Angular 2 如何防止事件触发摘要loop/detection 循环？

Question

我们正在使用 Angular 2 实现拖放功能。

我正在使用 dragover 事件来运行 preventDefault() 函数。这样 drop 事件就如 this question 中所解释的那样工作。

组件中的 onDragOver 函数正在处理 dragover 方法。

<div draggable="true"
    (dragover)="onDragOver($event)">
...

在组件中，此函数可防止默认行为允许将拖动的项目放在此目标上。

onDragOver(event) {
    event.preventDefault();
}

这按预期工作。 dragover 事件每隔几百毫秒就会触发一次。

但是，每次 onDragOver 函数被调用时，Angular 2 运行s 它的摘要周期。这会减慢应用程序。我想运行这个函数而不触发摘要循环。

我们为此使用的解决方法是订阅元素事件，然后运行在 Angular 2 的上下文之外对其进行如下操作：

constructor( ele: ElementRef, private ngZone: NgZone ) {
    this.ngZone.runOutsideAngular( () => {
        Observable.fromEvent(ele.nativeElement, "dragover")
            .subscribe( (event: Event) => {
                event.preventDefault();
            }
        );
    });
}

这很好用。但是有没有一种方法可以在不必直接访问 nativeElement 的情况下实现这一点？

Answer 1

您可以分离更改检测器以防止为组件调用更改检测

constructor(private cdRef:ChangeDetectorRef) {}
foo() {
  this.cdRef.detach();
  ...
  this.cdRef.attach();
}

Answer 2

1) 一个有趣的解决方案可能是覆盖 EventManager

自定义事件-manager.ts

import { Injectable, Inject, NgZone  } from '@angular/core';
import { EVENT_MANAGER_PLUGINS, EventManager } from '@angular/platform-browser';

@Injectable()
export class CustomEventManager extends EventManager {
  constructor(@Inject(EVENT_MANAGER_PLUGINS) plugins: any[], private zone: NgZone) {
    super(plugins, zone); 
  }

  addEventListener(element: HTMLElement, eventName: string, handler: Function): Function {
    if(eventName.endsWith('out-zone')) {
      eventName = eventName.split('.')[0];
      return this.zone.runOutsideAngular(() => 
          super.addEventListener(element, eventName, handler));
    } 

    return super.addEventListener(element, eventName, handler);
  }
}

app.module.ts

  ...
  providers: [
    { provide: EventManager, useClass: CustomEventManager }
  ]
})
export class AppModule {

用法：

<h1 (click.out-zone)="test()">Click outside ng zone</h1>

<div (dragover.out-zone)="onDragOver($event)">

Plunker Example

因此，通过上述解决方案，您可以使用这些选项之一来防止默认行为和运行事件在 angular 区域之外：

(dragover.out-zone)="$event.preventDefault()"
(dragover.out-zone)="false"
(dragover.out-zone)="!!0"

2) Rob Wormald 提供的另一种解决方案是 using blacklist for Zonejs

blacklist.ts

/// <reference types='zone.js/dist/zone.js' />

const BLACKLISTED_ZONE_EVENTS: string[] = [
  'addEventListener:mouseenter',
  'addEventListener:mouseleave',
  'addEventListener:mousemove',
  'addEventListener:mouseout',
  'addEventListener:mouseover',
  'addEventListener:mousewheel',
  'addEventListener:scroll',
  'requestAnimationFrame',
];

export const blacklistZone = Zone.current.fork({
  name: 'blacklist',
  onScheduleTask: (delegate: ZoneDelegate, current: Zone, target: Zone,
                   task: Task): Task => {

    // Blacklist scroll, mouse, and request animation frame events.
    if (task.type === 'eventTask' &&
        BLACKLISTED_ZONE_EVENTS.some(
            (name) => task.source.indexOf(name) > -1)) {
      task.cancelScheduleRequest();

      // Schedule task in root zone, note Zone.root != target,
      // "target" Zone is Angular. Scheduling a task within Zone.root will
      // prevent the infinite digest cycle from appearing.
      return Zone.root.scheduleTask(task);
    } else {
      return delegate.scheduleTask(target, task);
    }
  }
});

main.ts

import {blacklistZone} from './blacklist'

blacklistZone.run(() => {
  platformBrowser().bootstrapModuleFactory(...)
})

Plunker with blacklist

更新：

5.0.0-beta.7 (2017-09-13)

fix(platform-browser): run BLACK_LISTED_EVENTS outside of ngZone

关注

更新 2

Angular cli 包含用于禁用部分 macroTask/DomEvents 补丁的模板。

刚打开

polyfills.ts

您可以在那里找到以下代码

/**
 * By default, zone.js will patch all possible macroTask and DomEvents
 * user can disable parts of macroTask/DomEvents patch by setting following flags
 */

 // (window as any).__Zone_disable_requestAnimationFrame = true; // disable patch requestAnimationFrame
 // (window as any).__Zone_disable_on_property = true; // disable patch onProperty such as onclick
 // (window as any).__zone_symbol__BLACK_LISTED_EVENTS = ['scroll', 'mousemove']; // disable patch specified eventNames
 /*
 * in IE/Edge developer tools, the addEventListener will also be wrapped by zone.js
 * with the following flag, it will bypass `zone.js` patch for IE/Edge
 */
// (window as any).__Zone_enable_cross_context_check = true;

https://github.com/angular/devkit/blob/8651a94380eccef0e77b509ee9d2fff4030fbfc2/packages/schematics/angular/application/files/sourcedir/polyfills.ts#L55-L68

另请参阅：

https://angular.io/guide/browser-support#polyfills-for-non-cli-users

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