The QueryList is the return type of ViewChildren and contentChildren . QueryList stores the items returned by the viewChildren or contentChildren in a list. The Angular updates this list, whenever the state of the application change. It does it on each change detection.
The QueryList also Implements an iterable interface. Which means you can iterate over it using for (var i of items) or use it with ngFor in template *ngFor="let i of items".
Changes can be observed by subscribing to the changes Observable.
You can use the following methods & properties.
first: returns the first item in the list.
last: get the last item in the list.
length: get the length of the items.
changes: Is an observable. It emits a new value, whenever the Angular adds, removes or moves the child elements.
ViewChildren Example
In the example below, we have three input elements all using the ngModel directive We use the ViewChildren to get the QueryList of all input elements
Finally, we can use the this.modelRefList.forEach to loop through the QueryList and access each element.
import { ViewChild, Component, ViewChildren, QueryList, AfterViewInit } from '@angular/core';
import { NgModel } from '@angular/forms';
@Component({
selector: 'app-viewchildren1',
template: `
<h1>ViewChildren Example</h1>
<input name="firstName" [(ngModel)]="firstName">
<input name="midlleName" [(ngModel)]="middleName">
<input name="lastName" [(ngModel)]="lastName">
<button (click)="show()">Show</button>
`,
})
export class ViewChildrenExample1Component {
firstName;
middleName;
lastName;
@ViewChildren(NgModel) modelRefList: QueryList<NgModel>;
show() {
this.modelRefList.forEach(element => {
console.log(element)
//console.log(element.value)
});
}
}Listening for QueryList Changes
We can subscribe to the changes observable to find if any new elements are added/removed or moved.
In the example below, we have included ngIf directive to hide/show the input elements.
We subscribe to the changes observable in the component class. Every time we use the ngIf to hide or add the component, the changes observable emits the latest QueryList.
import { ViewChild, Component, ViewChildren, QueryList, AfterViewInit } from '@angular/core';
import { NgModel } from '@angular/forms';
@Component({
selector: 'app-viewchildren2',
template: `
<h1>ViewChildren Example</h1>
<input *ngIf="showFirstName" name="firstName" [(ngModel)]="firstName">
<input *ngIf="showMiddleName" name="midlleName" [(ngModel)]="middleName">
<input *ngIf="showlastName" name="lastName" [(ngModel)]="lastName">
<input type="checkbox" id="showFirstName" name="showFirstName" [(ngModel)]="showFirstName">
<input type="checkbox" id="showMiddleName" name="showMiddleName" [(ngModel)]="showMiddleName">
<input type="checkbox" id="showlastName" name="showlastName" [(ngModel)]="showlastName">
<button (click)="show()">Show</button>
`,
})
export class ViewChildrenExample2Component implements AfterViewInit {
firstName;
middleName;
lastName;
showFirstName=true;
showMiddleName=true;
showlastName=true;
@ViewChildren(NgModel) modelRefList: QueryList<NgModel>;
ngAfterViewInit() {
this,this.modelRefList.changes
.subscribe(data => {
console.log(data)
}
)
}
show() {
this.modelRefList.forEach(element => {
console.log(element)
//console.log(element.value)
});
}
}Methods:
1. get()
Returns the QueryList entry at index.
get(index: number): T | undefinedParameters indexnumber
Returns T | undefined
2. map()
map<U>(fn: (item: T, index: number, array: T[]) => U): U[]Parameters fn(item: T, index: number, array: T[]) => U
Returns U[]
3. filter()
filter(fn: (item: T, index: number, array: T[]) => boolean): T[]Parameters fn(item: T, index: number, array: T[]) => boolean
Returns T[]
4. find()
find(fn: (item: T, index: number, array: T[]) => boolean): T | undefinedParameters fn - (item: T, index: number, array: T[]) => boolean
Returns T | undefined
5. reduce()
reduce<U>(fn: (prevValue: U, curValue: T, curIndex: number, array: T[]) => U, init: U): UParameters
fn - (prevValue: U, curValue: T, curIndex: number, array: T[]) => UinitU
Returns U
6. forEach()
forEach(fn: (item: T, index: number, array: T[]) => void): voidParameters fn - (item: T, index: number, array: T[]) => void
Returns void
7. some()
some(fn: (value: T, index: number, array: T[]) => boolean): booleanParameters fn - (value: T, index: number, array: T[]) => boolean
Returns boolean
8. toArray()
Returns a copy of the internal results list as an Array.
toArray(): T[]ParametersThere are no parameters.
Returns T[]
9. toString()
toString(): stringParametersThere are no parameters.
Returns string
10. reset()
Updates the stored data of the query list, and resets the dirty flag to false, so that on change detection, it will not notify of changes to the queries, unless a new change occurs.
reset(resultsTree: (any[] | T)[], identityAccessor?: (value: T) => unknown): voidParameters resultsTree - (any[] | T)[] - The query results to store
identityAccessor - (value: T) => unknownOptional function for extracting stable object identity from a value in the array. This function is executed for each element of the query result list while comparing current query list with the new one (provided as a first argument of the reset function) to detect if the lists are different. If the function is not provided, elements are compared as is (without any pre-processing). Optional. Default is undefined.
Returns void
11. notifyOnChanges()
Triggers a change event by emitting on the changes EventEmitter.
notifyOnChanges(): voidParametersThere are no parameters.
Returns void
12. setDirty()
internal
setDirty()ParametersThere are no parameters.
13. destroy()
internal
destroy(): voidParametersThere are no parameters.
Returns void
Resource: tektutorialshub.com, angular
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