docs: edit two-way binding doc (#38952)

This commit edits copy and doc regions to clarify concepts.

PR Close #38952
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Kapunahele Wong 2020-09-23 11:02:04 -04:00 committed by atscott
parent b015d3e950
commit a87236292e
3 changed files with 52 additions and 45 deletions

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@ -1,7 +1,9 @@
<h1 id="two-way">Two-way Binding</h1> <h1 id="two-way">Two-way Binding</h1>
<div id="two-way-1"> <div id="two-way-1">
<!-- #docregion two-way-1 --> <!-- #docregion two-way-1 -->
<!-- #docregion two-way-syntax -->
<app-sizer [(size)]="fontSizePx"></app-sizer> <app-sizer [(size)]="fontSizePx"></app-sizer>
<!-- #enddocregion two-way-syntax-->
<div [style.font-size.px]="fontSizePx">Resizable Text</div> <div [style.font-size.px]="fontSizePx">Resizable Text</div>
<!-- #enddocregion two-way-1 --> <!-- #enddocregion two-way-1 -->
<label>FontSize (px): <input [(ngModel)]="fontSizePx"></label> <label>FontSize (px): <input [(ngModel)]="fontSizePx"></label>

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@ -5,9 +5,9 @@ import { Component, Input, Output, EventEmitter } from '@angular/core';
templateUrl: './sizer.component.html', templateUrl: './sizer.component.html',
styleUrls: ['./sizer.component.css'] styleUrls: ['./sizer.component.css']
}) })
// #docregion sizer-component
export class SizerComponent { export class SizerComponent {
@Input() size: number | string; @Input() size: number | string;
@Output() sizeChange = new EventEmitter<number>(); @Output() sizeChange = new EventEmitter<number>();
@ -18,5 +18,5 @@ export class SizerComponent {
this.size = Math.min(40, Math.max(8, +this.size + delta)); this.size = Math.min(40, Math.max(8, +this.size + delta));
this.sizeChange.emit(this.size); this.sizeChange.emit(this.size);
} }
} }
// #enddocregion sizer-component

