ngOnChanges only runs when the Input change comes from a template binding like <component [someInput]="aValue">. If you set it manually like this component.someInput = aValue, that happens outside the change detection cycle and you need to let Angular know somehow that you’ve changed something.

There are a couple ways to resolve this.

1. Directly make the changes that ngOnChanges would do. (This is what the update function does in the example.)
2. Manually trigger change detection.

constructor(private cd: ChangeDetectorRef) {}
public someFn() {
  this.someInput = aValue;
  this.cd.detectChanges();
  // ngOnChanges will be called
}

3. Use a getter/setter and don’t worry about ngOnChanges.

private _aValue;
public get aValue() { return this._aValue; }
public set aValue(newValue) {
  // logic
  this._aValue = newValue;
}