docs: rewrite event binding section and add example (#26162)

PR Close #26162

aio/content/examples/event-binding/e2e/src/app.e2e-spec.ts

@@ -0,0 +1,71 @@
+'use strict'; // necessary for es6 output in node
+
+import { browser, element, by, protractor } from 'protractor';
+
+describe('Event binding example', function () {
+
+  beforeEach(function () {
+    browser.get('');
+  });
+
+  let saveButton = element.all(by.css('button')).get(0);
+  let onSaveButton = element.all(by.css('button')).get(1);
+  let myClick = element.all(by.css('button')).get(2);
+  let deleteButton = element.all(by.css('button')).get(3);
+  let saveNoProp = element.all(by.css('button')).get(4);
+  let saveProp = element.all(by.css('button')).get(5);
+
+
+  it('should display Event Binding with Angular', function () {
+    expect(element(by.css('h1')).getText()).toEqual('Event Binding');
+  });
+
+  it('should display 6 buttons', function() {
+    expect(saveButton.getText()).toBe('Save');
+    expect(onSaveButton.getText()).toBe('on-click Save');
+    expect(myClick.getText()).toBe('click with myClick');
+    expect(deleteButton.getText()).toBe('Delete');
+    expect(saveNoProp.getText()).toBe('Save, no propagation');
+    expect(saveProp.getText()).toBe('Save with propagation');
+  });
+
+  it('should support user input', function () {
+    let input = element(by.css('input'));
+    let bindingResult = element.all(by.css('h4')).get(1);
+    expect(bindingResult.getText()).toEqual('Result: teapot');
+    input.sendKeys('abc');
+    expect(bindingResult.getText()).toEqual('Result: teapotabc');
+  });
+
+  it('should hide the item img', async () => {
+    let deleteButton = element.all(by.css('button')).get(3);
+    await deleteButton.click();
+    browser.switchTo().alert().accept();
+    expect(element.all(by.css('img')).get(0).getCssValue('display')).toEqual('none');
+  });
+
+  it('should show two alerts', async () => {
+    let parentDiv = element.all(by.css('.parent-div'));
+    let childDiv = element.all(by.css('div > div')).get(1);
+    await parentDiv.click();
+    browser.switchTo().alert().accept();
+    expect(childDiv.getText()).toEqual('Click me too! (child)');
+    await childDiv.click();
+    expect(browser.switchTo().alert().getText()).toEqual('Click me. Event target class is child-div');
+    browser.switchTo().alert().accept();
+  });
+
+  it('should show 1 alert from Save, no prop, button', async () => {
+    await saveNoProp.click();
+    expect(browser.switchTo().alert().getText()).toEqual('Saved. Event target is Save, no propagation');
+    browser.switchTo().alert().accept();
+  });
+
+  it('should show 2 alerts from Save w/prop button', async () => {
+    await saveProp.click();
+    expect(browser.switchTo().alert().getText()).toEqual('Saved.');
+    browser.switchTo().alert().accept();
+    expect(browser.switchTo().alert().getText()).toEqual('Saved.');
+    browser.switchTo().alert().accept();
+  });
+});

aio/content/examples/event-binding/src/app/app.component.css

@@ -0,0 +1,25 @@
+.group {
+  background-color: #dae8f9;
+  padding: 1rem;
+  margin: 1rem 0;
+}
+
+.parent-div {
+  background-color: #bdd1f7;
+  border: solid 1px rgb(25, 118, 210);
+  padding: 1rem;
+}
+
+.parent-div:hover {
+  background-color: #8fb4f9;
+}
+
+.child-div {
+  margin-top: 1rem;
+  background-color: #fff;
+  padding: 1rem;
+}
+
+.child-div:hover {
+  background-color: #eee;
+}

