ngx-on-change

Description

Fully generic decorator to replace Getter / Setter and ngOnChanges.

If the value of the class field changes, @OnChange decorator calls callback, whose name is supplied as a decorator argument. Callback function takes exactly the same values as in the case of Angular ngOnChanges: the current value after the change and SimpleChange

export type SimpleChange<V> = Readonly<{
  firstChange: boolean;
  previousValue: V;
  currentValue: V;
  isFirstChange: () => boolean;
}>;

Decorator will throw an error if :

• argument is not passed
• callback definition does not exist in the class
• callback definition is not a function Decorator doesn't have to be used on @Input property binding, but whatever class fields you want to track change.

##Installation First, you need to install the npm module:

npm install ngx-on-change --save

Arguments

callbackName: string - the name of the method to be called when value change

Usage

Instead of defining Getter / Setter for a class field, usually using an additional '_temp' field used only to be returned by the get method like below:

export class ConfigComponent {
  @Input() set config(value: Config) {
    this._config = value;
    ...
  }

  get config(): Config {
    return this._config;
  }

  _config: Config;
}

we can use the decorator on the class property and invoke the callback if the field value changes.

export class ConfigComponent {
  @Input()
  @OnChange('onConfigChange')
  config: Config;
    
  onConfigChange(newValue: Config, change: SimpleChange): void {
    consolg.log(newValue, change);
  }
}

Similarly, we can replace the ngOnChanges method

export class ConfigComponent extends OnChanges {
  @Input config1: Config;
  @Input config2: Config;
  @Input config3: Config;
          
  ngOnChanges({config1, config2, config3}}: SimpleChanges): void {
    if (config1.previousValue !=== config1.currentValue) {
      ...
    }
    if (config2.previousValue !=== config2.currentValue) {
      ...
    }   
    if (cconfig3.previousValue !=== config3.currentValue) {
      ...
    }
  }
}

to make the code more scaled, e.g. by getting rid of multiple if statements:

export class ConfigComponent {
  @Input @OnChange('onConfig1Change) config1: Config;
  @Input @OnChange('onConfig2Change) config2: Config;
  @Input @OnChange('onConfig3Change) config3: Config;
      
  onConfig1Change(config1: Config, change: SimpleChange) {
    ...
  }
  onConfig2Change(config2: Config, change: SimpleChange) {
    ...
  }
  onConfig3Change(config3: Config, change: SimpleChange) {
    ...
  }
}

Additional info

This decorator is instance-targeted meaning it is invoked per class instance rather than once per class.