@OnChange

A Typescript decorator to watch class properties changes

Installation

npm install on-property-change --save

Examples

Listening to a property changes

class Person {
  name: string;

  @OnChange('name')
  doStuff() {
      console.log(`Name has been changed:`, this.name);
  }
}

Usage

const p = new Person();
p.name = 'John';
p.name = 'Kyle';

Console output

Name has been changed: John
Name has been changed: Kyle

Listening to multiple properties changes

The doStuff method is called after both properties are initialised

class Person {
  public name: string;
  public age: number;

  @OnChange(['name', 'age'])
  public doStuff() {
      console.log(`${this.name} is ${this.age} years old`);
  }
}

Usage

const p = new Person();
p.name = 'John';
p.age = 18;
p.age = 22;

Console output

John is 18 years old
John is 22 years old

Bulk change

The bulk flag means to call the method only when all the properties have changed

class Point {
  public x: number;
  public y: number;

  @OnChange(['x', 'y'], { bulk: true })
  public move(): void {
    console.log(`Move to ${this.x}:${this.y}`);
  }
}

Usage

const p = new Point();
p.x = '5';
p.x = '3';  
p.y = 8;   // Move to 3:8
p.y = 16;
p.x = 10;  // Move to 10:16

Console output

Move to 3:8
Move to 10:16

Listening to multiple properties separately

You can have multiple decorated methods with any combinations of properties

class Person {
  name: string;
  age: number;

  @OnChange('name')
  doStuff() {
      console.log('change name')
  }

  @OnChange('age')
  doStuff2() {
      console.log('change age 1')
  }

  @OnChange('age')
  doStuff3() {
      console.log('change age 2')
  }
}

Usage

const p = new Person();
p.name = 'John';
p.age = 18;

Console output

change name
change age 1
change age 2

Optional method arguments

The doStuff method can have arguments. They are the same values as the class fields.

class Person {
  public name: string;
  public age: number;

  @OnChange(['name', 'age'])
  public doStuff(name: string, age: number) {
      console.log(`${name} is ${age} years old`);
  }
}

Compare with the previous value

The history flag allows you to get the previous value of the property.

class Person {
  name: string;

  @OnChange('name', { history: true })
  doStuff(name: PropertyChange<string>) {
      console.log(`User has changed name from ${name.previousValue} to ${name.currentValue}`);
  }
}

Usage

const p = new Person();
p.name = 'John';
p.name = 'Kyle';

Console output

User has changed name from undefined to John
User has changed name from John to Kyle

The full metadata looks like this:

export interface PropertyChange<T> {
    firstChange: boolean;
    previousValue: T;
    currentValue: T;
}

As a replacement for ngOnChanges in Angular projects

@Component({
    selector: 'app-person-card',
    templateUrl: './person-card.component.html',
    styleUrls: ['./person-card.component.css']
})
export class PersonCardComponent {
    @Input() name: string;

    @OnChange('name')
    doStuff() {
        // do stuff
    }
}