aio/content/examples/event-binding/src/app/app.component.html

@@ -0,0 +1,53 @@
+<h1 id="event-binding">Event Binding</h1>
+
+<div class="group">
+  <h3>Target event</h3>
+  <!-- #docregion event-binding-1 -->
+  <button (click)="onSave($event)">Save</button>
+  <!-- #enddocregion event-binding-1 -->
+
+  <!-- #docregion event-binding-2 -->
+  <button on-click="onSave($event)">on-click Save</button>
+  <!-- #enddocregion event-binding-2 -->
+
+  <!-- #docregion custom-directive -->
+  <h4>myClick is an event on the custom ClickDirective:</h4>
+  <button (myClick)="clickMessage=$event" clickable>click with myClick</button>
+  {{clickMessage}}
+  <!-- #enddocregion custom-directive -->
+
+</div>
+
+<div class="group">
+  <h3>$event and event handling statements</h3>
+  <h4>Result: {{currentItem.name}}</h4>
+
+  <!-- #docregion event-binding-3-->
+  <input [value]="currentItem.name"
+         (input)="currentItem.name=$event.target.value" >
+  without NgModel
+  <!-- #enddocregion event-binding-3-->
+</div>
+
+<div class="group">
+  <h3>Binding to a nested component</h3>
+  <h4>Custom events with EventEmitter</h4>
+  <!-- #docregion event-binding-to-component -->
+  <app-item-detail (deleteRequest)="deleteItem($event)" [item]="currentItem"></app-item-detail>
+    <!-- #enddocregion event-binding-to-component -->
+
+
+  <h4>Click to see event target class:</h4>
+<div class="parent-div" (click)="onClickMe($event)" clickable>Click me (parent)
+  <div class="child-div">Click me too! (child) </div>
+</div>
+
+<h3>Saves only once:</h3>
+<div (click)="onSave()" clickable>
+  <button (click)="onSave($event)">Save, no propagation</button>
+</div>
+
+<h3>Saves twice:</h3>
+<div (click)="onSave()" clickable>
+  <button (click)="onSave()">Save with propagation</button>
+</div>

aio/content/examples/event-binding/src/app/app.component.spec.ts

@@ -0,0 +1,27 @@
+import { TestBed, async } from '@angular/core/testing';
+import { AppComponent } from './app.component';
+describe('AppComponent', () => {
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({
+      declarations: [
+        AppComponent
+      ],
+    }).compileComponents();
+  }));
+  it('should create the app', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app).toBeTruthy();
+  }));
+  it(`should have as title 'Featured product:'`, async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app.title).toEqual('Featured product:');
+  }));
+  it('should render title in a p tag', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    fixture.detectChanges();
+    const compiled = fixture.debugElement.nativeElement;
+    expect(compiled.querySelector('p').textContent).toContain('Featured product:');
+  }));
+});

aio/content/examples/event-binding/src/app/app.component.ts

@@ -0,0 +1,29 @@
+import { Component } from '@angular/core';
+import { Item } from './item';
+
+@Component({
+  selector: 'app-root',
+  templateUrl: './app.component.html',
+  styleUrls: ['./app.component.css']
+})
+export class AppComponent {
+
+  currentItem = { name: 'teapot'} ;
+  clickMessage = '';
+
+  onSave(event: KeyboardEvent) {
+    const evtMsg = event ? ' Event target is ' + (<HTMLElement>event.target).textContent : '';
+    alert('Saved.' + evtMsg);
+    if (event) { event.stopPropagation(); }
+  }
+
+  deleteItem(item: Item) {
+    alert(`Delete the ${item}.`);
+  }
+
+  onClickMe(event: KeyboardEvent) {
+    const evtMsg = event ? ' Event target class is ' + (<HTMLElement>event.target).className  : '';
+    alert('Click me.' + evtMsg);
+  }
+
+}

aio/content/examples/event-binding/src/app/app.module.ts

@@ -0,0 +1,22 @@
+import { BrowserModule } from '@angular/platform-browser';
+import { NgModule } from '@angular/core';
+
+
+import { AppComponent } from './app.component';
+import { ItemDetailComponent } from './item-detail/item-detail.component';
+import { ClickDirective } from './click.directive';
+
+
+@NgModule({
+  declarations: [
+    AppComponent,
+    ItemDetailComponent,
+    ClickDirective
+  ],
+  imports: [
+    BrowserModule
+  ],
+  providers: [],
+  bootstrap: [AppComponent]
+})
+export class AppModule { }

aio/content/examples/event-binding/src/app/click.directive.ts

@@ -0,0 +1,18 @@
+/* tslint:disable use-output-property-decorator directive-class-suffix */
+import { Directive, ElementRef, EventEmitter, Output } from '@angular/core';
+
+@Directive({selector: '[myClick]'})
+export class ClickDirective {
+  @Output('myClick') clicks = new EventEmitter<string>(); //  @Output(alias) propertyName = ...
+
+  toggle = false;
+
+  constructor(el: ElementRef) {
+    el.nativeElement
+      .addEventListener('click', (event: Event) => {
+        this.toggle = !this.toggle;
+        this.clicks.emit(this.toggle ? 'Click!' : '');
+      });
+  }
+}
+

