angular-cn/aio/content/guide/component-communication.md

@title Component Interaction

@intro Share information between different directives and components.

@description {@a top}

This cookbook contains recipes for common component communication scenarios in which two or more components share information. {@a toc}

See the .

{@a parent-to-child}

Pass data from parent to child with input binding

HeroChildComponent has two input properties, typically adorned with @Input decorations.

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.

The running application displays three heroes:

Test it

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

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{@a parent-to-child-setter}

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.

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

Test it

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

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{@a parent-to-child-on-changes}

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.

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

Learn about ngOnChanges() in the LifeCycle Hooks chapter.

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

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

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

Test it

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

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{@a child-to-parent}

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 decoration as seen in this VoterComponent:

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.

The framework passes the event argument—represented by $event—to the handler method, and the method processes it:

Test it

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

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Parent interacts with child via local variable

A parent component cannot use data binding to read child properties or invoke child methods. You can 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 countdown-timer-example} The following is a child CountdownTimerComponent that repeatedly counts down to zero and launches a rocket. It has start and stop methods that control the clock and it displays a countdown status message in its own template.

The CountdownLocalVarParentComponent that hosts the timer component is as follows:

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

You can place a local variable, #timer, on the tag <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 we see the parent and child working together.

{@a countdown-tests}

Test it

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:

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{@a parent-to-view-child}

Parent calls an @ViewChild()

The local variable approach is simple and easy. 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 an instance of the parent component class must read or write child component values or must call child component 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 example. Neither its appearance nor its behavior will change. The child CountdownTimerComponent is the same as well.

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

Here is the parent, CountdownViewChildParentComponent:

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 via 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 app has to 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

Use the same countdown timer tests as before.

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{@a bidirectional-service}

Parent and children communicate via a service

A parent component and its children share a service whose interface enables bi-directional 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.

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:

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:

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 app because the lifetime of a AstronautComponent is the same as the lifetime of the app 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.

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

Test it

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

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