the view
Run change detection explicitly
So if you don't change input's reference you have to detect changes manually.
Here is the stackblitz that shows the difference between OnPush and Default
With ChangeDetectionStrategy.OnPush your child component will perform update only if data in #Input is really updates, so you need your data to be immutable, those after updating data Angular will perform ChangeDetector and update components view.
You can check this example: https://angular-cd-immutable-example.stackblitz.io
One of the way is to create a new reference to an array by using slice method.
A full stackblitz example can be seen here.
app.component.html
<button
(click)="people.push({firstName: '1', lastName: '1'}); people = people.slice()">
+</button>
<div>
<p> {{ people | json }} </p>
</div>
<br/>
<person [persons]="people"></person>
app.component.ts
import { Component } from '#angular/core';
#Component({
selector: 'my-app',
templateUrl: './app.component.html',
styleUrls: [ './app.component.css' ]
})
export class AppComponent {
people: any[] = [
{
firstName: "Alex1",
lastName: "Brown1",
age: 55,
},
{
firstName: "Foo2",
lastName: "Bar2",
age: 44,
},
{
firstName: "Fido3",
lastName: "Johnson3",
age: 14,
}
]
}
and person.component.html
<div *ngFor="let person of persons">
Hello I am {{person.firstName}} and I am {{ person.age}}
</div>
and person.component.ts
import { Component, OnInit, Input, ChangeDetectionStrategy,
ChangeDetectorRef } from '#angular/core';
class test {}
#Component({
selector: 'person',
templateUrl: './person.component.html',
styleUrls: ['./person.component.css'],
changeDetection: ChangeDetectionStrategy.OnPush
})
export class PersonComponent implements OnInit {
#Input() persons: any;
}
Related
I am trying to call view child of a child component from parent and getting undefined in the console.
see the image also see the stack blaze for the same
https://stackblitz.com/edit/angular-ivy-k4m2hp?file=src%2Fapp%2Fhello.component.ts
import { Component, Input, OnInit, ViewChild } from '#angular/core';
import { TestChildComponent } from './test-child/test-child.component';
#Component({
selector: 'hello',
template: `<h1>this is Hello component {{name}}!</h1>`,
styles: [`h1 { font-family: Lato; }`],
})
export class HelloComponent {
#Input() name: string;
#ViewChild('TestChildComponent') testChildComponent: TestChildComponent
ngOnInit() {
console.log('calling ngOninit in hello component');
console.log('going to call test child instance this.TestChildComponent.ngOninit')
console.log(this.testChildComponent);
}
}
Please help to get the child component
this.testChildComponent
So that i can call ngOnInit of child from parent.
this.testChildComponent.ngOnInit()
ViewChild element ref can be accessed in ngAfterViewInit() cycle the earliest.
Angular doc says we should use the child component injected by ViewChild in ngAfterViewInit. But sometimes you even can’t get it in ngAfterViewInit. The reason is, the child component is not created yet when AfterViewInit hook runs. For such a case you would have to wait more (using a setTimeout would work but it's a bad idea). Other option would be to have the child emit something to the parent, letting it know the child has been rendered and then the parent can query it.
But your case is that sibling HelloComponent wants to query sibling TestChildComponent it can't do that. TestChildComponent is just not in the scope for HelloComponent. Easiest solution would be to query TestChildComponent from the parent AppComponent.
You should also add #TestChildComponent to access the ref.
<app-test-child #TestChildComponent childname="{{ name }}"></app-test-child>
Working example: https://stackblitz.com/edit/angular-ivy-j5ceat?file=src%2Fapp%2Fapp.component.html
if you set your viewChild { static: true } you will be able to access it in ngOnInit
but in your sample the issue is due totestChildComponent is a child of app.component and not hello.component
<app-test-child childname="{{ name }}" #test></app-test-child>
app.component.ts
import { Component, VERSION, ViewChild } from '#angular/core';
import { TestChildComponent } from './test-child/test-child.component';
#Component({
selector: 'my-app',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css'],
})
export class AppComponent {
name = 'this is from app compoenent';
#ViewChild('test', { static: true }) testChildComponent: TestChildComponent;
ngOnInit() {
console.log('calling ngOninit in app component');
console.log(this.testChildComponent);
}
}
If you want to access testChildComponent from hello.component you will have to send it the component as an input for sample
following a working sample of accessing testChildComponent
https://stackblitz.com/edit/angular-ivy-tvwukg?file=src%2Fapp%2Fapp.component.html
I've implemented a child component to render a table based on a list provided via #Input(). The data is loaded via http, however the UI (child component) is not updated unless I wave my mouse over the screen. I've seen people post about implementing ngOnChanges() in my child, but I thought Angular was supposed to do this by default? Am I missing something? Why would the UI not update with this?
