ViewChild in Angular2

There are situations when a Parent Component needs to interact with Child Component so we will discuss a solution for those cases in this writeup.

To be more elaborate let us a take a small example. Suppose there is a small game which has multiple components as given below:

1. GameComponent - This is the parent component.
2. GameBoardComponent - The Game board component which has the actual game.
3. GameResetComponent - Component which would be responsible for resetting the game.

There are multiple ways to achieve the interaction between the above components, and one of them is ViewChild.

So, a user starts playing the game and after he notices no valid moves are left, he plans to reset the game. He clicks on the Reset button which is the part of the GameResetComponent(Child Component). The GameResetComponent(Child Component) then interacts with GameComponent (Parent Component) and would request for reset. The GameComponent(Parent Component) would then reset the game board by interacting with GameBoardComponent(Child Component) in order to reset the board.

Notice two interactions here:

1. Child Component to Parent Component : GameResetComponent -> GameComponent
2. Parent Component to Child Component : GameComponent -> GameBoardComponent

The scheme of interactions discussed above is based on the Mediator Design Pattern. Parent Component is acting as a central authority which is responsible for communication between child components.

Well, we would be discussing 2nd interaction in this blog i.e. Parent to Child. So suppose when a parent component needs to call a child component function, it can inject child component as a ViewChild in parent component.

Let's take a very small example to demonstrate this:

Below is a child component that has a very simple functionality - It has some text which can be shown/hidden and there is a Show/Hide Text button that toggles the visibility of that text.

Text is hidden by default and once the user clicks on the button, toggleVisibility function is called, which also sends the source in the parameters as from where the function has been called.

child.component.ts

import {Component} from '@angular/core';

@Component({
    selector: 'child-component',
    template: `
    <div>
        <h2>Child Component</h2>
        <div class="text">
            <span [hidden]="!showText">I am visible now! Thanks to {{visibilitySource}}</span>
        </div>
        <div>
            <button (click)="toggleVisibility('Child Component')">Show/Hide Text</button>
        </div>
    </div>
    `,
    styles: ['.text { margin-bottom: 10px; color:red}']
})

export class ChildComponent {
    showText:Boolean = false;
    visibilitySource:String = '';

    toggleVisibility(source) {
        this.showText = !this.showText;
        this.visibilitySource = source;
    }
}

And here is the parent component for that child component:

app.component.ts

import {Component} from '@angular/core';
import {ChildComponent} from './child.component';

@Component({
    selector: 'my-app',
    template: `
    <div>
        <h1>Parent Component</h1>
        <button (click)="showHideText()">Show/Hide Child Component Text</button>
        <child-component></child-component>
    </div>
    `,
    directives: [ChildComponent]
})

export class AppComponent {
    showHideText() {
        // TODO: Access child component to toggle text visibility
    }
}

Now assume, parent component also wants to show/hide the text displayed by the child component, so to achieve that we need to do the following:

1. Import ViewChild from @angular/core. So now first line of our app.component.ts would look like: import {Component,ViewChild} from '@angular/core'; In the above line we have imported the ViewChild, an annotation provided by Angular2 for getting reference of child components.

2. Let's add the following snippet to our AppComponent class: @ViewChild(ChildComponent) private childComponent:ChildComponent; We are querying the ChildComponent using @ViewChild property decoration and injecting it to private childComponent property.

This childComponent property will now provide us access to the child component. We know that our child component i.e. ChildComponent has a toggleVisibility function that shows/hides text and also displays the source which made it visible. In the code below we have defined the showHideText method which then calls the toggleVisibility function through childComponent property.

showHideText(){
    this.childComponent.toggleVisibility('Parent Component');
}

If we combine all the parts, our app.component.ts would now look as:

import {Component, ViewChild} from '@angular/core';
import {ChildComponent} from './child.component';

@Component({
    selector: 'my-app',
    template: `
    <div>
        <h1>Parent Component</h1>
        <button (click)="showHideText()">Show/Hide Child Component Text</button>
        <child-component></child-component>
    </div>
    `,
    directives: [ChildComponent]
})

export class AppComponent {
    @ViewChild(ChildComponent) private childComponent:ChildComponent;

    showHideText() {
        this.childComponent.toggleVisibility('Parent Component');
    }
}

In Angular2 there are multiple ways of interaction between components, ViewChild is just one of them!

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