# Component interaction

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This cookbook contains recipes for common component communication scenarios in which two or more components share information.

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<!--
# Contents

*   [Pass data from parent to child with input binding](guide/component-interaction#parent-to-child)
*   [Intercept input property changes with a setter](guide/component-interaction#parent-to-child-setter)
*   [Intercept input property changes with `ngOnChanges()`](guide/component-interaction#parent-to-child-on-changes)
*   [Parent calls an `@ViewChild()`](guide/component-interaction#parent-to-view-child)
*   [Parent and children communicate via a service](guide/component-interaction#bidirectional-service)
-->

**See the <live-example name="component-interaction"></live-example>**.

<a id="parent-to-child"></a>

## Pass data from parent to child with input binding

`HeroChildComponent` has two ***input properties***, typically adorned with [@Input() decorator](guide/inputs-outputs#input).

<code-example header="component-interaction/src/app/hero-child.component.ts" path="component-interaction/src/app/hero-child.component.ts"></code-example>

The second `@Input` aliases the child component property name `masterName` as `'master'`.

The `HeroParentComponent` nests the child `HeroChildComponent` inside an `*ngFor` repeater, binding its `master` string property to the child's `master` alias, and each iteration's `hero` instance to the child's `hero` property.

<code-example header="component-interaction/src/app/hero-parent.component.ts" path="component-interaction/src/app/hero-parent.component.ts"></code-example>

The running application displays three heroes:

<div class="lightbox">

<img alt="Parent-to-child" src="generated/images/guide/component-interaction/parent-to-child.png">

</div>

### Test it for Pass data from parent to child with input binding

E2E test that all children were instantiated and displayed as expected:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="parent-to-child"></code-example>

[Back to top](guide/component-interaction#top)

<a id="parent-to-child-setter"></a>

## Intercept input property changes with a setter

Use an input property setter to intercept and act upon a value from the parent.

The setter of the `name` input property in the child `NameChildComponent` trims the whitespace from a name and replaces an empty value with default text.

<code-example header="component-interaction/src/app/name-child.component.ts" path="component-interaction/src/app/name-child.component.ts"></code-example>

Here's the `NameParentComponent` demonstrating name variations including a name with all spaces:

<code-example header="component-interaction/src/app/name-parent.component.ts" path="component-interaction/src/app/name-parent.component.ts"></code-example>

<div class="lightbox">

<img alt="Parent-to-child-setter" src="generated/images/guide/component-interaction/setter.png">

</div>

### Test it for Intercept input property changes with a setter

E2E tests of input property setter with empty and non-empty names:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="parent-to-child-setter"></code-example>

[Back to top](guide/component-interaction#top)

<a id="parent-to-child-on-changes"></a>

## Intercept input property changes with `ngOnChanges()`

Detect and act upon changes to input property values with the `ngOnChanges()` method of the `OnChanges` lifecycle hook interface.

<div class="alert is-helpful">

You might prefer this approach to the property setter when watching multiple, interacting input properties.

Learn about `ngOnChanges()` in the [Lifecycle Hooks](guide/lifecycle-hooks) chapter.

</div>

This `VersionChildComponent` detects changes to the `major` and `minor` input properties and composes a log message reporting these changes:

<code-example header="component-interaction/src/app/version-child.component.ts" path="component-interaction/src/app/version-child.component.ts"></code-example>

The `VersionParentComponent` supplies the `minor` and `major` values and binds buttons to methods that change them.

<code-example header="component-interaction/src/app/version-parent.component.ts" path="component-interaction/src/app/version-parent.component.ts"></code-example>

Here's the output of a button-pushing sequence:

<div class="lightbox">

<img alt="Parent-to-child-onchanges" src="generated/images/guide/component-interaction/parent-to-child-on-changes.gif">

</div>

### Test it for Intercept input property changes with `ngOnChanges()`

Test that ***both*** input properties are set initially and that button clicks trigger the expected `ngOnChanges` calls and values:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="parent-to-child-onchanges"></code-example>

[Back to top](guide/component-interaction#top)

<a id="child-to-parent"></a>

## Parent listens for child event

The child component exposes an `EventEmitter` property with which it `emits` events when something happens.
The parent binds to that event property and reacts to those events.

The child's `EventEmitter` property is an ***output property***, typically adorned with an [@Output() decorator](guide/inputs-outputs#output) as seen in this `VoterComponent`:

<code-example header="component-interaction/src/app/voter.component.ts" path="component-interaction/src/app/voter.component.ts"></code-example>

Clicking a button triggers emission of a `true` or `false`, the boolean *payload*.

The parent `VoteTakerComponent` binds an event handler called `onVoted()` that responds to the child event payload `$event` and updates a counter.

<code-example header="component-interaction/src/app/votetaker.component.ts" path="component-interaction/src/app/votetaker.component.ts"></code-example>

The framework passes the event argument &mdash;represented by `$event`&mdash; to the handler method, and the method processes it:

<div class="lightbox">

<img alt="Child-to-parent" src="generated/images/guide/component-interaction/child-to-parent.gif">

</div>

### Test it for Parent listens for child event

Test that clicking the *Agree* and *Disagree* buttons update the appropriate counters:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="child-to-parent"></code-example>

[Back to top](guide/component-interaction#top)

## Parent interacts with child using *local variable*

A parent component cannot use data binding to read child properties or invoke child methods.
Do both by creating a template reference variable for the child element and then reference that variable *within the parent template* as seen in the following example.

<a id="countdown-timer-example"></a>

The following is a child `CountdownTimerComponent` that repeatedly counts down to zero and launches a rocket.
The `start` and `stop` methods control the clock and a countdown status message displays in its own template.

