Accessing Child Component’s API in Loklak Search

Loklak search being an angular application, comprises of components. Components provide us a way to organize the application in a more consistent way, along with providing the ability to reuse code in the application. Each component has two type of API’s public and private. Public API is the API which it exposes to the outer world for manipulating the working of the component, while private API is something which is local to the component and cannot be directly accessed by the outside world. Now when this distinction between the two is clear, it is important to state the need of these API’s, and why are they required in loklak search.

The components can never live in isolation, i.e. they have to communicate with their parent to be able to function properly. Same is the case with components of loklak search. They have to interact with others to make the application work. So how this, interaction looks like,

The rule of thumb here is, data flows down, events flow up. This is the core idea of all the SPA frameworks of modern times, unidirectional data flow, and these interactions can be seen everywhere in loklak search.

<feed-header
   [query]="query"
   (searchEvent)="doSearch($event)"></feed-header>

This is how a simple component’s API looks in loklak search. Here our component is FeedHeader and it exposes some of it’s API as inputs and outputs.

export class FeedHeaderComponent {

 @Input() query: string;

 @Output() searchEvent: EventEmitter<string> = new EventEmitter<string>();

  // Other methods and properties of the component
}

The FeedHeaderComponent ‘s class defines some inputs which it takes. These inputs are the data given to the component. Here the input is a simple query property, and the parent at the time of instantiating the component, passes the value to it’s child as [query]=”query”. This enables the one direction of API, from parent to child. Now, we also need a way for parent to be able to events generated by the child on interaction with user. For example, here we need to have a way to tell the parent to perform a search whenever user presses search button. For this the Output property searchEvent is used. The search event can be emitted by the child component independently. While the parent, if it wants to listen to child components simply do so by binding to the event and running a corresponding function whenever event is emitted (searchEvent)=”doSearch($event)”. Here the event which parent listens to is searchEvent and whenever such an event is emitted by the child a function doSearch is run by the parent. Thus this completes the event flow, from child to parent.

Now it is worth noticing that all these inputs for data and outputs for events is provided by the child component itself. They are the API of the child and parent’s job is just to bind to these inputs and outputs to bind to data and listen to events. This allows the component interactions in both directions.

@ViewChild and triggering child’s methods

The inputs are important to carry data from the parent to the child, declaratively but sometimes it is necessary for the parent to access the public API of it’s child more directly, specially the API methods to trigger an action. These methods require the way for the parent to access its child component. This is done by @ViewChild decorator. The child element which the parent wants access to, have to declare the component as, one of it’s attributes. Like in our example, the FeedHeaderComponent needs access to its child component SuggestBoxComponent, to show/hide suggest box as and when required. So here the feed header component gets the access to its child using viewchild decorator.

export class FeedHeaderComponent {

  @ViewChild(‘#suggestBox) suggestBox: SuggestBoxComponent;

  // Other properties and methods

  toggleSuggestBox() {

     this.suggestBox.toggle();
  }
}

The SuggestBoxComponent here has a public method toggle() which toggles the visibility state of the suggest box. This method is available as a component’s public API method. The parent of this component calls this method using the @ViewChild reference which it grabbed at the time of view instantiation.

export class SuggestBoxComponent {

  private suggestBoxVisible = true;

  public toggle() {

     this.suggestBoxVisible = !this.suggestBoxVisible;
  }
}

Resources and Links

  • Angular document pages
  • Basic Usage in Angular tour of heroes tutorial
  • In depth usage blog for Inputs and Outputs SitePoint Tutorial
  • Loklak Search Repo