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docs: add section on binding to passive events (#42292) 

fixes #41622

PR Close #42292
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David Shevitz 2021-05-24 22:33:44 +00:00 committed by Zach Arend
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aio/content/guide/event-binding.md
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@ -24,6 +24,26 @@ The event binding listens for the button's click events and calls the component'
<img src='generated/images/guide/template-syntax/syntax-diagram.svg' alt="Syntax diagram">
</div>
## Binding to passive events
Angular also supports passive event listeners. For example, you can 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.
```
(window as any)['__zone_symbol__PASSIVE_EVENTS'] = ['scroll'];
```
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`.
## Custom events with `EventEmitter`
[Directives](guide/built-in-directives) typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter) as follows.