Child code looks something like this:
child.component.ts
#Component({
selector: 'child',
templateUrl: './child.component.html',
styleUrls: ['./child.component.scss'],
})
export class ChildComponent implements {
#Input() data: any[] = [];
constructor() {}
}
child.component.html
<table>
<tr *ngFor="let item of data"><td>{{ item }}</td></tr>
</table>
Parent code that uses the component looks something like this:
parent.component.ts
#Component({
selector: 'parent',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.scss'],
})
export class ParentComponent implements OnInit {
data: string[] = [];
constructor(private endpointService: EndpointService) {}
ngOnInit() {
// response is a string array like: ['hello', 'world']
this.endpointService.loadData().subscribe((response) => {
this.data = response;
});
}
}
parent.component.html
<child [data]="data"></child>
============================= EDIT ==================================
I verified that it only fails to load when updating inside of the subscribe callback (if I set a static array, it loads just fine).
So it looks like I'm able to resolve this by running changeDetectorRef.detectChanges() in the parent component, but this feels hackish like I shouldn't have to do this. Is this a good way to resolve this? Or does this indicate something wrong with my implementation?
parent.component.ts
#Component({
selector: 'parent',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.scss'],
})
export class ParentComponent implements OnInit {
data: string[] = [];
constructor(private endpointService: EndpointService,
private changeDetectorRef: ChangeDetectorRef) {}
ngOnInit() {
// response is a string array like: ['hello', 'world']
this.endpointService.loadData().subscribe((response) => {
this.data = response;
this.changeDetectorRef.detectChanges();
});
}
}
You can also try to force change detection by forcing the value reference update via, for example, the spread operator:
this.endpointService.loadData().subscribe((response) => {
this.data = [...response];
});
hummm well .. when the component is rendered as first time it will show with the empty array becouse the api call stills happening and needs the onchanges method in child component in order to listen the complete api call and the list will re render
Seems that you have some other errors in template expressions which force the whole template to fail. Here's a stackblitz I've created and everything works: https://stackblitz.com/edit/angular-ivy-w2ptbb?file=src%2Fapp%2Fhello.component.ts
Do you have maybe some errors in console?
I replaced the service with a static string array and it worked well. I think there is problem with the observable subscription.
child.component.ts
import { Component, OnInit,Input } from '#angular/core';
#Component({
selector: 'child',
templateUrl: './child.component.html',
styleUrls: ['./child.component.css']
})
export class ChildComponent implements OnInit {
#Input() data: any[] = [];
constructor() { }
ngOnInit() {
}
}
parent.component.ts
import { Component, OnInit } from '#angular/core';
#Component({
selector: 'parent',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.css'],
})
export class ParentComponent implements OnInit {
data: string[] = [];
constructor() {}
ngOnInit() {
this.data = ['hello', 'world','aaa','ddd'];
}
}
How can I access a string of text given within the tags of a component
<my-custom-component>THIS TEXT</my-custom-component>
Within a template, I can use ng-content, or if it is an instance of some other class I can access it within the component definition like demonstrated in these examples. However I am interested in detecting if there is a string of text there or not, which I believe would make providedText undefined. However, I am always getting undefined.
#ContentChild(Element, { static: true }) providedText: Text | undefined;
I have tried Text as the first element passed to #ContentChild. Passing any will not work (I don't know why).
StackBlitz
I am interested mostly in finding if there is a string or undefined, but am also curious why ContentChild(Text... isn't working.
Edit:
I have added a potential solution, but it seems pretty imperfect, so I hope something better comes along.
