typescript - detect change of nested property for component input

Question

In a simplified way I have an Angular2 Component and an input object like this:

class MyObject{
 Prop1:string;
 Prop2:Number;
}  

@Component() 
export class MyComponent{
 @Input() myObject: MyObject;
 DoSomethingIfProp1Change(){
  console.log(myObject.Prop1);
 }
}

How can I detect if Prop1 was changed from Hostcomponent and then execute the DoSomethingIfProp1Change method from inside MyComponent?

Answer 1

In fact, by default Angular2 detects changes when object reference is updated not its content. But this default behavior can be changed by using the DoCheck interface.

In your case (detecting that a property was updated into the myObject object, you could use the following approach:

@Component({
  selector: 'my-component',
  (...)
}) 
export class MyComponent implements DoCheck {
  @Input() myObject: MyObject;
  differ: any;

  constructor(differs:  KeyValueDiffers) {
    this.differ = differs.find([]).create(null);
  }

  ngDoCheck() {
    var changes = this.differ.diff(this.myObject);

    if (changes) {
      changes.forEachChangedItem((elt) => {
        if (elt.key === 'prop1') {
          this.doSomethingIfProp1Change();
        }
      });
    }
  }

  doSomethingIfProp1Change() {
    console.log('doSomethingIfProp1Change');
  }
}

When the value of the prop1 property is updated, the doSomethingIfProp1Change method is called.

See this plunkr: http://plnkr.co/edit/uvOKMXQa9Ik8EiIhb60Y?p=preview.