Patrick Patrick - 6 months ago 28
AngularJS Question

Angular 2 & Typescript: Cannot access class instance from event handler

My use case requires me to add several copies of a child component to a template programmatically (think: iterating through an array with

*ngFor="let childComponent of componentArray"
). The children components all emit a
select
event; the twist is that each of these components has a different event handler within the parent component. Trying to be clever, I decided to store the event handler as a property of each member of
componentArray
. So my template is now something like:

<my-cmp *ngFor="let childComponent of componentArray" (select)="childComponent.callback()"></my-cmp>


My model contains:

componentArray = [{data:0, callback:this.onSelect0}, {data:1, callback:this.onSelect1}];

onSelect0(){/*do stuff specific to childComponent 0*/}
onSelect1(){/*do stuff specific to childComponent 1*/}


callback
is a reference to the class method I would like that particular childComponent's
select
event to trigger.
callback
is triggered properly; the problem is that from it I cannot access the rest of my component because
this
in that context refers to the
component
during that iteration of the loop.

It sounds more confusing than it is. I've found a workaround but it seems really clunky (I store the class instance as a property in each member of componentArray). I've made a plunkr accessible from http://plnkr.co/edit/VxxCR8hjUsxQ3SABWe8E?p=preview. Basically my question is: if I pass the event handler as an object property (
childComponent.callback
above), how can I access my class instance? Any feedback is welcome.

Answer

That's default JS/TS behavior if you pass a method reference directly. You can either use bind like methodName.bind(this) or a fat arrow function like () => methodName() to retain the scope of this.

In your Plunker just change this line

  private thing = {name:'ThingOne', onSelect: this.handlerOne };

to

  private thing = {name:'ThingOne', onSelect:() => this.handlerOne() };

Plunker example

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