diff --git a/aio/content/guide/event-binding.md b/aio/content/guide/event-binding.md index 092ceca65b6..cbaf4fd76c2 100644 --- a/aio/content/guide/event-binding.md +++ b/aio/content/guide/event-binding.md @@ -8,91 +8,62 @@ See the for a working example containing the code +## Prerequisites + +* [Basics of components](guide/architecture-components) +* [Basics of templates](guide/glossary#template) +* [Binding syntax](guide/binding-syntax) +* [Template statements](guide/template-statements) + ## Binding to events To bind to an event you use the Angular event binding syntax. This syntax consists of a target event name within parentheses to the left of an equal sign, and a quoted template statement to the right. -In the following example, the target event name is `click` and the template statement is `onSave()`. + +Create the following example; the target event name is `click` and the template statement is `onSave()`. - <button (click)="onSave()">Save</button> - The event binding listens for the button's click events and calls the component's `onSave()` method whenever a click occurs.
- Syntax diagram + Syntax diagram
-## Binding to passive events - -Angular also supports passive event listeners. -For example, use the following steps to make a scroll event passive. - -1. Create a file `zone-flags.ts` under `src` directory. -1. Add the following line into this file. - - - - (window as any)['__zone_symbol__PASSIVE_EVENTS'] = ['scroll']; - - - -1. In the `src/polyfills.ts` file, before importing `zone.js`, import the newly created `zone-flags`. - - - - import './zone-flags'; - import 'zone.js'; // Included with Angular CLI. - - - -After those steps, if you add event listeners for the `scroll` event, the listeners will be `passive`. - -## Custom events with `EventEmitter` - -[Directives](guide/built-in-directives) typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter) as follows. - -1. The directive creates an `EventEmitter` and exposes it as a property. -1. The directive then calls `EventEmitter.emit(data)` to emit an event, passing in message data, which can be anything. -1. Parent directives listen for the event by binding to this property and accessing the data through the `$event` object. - -Consider an `ItemDetailComponent` that presents item information and responds to user actions. -Although the `ItemDetailComponent` has a delete button, it doesn't contain the functionality to delete the hero. -It can only raise an event reporting the user's delete request. - - - -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 an `Item` object. - - - -The hosting parent component binds to the `deleteRequest` event of the `ItemDetailComponent` as follows. - - - -When the `deleteRequest` event fires, Angular calls the parent component's `deleteItem()` method with the item. - ### Determining an event target To determine an event target, Angular checks if the name of the target event matches an event property of a known directive. -In the following example, Angular checks to see if `myClick` is an event on the custom `ClickDirective`. - +Create the following example: (Angular checks to see if `myClick` is an event on the custom `ClickDirective`) -If the target event name, `myClick` fails to match an element event or an output property of `ClickDirective`, Angular reports an "unknown directive" error. + + +If the target event name, `myClick` fails to match an output property of `ClickDirective`, Angular will instead bind to the `myClick` event on the underlying DOM element. + +## Binding to passive events + +This is an advanced technique that is not necessary for most applications. You may find this useful if you want to optimize frequently occurring events that are causing performance problems. + +Angular also supports passive event listeners. For example, use the following steps to make a scroll event passive. + +1. Create a file `zone-flags.ts` under `src` directory. +2. Add the following line into this file. +3. In the `src/polyfills.ts` file, before importing zone.js, import the newly created `zone-flags`. + +``` +import './zone-flags'; +import 'zone.js'; // Included with Angular CLI. +``` + +After those steps, if you add event listeners for the `scroll` event, the listeners will be `passive`. ## What's next -For more information on how event binding works, see [How event binding works](guide/event-binding-concepts). +* For more information on how event binding works, see [How event binding works](guide/event-binding-concepts). +* [Property binding](guide/property-binding) +* [Text interpolation](guide/interpolation) +* [Two-way binding](guide/two-way-binding) - - - - - - -@reviewed 2022-02-28 +@reviewed 2022-05-10