Sjoerd Sjoerd - 1 month ago 29
AngularJS Question

Angular2 and class inheritance support

I am building an Angular2 application in Typescript and would like to use the class system functionality (read: class inheritance) offered by Typescript. However, it seems that Angular2 is not playing nice with derived classes. I am looking for some help in getting my application to work.

The problem I am facing is that I have a base class and derive a few child classes from there. When I am building up my component tree, I would like to be able to access the parent/children of the components (either way is fine). From what I understand, Angular2 offers two options to accomplish that:


  1. Inject the parent into the child component

  2. Use ContentChildren (or ViewChildren) to access the children of the component.



Both work fine if you know the type of the class you are working with (ChildComponent), but seem to fail when you try to use the base class of these components (BaseComponent) as selector.

To visualize it in some code (see this Plunker for a live demo), I have an application component/class as follows:

@Component({
selector: 'my-app',
template: `<parent-comp>
<child-comp1></child-comp1>
<child-comp1></child-comp1>
<child-comp2></child-comp2>
</parent-comp>`,
directives: [ParentComponent, ChildComponent1, ChildComponent2]
})
export class MyApplication {
}


The Base Class and Child Classes are defined as:

export class BaseComponent {
// Interesting stuff here
}

@Component({
selector: 'child-comp2',
template: '<div>child component #2</div>'
})
export class ChildComponent2 extends BaseComponent {
}

@Component({
selector: 'child-comp1',
template: '<div>child component #1</div>'
})
export class ChildComponent1 extends BaseComponent {
}


And the Parent class has some logic to count its children.

@Component({
selector: 'parent-comp',
template: `<div>Hello World</div>
<p>Number of Child Component 1 items: {{numComp1}}
<p>Number of Child Component 2 items: {{numComp2}}
<p>Number of Base Component items: {{numBase}}
<p><ng-content></ng-content>
`
})
export class ParentComponent implements AfterContentChecked {

@ContentChildren(ChildComponent1) contentChild1: QueryList<ChildComponent1>
@ContentChildren(ChildComponent2) contentChild2: QueryList<ChildComponent2>
@ContentChildren(BaseComponent) contentBase: QueryList<BaseComponent>
public numComp1:number
public numComp2:number
public numBase:number

ngAfterContentChecked() {
this.numComp1 = this.contentChild1.length
this.numComp2 = this.contentChild2.length
this.numBase = this.contentBase.length
}


(Again, you can see a live demo here)

The output for the first two counters is as expected. There are 2 ChildComponent1 and 1 ChildComponent2 children. Unfortunetely, the BaseComponent counter doesn't show the sum of these counters, but shows 0. It doesn't find any class that is of type BaseComponent in the children.

The same thing happens when ParentComponent also extends from BaseComponent and you want to Inject it into a ChildComponent. The Injector will require the specific type of the ParentComponent and can't work with the base class.

Any clues on how to work with derived classes in Angular2? Am I missing something or trying something impossible?

Answer

Try adding a provider to each derived component, aliasing the base component to the derived component:

@Component({
  selector: 'child-comp2',
  template: '<div>child component #2</div>',
  providers: [{provide: BaseComponent, useExisting: forwardRef(() => ChildComponent2) })]
})
export class ChildComponent2 extends BaseComponent {
}

@Component({
  selector: 'child-comp1',
  template: '<div>child component #1</div>',
  providers: [{provide: BaseComponent, useExisting: forwardRef(() => ChildComponent1) })]
})
export class ChildComponent1 extends BaseComponent {
}

Plunker: http://plnkr.co/edit/5gb5E4curAE2EfH2lNZQ?p=preview