82 lines
4.1 KiB
Markdown
82 lines
4.1 KiB
Markdown
# Two-way binding
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Two-way binding gives components in your application a way to share data.
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Use two-way binding to listen for events and update values simultaneously between parent and child components.
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<div class="alert is-helpful">
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See the <live-example></live-example> for a working example containing the code snippets in this guide.
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</div>
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## Prerequisites
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To get the most out of two-way binding, you should have a basic understanding of the following concepts:
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* [Property binding](guide/property-binding)
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* [Event binding](guide/event-binding)
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* [Inputs and Outputs](guide/inputs-outputs)
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<hr>
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Two-way binding combines property binding with event binding:
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* [Property binding](guide/property-binding) sets a specific element property.
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* [Event binding](guide/event-binding) listens for an element change event.
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## Adding two-way data binding
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Angular's two-way binding syntax is a combination of square brackets and parentheses, `[()]`.
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The `[()]` syntax combines the brackets of property binding, `[]`, with the parentheses of event binding, `()`, as follows.
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<code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html" region="two-way-syntax"></code-example>
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## How two-way binding works
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For two-way data binding to work, the `@Output()` property must use the pattern, `inputChange`, where `input` is the name of the `@Input()` property.
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For example, if the `@Input()` property is `size`, the `@Output()` property must be `sizeChange`.
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The following `sizerComponent` has a `size` value property and a `sizeChange` event.
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The `size` property is an `@Input()`, so data can flow into the `sizerComponent`.
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The `sizeChange` event is an `@Output()`, which allows data to flow out of the `sizerComponent` to the parent component.
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Next, there are two methods, `dec()` to decrease the font size and `inc()` to increase the font size.
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These two methods use `resize()` to change the value of the `size` property within min/max value constraints, and to emit an event that conveys the new `size` value.
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<code-example path="two-way-binding/src/app/sizer/sizer.component.ts" region="sizer-component" header="src/app/sizer.component.ts"></code-example>
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The `sizerComponent` template has two buttons that each bind the click event to the `inc()` and `dec()` methods.
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When the user clicks one of the buttons, the `sizerComponent` calls the corresponding method.
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Both methods, `inc()` and `dec()`, call the `resize()` method with a `+1` or `-1`, which in turn raises the `sizeChange` event with the new size value.
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<code-example path="two-way-binding/src/app/sizer/sizer.component.html" header="src/app/sizer.component.html"></code-example>
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In the `AppComponent` template, `fontSizePx` is two-way bound to the `SizerComponent`.
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<code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html" region="two-way-1"></code-example>
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In the `AppComponent`, `fontSizePx` establishes the initial `SizerComponent.size` value by setting the value to `16`.
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<code-example path="two-way-binding/src/app/app.component.ts" header="src/app/app.component.ts" region="font-size"></code-example>
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Clicking the buttons updates the `AppComponent.fontSizePx`.
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The revised `AppComponent.fontSizePx` value updates the style binding, which makes the displayed text bigger or smaller.
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The two-way binding syntax is shorthand for a combination of property binding and event binding.
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The `SizerComponent` binding as separate property binding and event binding is as follows.
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<code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html (expanded)" region="two-way-2"></code-example>
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The `$event` variable contains the data of the `SizerComponent.sizeChange` event.
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Angular assigns the `$event` value to the `AppComponent.fontSizePx` when the user clicks the buttons.
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<div class="callout is-helpful">
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<header>Two-way binding in forms</header>
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Because no native HTML element follows the `x` value and `xChange` event pattern, two-way binding with form elements requires `NgModel`.
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For more information on how to use two-way binding in forms, see Angular [NgModel](guide/built-in-directives#ngModel).
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</div>
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