Comparing Input and Output Event Binding

Question

Comparing Input and Output Event Binding

Can you provide reasons why using @Output for events is more advantageous than passing an @Input function in Angular 2+?

Utilizing @Input:

Parent Template:

<my-component [customEventFunction]=myFunction></my-component>

Inside parent-component.ts:

myFunction = () => {
  console.log("Hello world")
}

Inside my-component.ts

@Input() customEventFunction: Function;

someFunctionThatTriggersTheEvent() {
  this.customEventFunction();
}

Opting for @Output:

Parent Template:

<my-component (onCustomEvent)=myFunction()></my-component>

Inside parent-component.ts:

myFunction() {
  console.log("Hello world")
}

Inside my-component.ts

@Output() onCustomEvent: EventEmitter<any> = new EventEmitter<any>();

someFunctionThatTriggersTheEvent() {
  this.onCustomEvent.emit();
}

Both methods achieve the same outcome, but it seems that the @Output approach is more commonly used in various Angular packages. Some may argue that with an Input, one can verify if the function exists before triggering the event conditionally.

What are your insights on this matter?

angular typescript angular2-components angular5 typescript-decorator

Answer 1

Answer №1

Benefits of using @Output event binding:

Utilizing @Output to define an event clearly indicates that it expects callback methods to handle the event, following Angular's standard mechanism and syntax.
Multiple event handlers can subscribe to the @Output event compared to defining an @Input property that only allows one callback function. Adding a second event handler with @Input would replace the first one, similar to setting onmousemove="doSomething()" in standard DOM event handling. In contrast, @Output event binding allows for multiple event handlers akin to calling
```
btn.addEventListener("mousemove", ...)
```
.

Answer 2

Benefits of using @Output event binding:

Utilizing @Output to define an event clearly indicates that it expects callback methods to handle the event, following Angular's standard mechanism and syntax.
Multiple event handlers can subscribe to the @Output event compared to defining an @Input property that only allows one callback function. Adding a second event handler with @Input would replace the first one, similar to setting onmousemove="doSomething()" in standard DOM event handling. In contrast, @Output event binding allows for multiple event handlers akin to calling
```
btn.addEventListener("mousemove", ...)
```
.

Answer 3

Answer №2

@Sajeetharan's response is not entirely accurate; there is indeed a significant functional discrepancy: the execution context. Let's examine this situation:

@Component({
  selector: 'app-example',
  template: `<button (click)="runFn()">Click Me</button>`,
})
export class ExampleComponent {
  @Input() public fn: any;

  public runFn(): void {
    this.fn();
  }
}

@Component({
  selector: 'app',
  template: `<app-example [fn]="myFn"></app-example>`,
})
export class AppComponent {
  public state = 42;

  // Utilizing arrow syntax will actually trigger an alert of "42" because
  // arrow functions do not possess their own unique "this" context.
  //
  // public myFn = () => window.alert(this.state);

  public myFn(): void {
    // This will unfortunately prompt an alert of "undefined" since this function
    // is executed within the child component's scope!
    window.alert(this.state);
  }
}

This can complicate the use of @Input() properties for passing functions, disrupting the principle of least astonishment and potentially introducing subtle bugs.

Naturally, there are situations where a specific context might not be necessary. For instance, in a searchable list component that accepts intricate data as items and requires a fnEquals function to determine if the search input text matches an item. Nevertheless, such scenarios are often better managed with more versatile mechanisms (like content projection, etc.), boosting reusability.

Answer 4

@Sajeetharan's response is not entirely accurate; there is indeed a significant functional discrepancy: the execution context. Let's examine this situation:

@Component({
  selector: 'app-example',
  template: `<button (click)="runFn()">Click Me</button>`,
})
export class ExampleComponent {
  @Input() public fn: any;

  public runFn(): void {
    this.fn();
  }
}

@Component({
  selector: 'app',
  template: `<app-example [fn]="myFn"></app-example>`,
})
export class AppComponent {
  public state = 42;

  // Utilizing arrow syntax will actually trigger an alert of "42" because
  // arrow functions do not possess their own unique "this" context.
  //
  // public myFn = () => window.alert(this.state);

  public myFn(): void {
    // This will unfortunately prompt an alert of "undefined" since this function
    // is executed within the child component's scope!
    window.alert(this.state);
  }
}

This can complicate the use of @Input() properties for passing functions, disrupting the principle of least astonishment and potentially introducing subtle bugs.

Naturally, there are situations where a specific context might not be necessary. For instance, in a searchable list component that accepts intricate data as items and requires a fnEquals function to determine if the search input text matches an item. Nevertheless, such scenarios are often better managed with more versatile mechanisms (like content projection, etc.), boosting reusability.

Answer 5

Answer №3

There are essentially no distinctions in terms of functionality, however

(i)When utilizing @input, you benefit from being able to specify the type and whether it is private or public

(ii)As pointed out by @ConnorsFan in the comment, a key advantage of using @Output is that multiple subscribers can handle the Output event, whereas only one handler can be provided for the @Input property.

Answer 6

There are essentially no distinctions in terms of functionality, however

(i)When utilizing @input, you benefit from being able to specify the type and whether it is private or public

(ii)As pointed out by @ConnorsFan in the comment, a key advantage of using @Output is that multiple subscribers can handle the Output event, whereas only one handler can be provided for the @Input property.

Comparing Input and Output Event Binding

Answer №1

Answer №2

Answer №3

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