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# Two-way binding `[(...)]` # Two-way binding
Two-way binding gives your app a way to share data between a component class and Two-way binding gives components in your application a way to share data.
its template. Use two-way binding binding to listen for events and update values simultaneously between parent and child components.
<div class="alert is-helpful"> <div class="alert is-helpful">
@ -9,68 +9,73 @@ See the <live-example></live-example> for a working example containing the code
</div> </div>
## Basics of two-way binding ## Prerequisites
Two-way binding does two things: To get the most out of two-way binding, you should have a basic understanding of the following concepts:
1. Sets a specific element property. * [Property binding](guide/property-binding)
1. Listens for an element change event. * [Event binding](guide/event-binding)
* [Inputs and Outputs](guide/inputs-outputs)
Angular offers a special _two-way data binding_ syntax for this purpose, `[()]`. <hr>
The `[()]` syntax combines the brackets
of property binding, `[]`, with the parentheses of event binding, `()`.
<div class="callout is-important"> Two-way binding combines property binding with event binding:
<header> * [Property binding](guide/property-binding) sets a specific element property.
[( )] = banana in a box * [Event binding](guide/event-binding) listens for an element change event.
</header>
Visualize a *banana in a box* to remember that the parentheses go _inside_ the brackets. ## Adding two-way data binding
</div> Angular's two-way binding syntax is a combination of square brackets and parentheses, `[()]`.
The `[()]` syntax combines the brackets of property binding, `[]`, with the parentheses of event binding, `()`, as follows.
The `[()]` syntax is easy to demonstrate when the element has a settable <code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html" region="two-way-syntax"></code-example>
property called `x` and a corresponding event named `xChange`.
Here's a `SizerComponent` that fits this pattern.
It has a `size` value property and a companion `sizeChange` event:
<code-example path="two-way-binding/src/app/sizer/sizer.component.ts" header="src/app/sizer.component.ts"></code-example> ## How two-way binding works
For two-way data binding to work, the `@Output()` property must use the pattern, `inputChange`, where `input` is the name of the `@Input()` property.
For example, if the `@Input()` property is `size`, the `@Output()` property must be `sizeChange`.
The following `sizerComponent` has a `size` value property and a `sizeChange` event.
The `size` property is an `@Input()`, so data can flow into the `sizerComponent`.
The `sizeChange` event is an `@Output()`, which allows data to flow out of the `sizerComponent` to the parent component.
Next, there are two methods, `dec()` to decrease the font size and `inc()` to increase the font size.
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.
<code-example path="two-way-binding/src/app/sizer/sizer.component.ts" region="sizer-component" header="src/app/sizer.component.ts"></code-example>
The `sizerComponent` template has two buttons that each bind the click event to the `inc()` and `dec()` methods.
When the user clicks one of the buttons, the `sizerComponent` calls the corresponding method.
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.
<code-example path="two-way-binding/src/app/sizer/sizer.component.html" header="src/app/sizer.component.html"></code-example> <code-example path="two-way-binding/src/app/sizer/sizer.component.html" header="src/app/sizer.component.html"></code-example>
The initial `size` is an input value from a property binding.
Clicking the buttons increases or decreases the `size`, within
min/max value constraints,
and then raises, or emits, the `sizeChange` event with the adjusted size.
Here's an example in which the `AppComponent.fontSizePx` is two-way bound to the `SizerComponent`: In the `AppComponent` template, `fontSizePx` is two-way bound to the `SizerComponent`.
<code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html (two-way-1)" region="two-way-1"></code-example> <code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html" region="two-way-1"></code-example>
The `AppComponent.fontSizePx` establishes the initial `SizerComponent.size` value. In the `AppComponent`, `fontSizePx` establishes the initial `SizerComponent.size` value by setting the value to `16`.
<code-example path="two-way-binding/src/app/app.component.ts" header="src/app/app.component.ts" region="font-size"></code-example> <code-example path="two-way-binding/src/app/app.component.ts" header="src/app/app.component.ts" region="font-size"></code-example>
Clicking the buttons updates the `AppComponent.fontSizePx` via the two-way binding. Clicking the buttons updates the `AppComponent.fontSizePx`.
The revised `AppComponent.fontSizePx` value flows through to the _style_ binding, The revised `AppComponent.fontSizePx` value updates the style binding, which makes the displayed text bigger or smaller.
making the displayed text bigger or smaller.
The two-way binding syntax is really just syntactic sugar for a _property_ binding and an _event_ binding. The two-way binding syntax is shorthand for a combination of property binding and event binding.
Angular desugars the `SizerComponent` binding into this: The `SizerComponent` binding as separate property binding and event binding is as follows.
<code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html (two-way-2)" region="two-way-2"></code-example> <code-example path="two-way-binding/src/app/app.component.html" header="src/app/app.component.html (expanded)" region="two-way-2"></code-example>
The `$event` variable contains the payload of the `SizerComponent.sizeChange` event. The `$event` variable contains the data of the `SizerComponent.sizeChange` event.
Angular assigns the `$event` value to the `AppComponent.fontSizePx` when the user clicks the buttons. Angular assigns the `$event` value to the `AppComponent.fontSizePx` when the user clicks the buttons.
## Two-way binding in forms <div class="callout is-helpful">
The two-way binding syntax is a great convenience compared to <header>Two-way binding in forms</header>
separate property and event bindings. It would be convenient to
use two-way binding with HTML form elements like `<input>` and
`<select>`. However, no native HTML element follows the `x`
value and `xChange` event pattern.
For more on how to use two-way binding in forms, see Because no native HTML element follows the `x` value and `xChange` event pattern, two-way binding with form elements requires `NgModel`.
Angular [NgModel](guide/built-in-directives#ngModel). For more information on how to use two-way binding in forms, see Angular [NgModel](guide/built-in-directives#ngModel).
</div>