Edit 2:
I now understand that #ContentChild is not a mechanism for selecting whatever native HTML I want without wiring it up to Angular’s dependency graph with a ref, directive, etc.
I am still curious if my proposed solution below is a bad idea for any reason.
My solution for now (since I wish to capture all transcluded content) is to wrap ng-content in a containing element, then get its innerText.
#Component({
selector: "app-parent",
template: `
<span #transcludedContainerRef>
<ng-content></ng-content>
</span>
`
})
export class ParentComponent implements AfterViewInit {
#ViewChild("transcludedContainerRef", { static: false })
transcludedContainerRef: ElementRef | undefined;
buttonText: string;
ngAfterViewInit() {
const isButtonTextPresent = this.transcludedContainerRef.nativeElement
.innerText;
if (isButtonTextPresent) {
console.log(isButtonTextPresent); // successfully logs content
}else {
console.log('No text set');
}
}
}
It does feel hacky, but it works. I am holding out for something better.
it's difficult if I don't know about your <my-custom-component>
In general if your custom component it's only
<ng-content></ng-content>
You can inject in constructor the elementRef
constructor(public el:ElementRef){}
From a parent
<hello >
Start editing to see some magic happen :)
</hello>
You can use
#ViewChild(HelloComponent,{static:false}) helloComponent:HelloComponent
click()
{
console.log(this.helloComponent.el.nativeElement.innerHTML)
}
If your component has any variable -or ViewContent-, you can access this variables in a similar way
So the other way to read the inner text from the component is that child component emit the value whatever it get's as input from other component. See below:
hello.component.ts
import { Component, Input, Output, EventEmitter, OnInit } from '#angular/core';
#Component({
selector: 'hello',
template: `<h1>Hello {{name}}!</h1>`,
styles: [`h1 { font-family: Lato; }`]
})
export class HelloComponent implements OnInit {
#Input() name: string;
#Output() innerText: EventEmitter<string> = new EventEmitter();
ngOnInit() {
this.innerText.emit(this.name);
}
}
app.component.ts
import { Component, ContentChild, AfterContentInit, OnInit } from "#angular/core";
#Component({
selector: "app-parent",
template: "content from <code>app-parent</code>"
})
export class ParentComponent implements AfterContentInit {
#ContentChild(Element, { static: true }) providedText: Text | undefined;
ngAfterContentInit() {
console.log("ngAfterContentInit Content text: ", this.providedText);
}
}
#Component({
selector: "my-app",
templateUrl: "./app.component.html",
styleUrls: ["./app.component.css"]
})
export class AppComponent {
name = "Angular";
_innerText: string;
ngAfterContentInit() {}
get childContent(): string {
return this._innerText;
}
set childContent(text) {
this._innerText = text;
}
innerTextFn(innertext: string) {
this.childContent = innertext;
console.log('Event: ', innertext);
}
}
app.component.html
<hello name="{{ name }}" (innerText)="innerTextFn($event)"></hello>
<app-parent>This is the content text</app-parent>
Here is stackblitz url to check: https://stackblitz.com/edit/angular-bacizp
I hope this may helpful for you and if yes then accept this as correct answer.
Trying to do child to parent communication with #Output event emitter but is no working
here is the child component
import { Component, OnInit, Output, Input, EventEmitter } from '#angular/core';
#Component({
selector: 'app-emiter',
templateUrl: './emiter.component.html',
styleUrls: ['./emiter.component.css']
})
export class EmiterComponent implements OnInit {
#Output() emitor: EventEmitter<any>
constructor() { this.emitor = new EventEmitter()}
touchHere(){this.emitor.emit('Should Work');
console.log('<><><><>',this.emitor) // this comes empty
}
ngOnInit() {
}
}
this is the html template
<p>
<button (click)=" touchHere()" class="btn btn-success btn-block">touch</button>
</p>
The console.log inside the touchHere it shows nothing
even if I put this inside the parent component it show nothing as well
parent component
import { Component , OnInit} from '#angular/core';
// service I use for other stuff//
import { SenderService } from './sender.service';
// I dont know if I have to import this but did it just in case
import { EmiterComponent } from './emiter/emiter.component'
#Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent {
title = 'app';
user: any;
touchThis(message: string) {
console.log('Not working: ${message}');
}
constructor(private mySessionService: SenderService) { }
}
and here is the html template
<div>
<app-emiter>(touchHere)='touchThis($event)'</app-emiter>
</div>
Parent component template:
<app-emitor (emitor)='touchThis($event)'></app-emiter>
In parent template #Output should be 'called', not the child method.
