Angular2 @Input to a property with get/set

215
votes

I have an Angular2 component in that component it currently has a bunch fields that have @Input() applied before them to allow binding to that property, i.e.

@Input() allowDay: boolean;

What I would like to do is actually bind to a property with get/set, so that I can do some other logic in the setter, something like the following

_allowDay: boolean;
get allowDay(): boolean {
    return this._allowDay;
}
set allowDay(value: boolean) {
     this._allowDay = value;
     this.updatePeriodTypes();
}

how would I do this in Angular2?

Based on Thierry Templier suggestion I changed it to, but that throws the error Can't bind to 'allowDay' since it isn't a known native property :

//@Input() allowDay: boolean;
_allowDay: boolean;
get allowDay(): boolean {
    return this._allowDay;
}
@Input('allowDay') set allowDay(value: boolean) {
    this._allowDay = value;
    this.updatePeriodTypes();
}
4
angular
How and where to do you bind to [allowDay]="....". If the field (setter) name and the property name you want to use for binding are the same, you can omit the parameter for @Input(...)`. – Günter Zöchbauer
I would be curious to see how yo set up your unit test if you went the route of using get set as shown in the accepted answer. – Winnemucca
Whatever you end up doing make sure to put a breakpoint, or debug statement, or counter inside your setter to make sure it is only firing once as expected. I just found mine was being updated for every change detection run causing horrible performance and quirky behavior. – Simon_Weaver

4 Answers

351
votes

You could set the @Input on the setter directly, as described below:

_allowDay: boolean;
get allowDay(): boolean {
    return this._allowDay;
}
@Input() set allowDay(value: boolean) {
    this._allowDay = value;
    this.updatePeriodTypes();
}

See this Plunkr: https://plnkr.co/edit/6miSutgTe9sfEMCb8N4p?p=preview.

71
votes

If you are mainly interested in implementing logic to the setter only:

import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

// [...]

export class MyClass implements OnChanges {
  @Input() allowDay: boolean;

  ngOnChanges(changes: SimpleChanges): void {
    if(changes['allowDay']) {
      this.updatePeriodTypes();
    }
  }
}

The import of SimpleChanges is not needed if it doesn't matter which input property was changed or if you have only one input property.

Angular Doc: OnChanges

otherwise:

private _allowDay: boolean;

@Input() set allowDay(value: boolean) {
  this._allowDay = value;
  this.updatePeriodTypes();
}
get allowDay(): boolean {
  // other logic
  return this._allowDay;
}
9
votes

@Paul Cavacas, I had the same issue and I solved by setting the Input() decorator above the getter.

  @Input('allowDays')
  get in(): any {
    return this._allowDays;
  }

  //@Input('allowDays')
  // not working
  set in(val) {
    console.log('allowDays = '+val);
    this._allowDays = val;
  }

See this plunker: https://plnkr.co/edit/6miSutgTe9sfEMCb8N4p?p=preview

1
votes

Updated accepted answer to angular 7.0.1 on stackblitz here: https://stackblitz.com/edit/angular-inputsetter?embed=1&file=src/app/app.component.ts

directives are no more in Component decorator options. So I have provided sub directive to app module.

thank you @thierry-templier!