<code-example header="component-interaction/src/app/countdown-timer.component.ts" path="component-interaction/src/app/countdown-timer.component.ts"></code-example>

The `CountdownLocalVarParentComponent` that hosts the timer component is as follows:

<code-example header="component-interaction/src/app/countdown-parent.component.ts" path="component-interaction/src/app/countdown-parent.component.ts" region="lv"></code-example>

The parent component cannot data bind to the child's `start` and `stop` methods nor to its `seconds` property.

Place a local variable, `#timer`, on the tag `<app-countdown-timer>` representing the child component.
That gives you a reference to the child component and the ability to access *any of its properties or methods* from within the parent template.

This example wires parent buttons to the child's `start` and `stop` and uses interpolation to display the child's `seconds` property.

Here, the parent and child are working together.

<div class="lightbox">

<img alt="countdown timer" src="generated/images/guide/component-interaction/countdown-timer-anim.gif">

</div>

<a id="countdown-tests"></a>

### Test it for Parent interacts with child using *local variable*

Test that the seconds displayed in the parent template match the seconds displayed in the child's status message.
Test also that clicking the *Stop* button pauses the countdown timer:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="countdown-timer-tests"></code-example>

[Back to top](guide/component-interaction#top)

<a id="parent-to-view-child"></a>

## Parent calls an `@ViewChild()`

The *local variable* approach is straightforward.
But it is limited because the parent-child wiring must be done entirely within the parent template.
The parent component *itself* has no access to the child.

You can't use the *local variable* technique if the parent component's *class* relies on the child component's *class*.
The parent-child relationship of the components is not established within each component's respective *class* with the *local variable* technique.
Because the *class* instances are not connected to one another, the parent *class* cannot access the child *class* properties and methods.

When the parent component *class* requires that kind of access, ***inject*** the child component into the parent as a *ViewChild*.

The following example illustrates this technique with the same [Countdown Timer](guide/component-interaction#countdown-timer-example) example.
Neither its appearance nor its behavior changes.
The child [CountdownTimerComponent](guide/component-interaction#countdown-timer-example) is the same as well.

<div class="alert is-helpful">

The switch from the *local variable* to the *ViewChild* technique is solely for the purpose of demonstration.

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Here is the parent, `CountdownViewChildParentComponent`:

<code-example header="component-interaction/src/app/countdown-parent.component.ts" path="component-interaction/src/app/countdown-parent.component.ts" region="vc"></code-example>

It takes a bit more work to get the child view into the parent component *class*.

First, you have to import references to the `ViewChild` decorator and the `AfterViewInit` lifecycle hook.

Next, inject the child `CountdownTimerComponent` into the private `timerComponent` property using the `@ViewChild` property decoration.

The `#timer` local variable is gone from the component metadata.
Instead, bind the buttons to the parent component's own `start` and `stop` methods and present the ticking seconds in an interpolation around the parent component's `seconds` method.

These methods access the injected timer component directly.

The `ngAfterViewInit()` lifecycle hook is an important wrinkle.
The timer component isn't available until *after* Angular displays the parent view.
So it displays `0` seconds initially.

Then Angular calls the `ngAfterViewInit` lifecycle hook at which time it is *too late* to update the parent view's display of the countdown seconds.
Angular's unidirectional data flow rule prevents updating the parent view's in the same cycle.
The application must *wait one turn* before it can display the seconds.

Use `setTimeout()` to wait one tick and then revise the `seconds()` method so that it takes future values from the timer component.

### Test it for Parent calls an `@ViewChild()`

Use [the same countdown timer tests](guide/component-interaction#countdown-tests) as before.

[Back to top](guide/component-interaction#top)

<a id="bidirectional-service"></a>

## Parent and children communicate using a service

A parent component and its children share a service whose interface enables bidirectional communication *within the family*.

The scope of the service instance is the parent component and its children.
Components outside this component subtree have no access to the service or their communications.

This `MissionService` connects the `MissionControlComponent` to multiple `AstronautComponent` children.

<code-example header="component-interaction/src/app/mission.service.ts" path="component-interaction/src/app/mission.service.ts"></code-example>

The `MissionControlComponent` both provides the instance of the service that it shares with its children \(through the `providers` metadata array\) and injects that instance into itself through its constructor:

<code-example header="component-interaction/src/app/missioncontrol.component.ts" path="component-interaction/src/app/missioncontrol.component.ts"></code-example>

The `AstronautComponent` also injects the service in its constructor.
Each `AstronautComponent` is a child of the `MissionControlComponent` and therefore receives its parent's service instance:

<code-example header="component-interaction/src/app/astronaut.component.ts" path="component-interaction/src/app/astronaut.component.ts"></code-example>

<div class="alert is-helpful">

Notice that this example captures the `subscription` and `unsubscribe()` when the `AstronautComponent` is destroyed.
This is a memory-leak guard step.
There is no actual risk in this application because the lifetime of a `AstronautComponent` is the same as the lifetime of the application itself.
That *would not* always be true in a more complex application.

You don't add this guard to the `MissionControlComponent` because, as the parent,
it controls the lifetime of the `MissionService`.

</div>

The *History* log demonstrates that messages travel in both directions between the parent `MissionControlComponent` and the `AstronautComponent` children, facilitated by the service:

<div class="lightbox">

<img alt="bidirectional-service" src="generated/images/guide/component-interaction/bidirectional-service.gif">

</div>

### Test it for Parent and children communicate using a service

Tests click buttons of both the parent `MissionControlComponent` and the `AstronautComponent` children and verify that the history meets expectations:

<code-example header="component-interaction/e2e/src/app.e2e-spec.ts" path="component-interaction/e2e/src/app.e2e-spec.ts" region="bidirectional-service"></code-example>

[Back to top](guide/component-interaction#top)

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@reviewed 2022-02-28
