334 lines
13 KiB
Plaintext
334 lines
13 KiB
Plaintext
include ../_util-fns
|
|
|
|
<a id="top"></a>
|
|
: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.
|
|
|
|
<a id="toc"></a>
|
|
:marked
|
|
## Table of 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 listens for child event](#child-to-parent)
|
|
|
|
[Parent interacts with child via a *local variable*](#parent-to-child-local-var)
|
|
|
|
[Parent calls a *ViewChild*](#parent-to-view-child)
|
|
|
|
[Parent and children communicate via a service](#bidirectional-service)
|
|
|
|
:marked
|
|
**See the [live example](/resources/live-examples/cb-component-communication/ts/plnkr.html)**.
|
|
|
|
.l-main-section
|
|
<a id="parent-to-child"></a>
|
|
:marked
|
|
## Pass data from parent to child with input binding
|
|
|
|
`HeroChildComponent` has two ***input properties***,
|
|
typically adorned with [@Input decorations](/docs/ts/latest/guide/template-syntax.html#inputs-outputs).
|
|
|
|
+makeExample('cb-component-communication/ts/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/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.js', 'parent-to-child')
|
|
|
|
:marked
|
|
[Back to top](#top)
|
|
|
|
.l-main-section
|
|
<a id="parent-to-child-setter"></a>
|
|
: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/app/name-child.component.ts')
|
|
|
|
:marked
|
|
Here's the `NameParentComponent` demonstrating name variations including a name with all spaces:
|
|
|
|
+makeExample('cb-component-communication/ts/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.js', 'parent-to-child-setter')
|
|
|
|
:marked
|
|
[Back to top](#top)
|
|
|
|
.l-main-section
|
|
<a id="parent-to-child-on-changes"></a>
|
|
: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
|
|
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/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/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.js', 'parent-to-child-onchanges')
|
|
|
|
:marked
|
|
[Back to top](#top)
|
|
|
|
.l-main-section
|
|
<a id="child-to-parent"></a>
|
|
: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](/docs/ts/latest/guide/template-syntax.html#inputs-outputs)
|
|
as seen in this `VoterComponent`:
|
|
|
|
+makeExample('cb-component-communication/ts/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 (`onVoted`) that responds to the child event
|
|
payload (`$event`) and updates a counter.
|
|
|
|
+makeExample('cb-component-communication/ts/app/votetaker.component.ts')
|
|
|
|
:marked
|
|
The framework passes the event argument — represented by `$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.js', 'child-to-parent')
|
|
|
|
:marked
|
|
[Back to top](#top)
|
|
|
|
parent-to-child-local-var
|
|
.l-main-section
|
|
<a id="parent-to-child-local-var"></a>
|
|
:marked
|
|
## Parent interacts with child via *local variable*
|
|
|
|
A parent component cannot use data binding to read child properties
|
|
or invoke child methods. We can do both
|
|
by creating a template local 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>
|
|
We have 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/app/countdown-timer.component.ts')
|
|
:marked
|
|
Let's see the `CountdownLocalVarParentComponent` that hosts the timer component.
|
|
|
|
+makeExample('cb-component-communication/ts/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.
|
|
|
|
We can place a local variable (`#timer`) on the tag (`<countdown-timer>`) representing the child component.
|
|
That gives us a reference to the child component itself and the ability to access
|
|
*any of its properties or methods* from within the parent template.
|
|
|
|
In this example, we wire parent buttons to the child's `start` and `stop` and
|
|
use 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.js', 'countdown-timer-tests')
|
|
|
|
:marked
|
|
[Back to top](#top)
|
|
|
|
.l-main-section
|
|
<a id="parent-to-view-child"></a>
|
|
:marked
|
|
## Parent calls a *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.
|
|
|
|
We 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,
|
|
we ***inject*** the child component into the parent as a *ViewChild*.
|
|
|
|
We'll illustrate this technique with the same [Countdown Timer](#countdown-timer-example) example.
|
|
We won't change its appearance or behavior.
|
|
The child [CountdownTimerComponent](#countdown-timer-example) is the same as well.
|
|
.l-sub-section
|
|
:marked
|
|
We are switching from the *local variable* to the *ViewChild* technique
|
|
solely for the purpose of demonstration.
|
|
:marked
|
|
Here is the parent, `CountdownViewChildParentComponent`:
|
|
+makeExample('cb-component-communication/ts/app/countdown-parent.component.ts', 'vc')
|
|
:marked
|
|
It takes a bit more work to get the child view into the parent component classs.
|
|
|
|
We import references to the `ViewChild` decorator and the `AfterViewInit` lifecycle hook.
|
|
|
|
We 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 we 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 we display `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 us from updating the parent view's
|
|
in the same cycle. We have to *wait one turn* before we can display the seconds.
|
|
|
|
We 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
|
|
<a id="bidirectional-service"></a>
|
|
: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/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/app/missioncontrol.component.ts')
|
|
|
|
:marked
|
|
The `AstronoutComponent` also injects the service in its constructor.
|
|
Each `AstronoutComponent` is a child of the `MissionControlComponent` and therefore receives its parent's service instance:
|
|
|
|
+makeExample('cb-component-communication/ts/app/astronaut.component.ts')
|
|
|
|
.l-sub-section
|
|
:marked
|
|
Notice that we capture 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.
|
|
|
|
We do not 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 `AstronoutComponent` 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 `AstronoutComponent` children
|
|
and verify that the *History* meets expectations:
|
|
|
|
+makeExample('cb-component-communication/e2e-spec.js', 'bidirectional-service')
|
|
|
|
:marked
|
|
[Back to top](#top)
|