Angular 2 how to keep event from triggering digest loop/detection cycle?

Question

We are implementing drag and drop functionality with Angular 2.

I\'m using the dragover event just to run the preventDefault() function. S

Answer 1

You can detach the change detector to prevent change detection to be invoked for a component

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

Answer 2

1) One interesting solution might be overriding EventManager

custom-event-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 {

Usage:

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

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

Plunker Example

So with solution above you can use one of these options to prevent default behavior and run event outside angular zone:

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

2) One more solution provided by Rob Wormald is 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

Update:

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

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

Follow

• https://github.com/angular/angular/issues/19989

• https://github.com/angular/angular/commit/e82812b9a9992bc275bba54575f9d780753c1f8f

• https://github.com/angular/angular/pull/21681

Update 2

Angular cli includes template for disabling parts of macroTask/DomEvents patch.

Just open

polyfills.ts

You can find there the following code

/**
 * 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

See also:

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