Detect when input value changed in directive

You need to make an input property of input and then use the ngOnChanges hook to tell when the input property changes.

@Directive({
    selector: '[number]'
})
export class NumberDirective implements OnChanges {
    @Input() public number: any;
    @Input() public input: any;

    ngOnChanges(changes: SimpleChanges){
      if(changes.input){
        console.log('input changed');
      }
    }
}

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There is a better way to detect when an Input property changes, it's considered a best practice and it's also used in the *ngIf implementation.

You just need to postpone the keyword set after Input(), in this way you combine the @Input() decorator with a setter, and it gets called all the times the value changes.

_rows: number;
@Input() set rows(value: number) {
  if (this.value < 0) {
    console.warn('The number of rows must be positive, but got ' + this.value);
    console.warn('Setting default: 2');
    this._rows = 2;
    return;
  }
 
  this._rows = value;
}

If you want to compare the new value with the previous value, you must store the variable in a class property, and retrieve it the second time the method got called:

private previousValue: any = T;

@Input() set myInputName(value: T) {
  console.log(`Previous value was: ${this.previousValue}`);
  console.log(`New value is: ${value}`);
  this.previousValue = value;  
}

As of Angular 9+ I can verify that the snippet below is the correct solution. It's best to avoid ngOnChanges whenever possible for several reasons and use the correct host listener event instead. The below example will get the current input value on keyup. You can easily customize this code snippet for whatever you need on an input directive.

  @Directive({
      selector: '[appMyDirective]',
  })
  export class MyDirective {
    // Add whatever event you want to track here
    @HostListener('keyup', ['$event']) public onKeyup(event: KeyboardEvent): void {
      const value = (event.target as HTMLInputElement).value;
      console.log(value);
    }
  }