include ../_util-fns :marked This cookbook contains recipes for common component communication scenarios in which two or more components share information. // .l-sub-section :marked For an in-depth look at each fundamental concepts in component communication, we can find detailed description and samples in the [Component Communication]() document. :marked # Contents * [Pass data from parent to child with input binding](#parent-to-child) * [Intercept input property changes with a setter](#parent-to-child-setter) * [Intercept input property changes with `ngOnChanges()`](#parent-to-child-on-changes) * [Parent calls an `@ViewChild()`](#parent-to-view-child) * [Parent and children communicate via a service](#bidirectional-service) :marked **See the **. .l-main-section :marked ## Pass data from parent to child with input binding `HeroChildComponent` has two ***input properties***, typically adorned with [@Input decorations](../guide/template-syntax.html#inputs-outputs). +makeExample('cb-component-communication/ts/src/app/hero-child.component.ts') :marked 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. +makeExample('cb-component-communication/ts/src/app/hero-parent.component.ts') :marked The running application displays three heroes: figure.image-display img(src="/resources/images/cookbooks/component-communication/parent-to-child.png" alt="Parent-to-child") :marked ### Test it E2E test that all children were instantiated and displayed as expected: +makeExample('cb-component-communication/e2e-spec.ts', 'parent-to-child') :marked [Back to top](#top) .l-main-section :marked ## 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. +makeExample('cb-component-communication/ts/src/app/name-child.component.ts') :marked Here's the `NameParentComponent` demonstrating name variations including a name with all spaces: +makeExample('cb-component-communication/ts/src/app/name-parent.component.ts') figure.image-display img(src="/resources/images/cookbooks/component-communication/setter.png" alt="Parent-to-child-setter") :marked ### Test it E2E tests of input property setter with empty and non-empty names: +makeExample('cb-component-communication/e2e-spec.ts', 'parent-to-child-setter') :marked [Back to top](#top) .l-main-section :marked ## 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. .l-sub-section :marked You may prefer this approach to the property setter when watching multiple, interacting input properties. Learn about `ngOnChanges()` in the [LifeCycle Hooks](../guide/lifecycle-hooks.html) chapter. :marked This `VersionChildComponent` detects changes to the `major` and `minor` input properties and composes a log message reporting these changes: +makeExample('cb-component-communication/ts/src/app/version-child.component.ts') :marked The `VersionParentComponent` supplies the `minor` and `major` values and binds buttons to methods that change them. +makeExample('cb-component-communication/ts/src/app/version-parent.component.ts') :marked Here's the output of a button-pushing sequence: figure.image-display img(src="/resources/images/cookbooks/component-communication/parent-to-child-on-changes.gif" alt="Parent-to-child-onchanges") :marked ### Test it Test that ***both*** input properties are set initially and that button clicks trigger the expected `ngOnChanges` calls and values: +makeExample('cb-component-communication/e2e-spec.ts', 'parent-to-child-onchanges') :marked [Back to top](#top) .l-main-section :marked ## 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](../guide/template-syntax.html#inputs-outputs) as seen in this `VoterComponent`: +makeExample('cb-component-communication/ts/src/app/voter.component.ts') :marked 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 `$event` and updates a counter. +makeExample('cb-component-communication/ts/src/app/votetaker.component.ts') :marked The framework passes the event argument—represented by `$event`—to the handler method, and the method processes it: figure.image-display img(src="/resources/images/cookbooks/component-communication/child-to-parent.gif" alt="Child-to-parent") :marked ### Test it Test that clicking the *Agree* and *Disagree* buttons update the appropriate counters: +makeExample('cb-component-communication/e2e-spec.ts', 'child-to-parent') :marked [Back to top](#top) .l-main-section#parent-to-child-local-var :marked ## 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. 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. +makeExample('cb-component-communication/ts/src/app/countdown-timer.component.ts') :marked The `CountdownLocalVarParentComponent` that hosts the timer component is as follows: +makeExample('cb-component-communication/ts/src/app/countdown-parent.component.ts', 'lv') :marked 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 `` 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. figure.image-display img(src="/resources/images/cookbooks/component-communication/countdown-timer-anim.gif" alt="countdown timer") a(id="countdown-tests") :marked ### 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: +makeExample('cb-component-communication/e2e-spec.ts', 'countdown-timer-tests') :marked [Back to top](#top) .l-main-section :marked ## 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](#countdown-timer-example) example. Neither its appearance nor its behavior will change. The child [CountdownTimerComponent](#countdown-timer-example) is the same as well. .l-sub-section :marked The switch from the *local variable* to the *ViewChild* technique is solely for the purpose of demonstration. :marked Here is the parent, `CountdownViewChildParentComponent`: +makeExample('cb-component-communication/ts/src/app/countdown-parent.component.ts', 'vc') :marked 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](#countdown-tests) as before. :marked [Back to top](#top) .l-main-section :marked ## 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. +makeExample('cb-component-communication/ts/src/app/mission.service.ts') :marked 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: +makeExample('cb-component-communication/ts/src/app/missioncontrol.component.ts') :marked 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: +makeExample('cb-component-communication/ts/src/app/astronaut.component.ts') .l-sub-section :marked 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`. :marked The *History* log demonstrates that messages travel in both directions between the parent `MissionControlComponent` and the `AstronautComponent` children, facilitated by the service: figure.image-display img(src="/resources/images/cookbooks/component-communication/bidirectional-service.gif" alt="bidirectional-service") :marked ### Test it Tests click buttons of both the parent `MissionControlComponent` and the `AstronautComponent` children and verify that the history meets expectations: +makeExample('cb-component-communication/e2e-spec.ts', 'bidirectional-service') :marked [Back to top](#top)