aio/content/examples/event-binding/src/app/item-detail/item-detail.component.css

@@ -0,0 +1,11 @@
+.detail {
+  border: 1px solid rgb(25, 118, 210);
+  padding: 1rem;
+  margin: 1rem 0;
+}
+
+img {
+  max-width: 100px;
+  display: block;
+  padding: 1rem 0;
+}

aio/content/examples/event-binding/src/app/item-detail/item-detail.component.html

@@ -0,0 +1,9 @@
+<div class="detail">
+  <p>This is the ItemDetailComponent</p>
+  <!-- #docregion line-through -->
+  <img src="{{itemImageUrl}}" [style.display]="displayNone">
+  <span [style.text-decoration]="lineThrough">{{ item.name }}
+  </span>
+  <button (click)="delete()">Delete</button>
+  <!-- #enddocregion line-through -->
+</div>

aio/content/examples/event-binding/src/app/item-detail/item-detail.component.spec.ts

@@ -0,0 +1,25 @@
+import { async, ComponentFixture, TestBed } from '@angular/core/testing';
+
+import { ItemDetailComponent } from './item-detail.component';
+
+describe('ItemDetailComponent', () => {
+  let component: ItemDetailComponent;
+  let fixture: ComponentFixture<ItemDetailComponent>;
+
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({
+      declarations: [ ItemDetailComponent ]
+    })
+    .compileComponents();
+  }));
+
+  beforeEach(() => {
+    fixture = TestBed.createComponent(ItemDetailComponent);
+    component = fixture.componentInstance;
+    fixture.detectChanges();
+  });
+
+  it('should create', () => {
+    expect(component).toBeTruthy();
+  });
+});

aio/content/examples/event-binding/src/app/item-detail/item-detail.component.ts

@@ -0,0 +1,30 @@
+/* tslint:disable use-input-property-decorator use-output-property-decorator */
+import { Component, EventEmitter, Input, Output } from '@angular/core';
+
+import { Item } from '../item';
+
+@Component({
+  selector: 'app-item-detail',
+  styleUrls: ['./item-detail.component.css'],
+  templateUrl: './item-detail.component.html'
+})
+export class ItemDetailComponent {
+
+  @Input() item;
+  itemImageUrl = 'assets/teapot.svg';
+  lineThrough = '';
+  displayNone = '';
+  @Input() prefix = '';
+
+  // #docregion deleteRequest
+  // This component makes a request but it can't actually delete a hero.
+  @Output() deleteRequest = new EventEmitter<Item>();
+
+  delete() {
+    this.deleteRequest.emit(this.item.name);
+    this.displayNone = this.displayNone ? '' : 'none';
+    this.lineThrough = this.lineThrough ? '' : 'line-through';
+  }
+  // #enddocregion deleteRequest
+
+}

aio/content/examples/event-binding/src/app/item.ts

@@ -0,0 +1,4 @@
+export class Item {
+  name: '';
+}
+

aio/content/examples/event-binding/src/index.html

@@ -0,0 +1,14 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <title>EventBinding</title>
+  <base href="/">
+
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <link rel="icon" type="image/x-icon" href="favicon.ico">
+</head>
+<body>
+  <app-root></app-root>
+</body>
+</html>

aio/content/examples/event-binding/src/main.ts

@@ -0,0 +1,12 @@
+import { enableProdMode } from '@angular/core';
+import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
+
+import { AppModule } from './app/app.module';
+import { environment } from './environments/environment';
+
+if (environment.production) {
+  enableProdMode();
+}
+
+platformBrowserDynamic().bootstrapModule(AppModule)
+  .catch(err => console.log(err));

aio/content/examples/event-binding/stackblitz.json

@@ -0,0 +1,10 @@
+{
+  "description": "Event Binding",
+  "files": [
+    "!**/*.d.ts",
+    "!**/*.js",
+    "!**/*.[1,2].*"
+  ],
+  "file": "src/app/app.component.ts",
+  "tags": ["Event Binding"]
+}

aio/content/guide/template-syntax.md

@@ -797,7 +797,6 @@ content harmlessly.
 