Also, see: https://angular.io/guide/component-interaction#parent-listens-for-child-event
Here’s an example of how we write a component that has outputs:
#Component({
selector: 'single-component',
template: `<button (click)="liked()">Like it?</button>`
})
class SingleComponent {
#Output() putRingOnIt: EventEmitter<string>;
constructor() {
this.putRingOnIt = new EventEmitter();
}
liked(): void {
this.putRingOnIt.emit("oh oh oh");
}
}
Notice that we did all three steps: 1. specified outputs, 2. created an EventEmitter that we attached
to the output property putRingOnIt and 3. Emitted an event when liked is called.
If we wanted to use this output in a parent component we could do something like this:
#Component({
selector: 'club',
template: `
<div>
<single-component
(putRingOnIt)="ringWasPlaced($event)"
></single-component>
</div>`
})
class ClubComponent {
ringWasPlaced(message: string) { console.log(`Put your hands up: ${message}`);
} }
// logged -> "Put your hands up: oh oh oh"
Again, notice that:
putRingOnIt comes from the outputs of SingleComponent
ringWasPlaced is a function on the ClubComponent
$event contains the thing that wasemitted, in this case a string
<app-emiter (emitor)="touchThis($event)" ></app-emiter>
By using #Output() you should apply the event you need to emit in the directive of the emitter component.Adding the name of the variable to the the directive and but the emitted over function inside the quotation passing the $event.
touchHere() is the method from which you are binding some value to emit with your EventEmitter. And your EventEmitter is 'emitor'.
So your code will work if you simply do the below:
<app-emiter (emitor)='touchThis($event)'></app-emiter>
I have the following component below:
#Component({
selector: 'myselector',
providers: [ ],
directives: [ ChildComponent],
pipes: [ ],
template: '<myselector>This is {{testEmitter}}</myselector>'
})
export class ParentComponent{
#Input() testEmitter;
constructor(){
}
}
//My Child class goes as such:
#Component({
selector: 'childselector',
templateUrl: '<childselector><input type="text" (focus)="beginTest()"/></childselector>',
pipes: [],
directives: []
})
export class ChildComponent{
#Output() testEmitter: EventEmitter = new EventEmitter();
startTest: boolean = false;
constructor() {
}
beginTest(){
this.startTest = !this.startTest;
this.testEmitter.emit(this.startTest);
}
}
I am just trying to figure out how to display the value of the this.startTest variable from the ChildComponent to the ParentComponent. Right now, the {{testEmitter}} doesn't show anything in my ParentComponent html. I feel like I'm close. Your help is appreciated!
This code doesn't seem to make much sense.
#Component({
selector: 'myselector',
providers: [ ],
directives: [ ChildComponent],
pipes: [ ],
template: '<myselector>This is {{testEmitter}}</myselector>'
})
The template uses the selector <myselector> of the component it is the template of. While there are scenarios where this make sense (recursive structures like trees) this doesn't seem to be the intention here.
Also directives and pipes on #Component() are gone since a while and should be added to #NgModule() instead.
Obviously you are using some beta or RC version instead of final of Angular2 where this is still supported. I'd suggest to update to the newest Angular2 version.
This might be what you want instead:
#Component({
selector: 'parentselector',
directives: [ChildComponent],
template: '<childselector (testEmitter)="testEmitter=$event">This is {{testEmitter}}</childselector>'
})
export class ParentComponent{
#Input() testEmitter;
}
#Component({
selector: 'childselector',
templateUrl: '<input type="text" (focus)="beginTest()"/>',
})
export class ChildComponent{
#Output() testEmitter: EventEmitter = new EventEmitter();
startTest: boolean = false;
beginTest(){
this.startTest = !this.startTest;
this.testEmitter.emit(this.startTest);
}
}