There is a component that has the following input:
@Input() list: Array<string>I want to know how I can detect when the parent component changes the value of this input.
There is a component that has the following input:
@Input() list: Array<string>I want to know how I can detect when the parent component changes the value of this input.
To keep track of updates in an array (such as adding or removing elements), you can utilize the ngDoCheck method. Angular2 does not automatically detect changes in arrays because it focuses on references rather than content.
@Component({
(...)
})
export class SomeComponent implements DoCheck {
@Input() list: Array<string>;
constructor(private differ:KeyValueDiffer) {
}
ngDoCheck() {
var changes = this.differ.diff(this.list);
if (changes) {
changes.forEachAddedItem(r => {
// Handle element added to the input array
});
changes.forEachRemovedItem(r => {
// Handle element removed from the input array
});
}
}
The ngFor directive depends on this behavior to function properly.
For a demonstration, check out this plunkr: http://plnkr.co/edit/LVQqpjbLENZ7AZ2a6OTt?p=preview.
Learn how to implement the OnChanges interface in Angular.
ngOnChanges(changes) {
console.log(changes);
}
The ngOnChanges method is triggered only when a value is changed through Angular binding.
If you need to perform actions when the value changes by other means, you can use a getter and setter like this:
_list:Array<string>;
@Input()
set input(newVal:Array<string>) {
this._list = newVal;
doSomething();
}
get input(): Array<string> {
return this._list;
}
Note that this approach will not detect changes within the same array, only when a different array is assigned to _list.
When it comes to "changes", the approach you take depends on whether you are creating a new array for each change (following the immutable pattern) or manipulating the same array by modifying its elements.
If you are adopting the immutable pattern, you can utilize the OnChanges lifecycle hook as suggested by Günter. Otherwise, Thierry recommends using the DoCheck hook. Keep in mind that ngDoCheck() is invoked every time change detection occurs, so it should only be used when necessary.
The Angular Lifecycle Hooks dev guide offers comprehensive coverage of these scenarios – OnChanges, DoCheck – although it may not provide guidance on integrating the IterableDiffer with DoCheck like Thierry's response does.
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