 
 <hr/>
-
 {@a other-bindings}
 
 ## Attribute, class, and style bindings
@@ -944,56 +943,44 @@ Note that a _style property_ name can be written in either
 
 {@a event-binding}
 
-## Event binding  ( <span class="syntax">(event)</span> )
+## Event binding `(event)`
 
-The bindings directives you've met so far flow data in one direction: **from a component to an element**.
+Event binding allows you to listen for certain events such as
+keystrokes, mouse movements, clicks, and touches. For an example
+demonstrating all of the points in this section, see the <live-example name="event-binding">event binding example</live-example>.
 
-Users don't just stare at the screen. They enter text into input boxes. They pick items from lists.
-They click buttons. Such user actions may result in a flow of data in the opposite direction:
-**from an element to a component**.
-
-The only way to know about a user action is to listen for certain events such as
-keystrokes, mouse movements, clicks, and touches.
-You declare your interest in user actions through Angular event binding.
-
-Event binding syntax consists of a **target event** name
+Angular event binding syntax consists of a **target event** name
 within parentheses on the left of an equal sign, and a quoted
-[template statement](guide/template-syntax#template-statements) on the right.
+template statement on the right.
 The following event binding listens for the button's click events, calling
 the component's `onSave()` method whenever a click occurs:
 
-<code-example path="template-syntax/src/app/app.component.html" region="event-binding-1" header="src/app/app.component.html" linenums="false">
-</code-example>
+<figure>
+  <img src='generated/images/guide/event-binding/syntax-diagram.svg' alt="Syntax diagram">
+</figure>
 
 ### Target event
 
-A **name between parentheses** &mdash; for example, `(click)` &mdash;
-identifies the target event. In the following example, the target is the button's click event.
+As above, the target is the button's click event.
 
-<code-example path="template-syntax/src/app/app.component.html" region="event-binding-1" header="src/app/app.component.html" linenums="false">
+<code-example path="event-binding/src/app/app.component.html" region="event-binding-1" header="src/app/app.component.html" linenums="false">
 </code-example>
 
-Some people prefer the `on-` prefix alternative, known as the **canonical form**:
+Alternatively, use the `on-` prefix, known as the canonical form:
 
-<code-example path="template-syntax/src/app/app.component.html" region="event-binding-2" header="src/app/app.component.html" linenums="false">
+<code-example path="event-binding/src/app/app.component.html" region="event-binding-2" header="src/app/app.component.html" linenums="false">
 </code-example>
 
 Element events may be the more common targets, but Angular looks first to see if the name matches an event property
 of a known directive, as it does in the following example:
 
-<code-example path="template-syntax/src/app/app.component.html" region="event-binding-3" header="src/app/app.component.html" linenums="false">
+<code-example path="event-binding/src/app/app.component.html" region="custom-directive" header="src/app/app.component.html" linenums="false">
 </code-example>
 
-<div class="alert is-helpful">
-
-The `myClick` directive is further described in the section
-on [aliasing input/output properties](guide/template-syntax#aliasing-io).
-
-</div>
-
 If the name fails to match an element event or an output property of a known directive,
 Angular reports an “unknown directive” error.
 
+
 ### *$event* and event handling statements
 
 In an event binding, Angular sets up an event handler for the target event.
@@ -1003,72 +990,73 @@ The template statement typically involves a receiver, which performs an action
 in response to the event, such as storing a value from the HTML control
 into a model.
 
-The binding conveys information about the event, including data values, through
-an **event object named `$event`**.
+The binding conveys information about the event. This information can include data values such as an event object, string, or number named `$event`.
 
-The shape of the event object is determined by the target event.
+The target event determines the shape of the `$event` object.
 If the target event is a native DOM element event, then `$event` is a
 [DOM event object](https://developer.mozilla.org/en-US/docs/Web/Events),
 with properties such as `target` and `target.value`.
 
 Consider this example:
 
-<code-example path="template-syntax/src/app/app.component.html" region="without-NgModel" header="src/app/app.component.html" linenums="false">
+<code-example path="event-binding/src/app/app.component.html" region="event-binding-3" header="src/app/app.component.html" linenums="false">
 </code-example>
 
-This code sets the input box `value` property by binding to the `name` property.
-To listen for changes to the value, the code binds to the input box's `input` event.
+This code sets the `<input>` `value` property by binding to the `name` property.
+To listen for changes to the value, the code binds to the `input`
+event of the `<input>` element.
 When the user makes changes, the `input` event is raised, and the binding executes
 the statement within a context that includes the DOM event object, `$event`.
 
 To update the `name` property, the changed text is retrieved by following the path `$event.target.value`.
 
-If the event belongs to a directive (recall that components are directives),
-`$event` has whatever shape the directive decides to produce.
+If the event belongs to a directive&mdash;recall that components
+are directives&mdash;`$event` has whatever shape the directive produces.
 
-{@a eventemitter}
 
-{@a custom-event}
-
-### Custom events with <span class="syntax">EventEmitter</span>
+### Custom events with `EventEmitter`
 
 Directives typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter).
 The directive creates an `EventEmitter` and exposes it as a property.
 The directive calls `EventEmitter.emit(payload)` to fire an event, passing in a message payload, which can be anything.
 Parent directives listen for the event by binding to this property and accessing the payload through the `$event` object.
 
-Consider a `HeroDetailComponent` that presents hero information and responds to user actions.
-Although the `HeroDetailComponent` has a delete button it doesn't know how to delete the hero itself.
-The best it can do is raise an event reporting the user's delete request.
+Consider an `ItemDetailComponent` that presents item information and responds to user actions.
+Although the `ItemDetailComponent` has a delete button, it doesn't know how to delete the hero. It can only raise an event reporting the user's delete request.
 
-Here are the pertinent excerpts from that `HeroDetailComponent`:
+Here are the pertinent excerpts from that `ItemDetailComponent`:
 
-<code-example path="template-syntax/src/app/hero-detail.component.ts" linenums="false" header="src/app/hero-detail.component.ts (template)" region="template-1">
+
+<code-example path="event-binding/src/app/item-detail/item-detail.component.html" linenums="false" header="src/app/item-detail/item-detail.component.ts (template)" region="line-through">
 </code-example>
 
-<code-example path="template-syntax/src/app/hero-detail.component.ts" linenums="false" header="src/app/hero-detail.component.ts (deleteRequest)" region="deleteRequest">
+<code-example path="event-binding/src/app/item-detail/item-detail.component.ts" linenums="false" header="src/app/item-detail/item-detail.component.ts (deleteRequest)" region="deleteRequest">
 </code-example>
 
+
 The component defines a `deleteRequest` property that returns an `EventEmitter`.
 When the user clicks *delete*, the component invokes the `delete()` method,
-telling the `EventEmitter` to emit a `Hero` object.
+telling the `EventEmitter` to emit an `Item` object.
 
-Now imagine a hosting parent component that binds to the `HeroDetailComponent`'s `deleteRequest` event.
+Now imagine a hosting parent component that binds to the `deleteRequest` event
+of the `ItemDetailComponent`.
 
-<code-example path="template-syntax/src/app/app.component.html" linenums="false" header="src/app/app.component.html (event-binding-to-component)" region="event-binding-to-component">
+<code-example path="event-binding/src/app/app.component.html" linenums="false" header="src/app/app.component.html (event-binding-to-component)" region="event-binding-to-component">
 </code-example>
 
-When the `deleteRequest` event fires, Angular calls the parent component's `deleteHero` method,
-passing the *hero-to-delete* (emitted by `HeroDetail`) in the `$event` variable.
+When the `deleteRequest` event fires, Angular calls the parent component's
+`deleteItem()` method, passing the *item-to-delete* (emitted by `ItemDetail`)
+in the `$event` variable.
 
 ### Template statements have side effects
 
-The `deleteHero` method has a side effect: it deletes a hero.
-Template statement side effects are not just OK, but expected.
+Though [template expressions](guide/template-syntax#template-expressions) shouldn't have [side effects](guide/template-syntax#avoid-side-effects), template
+statements usually do. The `deleteItem()` method does have
+a side effect: it deletes an item.
 
-Deleting the hero updates the model, perhaps triggering other changes
-including queries and saves to a remote server.
-These changes percolate through the system and are ultimately displayed in this and other views.
+Deleting an item updates the model, and depending on your code, triggers
+other changes including queries and saving to a remote server.
+These changes propagate through the system and ultimately display in this and other views.
 
 
 <hr/>

aio/content/images/guide/event-binding/syntax-diagram.svg

@@ -0,0 +1 @@
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