7

在 Angular 中如何为同一个表达式绑定多个事件呢?如果我们这样做可能会是这样的:

<div>
   <button (click, mouseover)="onClick()">Click me</button>
</div>

在继续分析绑定多个事件之前,我们先来分析一下,如果在模板中绑定一个事件如 click 事件,Angular 是如何工作的?

<div>
   <button (click)="onClick()">Click me</button>
</div>

Angular 在解析 DOM 树的时候,对于事件绑定它会调用 DomRenderer 实例的 listen() 方法,进行事件绑定,listen() 方法具体实现如下:

// angular2/packages/platform-browser/src/dom/dom_renderer.ts
class DefaultDomRenderer2 implements Renderer2 {
    ....
    listen(target: 'window'|'document'|'body'|any, event: string, 
      callback: (event: any) => boolean):
          () => void {
        checkNoSyntheticProp(event, 'listener');
        if (typeof target === 'string') {
          return <() => void>this.eventManager.addGlobalEventListener(
              target, event, decoratePreventDefault(callback));
        }
        return <() => void>this.eventManager.addEventListener(
                   target, event, decoratePreventDefault(callback)) as() => void;
    }
}

通过源码我们发现,不管走哪条分支,最终都是调用 this.eventManager 对象的方法设置事件监听。这里的 this.eventManager 是什么?它是 Angular 中的事件管理器 EventManager,我们先来会会它。

EventManager (事件管理器)

在 Angular 中所有的事件绑定都是由一个事件管理器来驱动,事件管理器本身由多个事件插件提供支持。Angular 中内置的事件插件如下:

  • KeyEventsPlugin - 处理键盘事件

  • HammerGesturesPlugin - 处理手势

  • DomEventsPlugin - 处理 DOM 事件

看完上面的内容,相信很多人也会有疑问 - EventManager 到底是如何管理不同事件的呢?要揭开这背后的秘密,我们的唯一途径就是看源码,因为它是最诚实的,它对你毫无保留,此刻脑海中突然想起一首歌:

美丽的神话

解开我 最神秘的等待
星星坠落 风在吹动
终于再将你拥入怀中
….

爱是心中唯一不变美丽的神话

放松一下,马上回到正题 - EventManager 类:

EventManager 类

// angular2/packages/platform-browser/src/dom/events/event_manager.ts
export class EventManager {
  // EventManagerPlugin列表
  private _plugins: EventManagerPlugin[]; 
  // 缓存已匹配的eventName与对应的插件
  private _eventNameToPlugin = new Map<string, EventManagerPlugin>();

  constructor(
    @Inject(EVENT_MANAGER_PLUGINS) plugins: EventManagerPlugin[], 
    private _zone: NgZone) {
        plugins.forEach(p => p.manager = this);
        /**
         * {provide: EVENT_MANAGER_PLUGINS, useClass: DomEventsPlugin, multi: true},
         * {provide: EVENT_MANAGER_PLUGINS, useClass: KeyEventsPlugin, multi: true},
         * {provide: EVENT_MANAGER_PLUGINS, useClass: HammerGesturesPlugin, multi: true}
         * 
         * slice(): 创建新的plugins数组
         * reverse(): 让DomEventsPlugin插件作为列表最后一项,因为它能够处理所有的事件。
        */
        this._plugins = plugins.slice().reverse();
  }

  // 获取能处理eventName的插件,并调用对应插件提供的addEventListener()方法
  addEventListener(element: HTMLElement, eventName: string,
    handler: Function): Function {
        const plugin = this._findPluginFor(eventName);
        return plugin.addEventListener(element, eventName, handler);
  }

  // 获取能处理eventName的插件,并调用对应插件提供的addGlobalEventListener()方法
  addGlobalEventListener(target: string, eventName: string, 
    handler: Function): Function {
        const plugin = this._findPluginFor(eventName);
        return plugin.addGlobalEventListener(target, eventName, handler);
  }

  // 获取NgZone
  getZone(): NgZone { return this._zone; }

  /** @internal */
  _findPluginFor(eventName: string): EventManagerPlugin {
    // 优先从_eventNameToPlugin对象中获取eventName对应的EventManagerPlugin
    const plugin = this._eventNameToPlugin.get(eventName);  
    if (plugin) {
      return plugin;
    }

    // 遍历插件列表,判断当前插件是否支持eventName对应的事件名
    const plugins = this._plugins;
    for (let i = 0; i < plugins.length; i++) {
      const plugin = plugins[i];
      if (plugin.supports(eventName)) {
        this._eventNameToPlugin.set(eventName, plugin);
        return plugin;
      }
    }
    throw new Error(`No event manager plugin found for event ${eventName}`);
  }
}

相关说明

  • 在 addEventListener() 或 addGlobalEventListener() 方法内部都会调用 _findPluginFor() 方法,查询对应的能够处理 eventName 对应的 EventManagerPlugin 插件对象。

  • _findPluginFor() 方法中,会遍历插件列表,然后以 eventName 作为参数调用插件对象提供的 supports() 方法,判断当前是否能够处理 eventName 对应的事件。因此对于 EventManagerPlugin 插件对象,如果要声明能够处理某类事件,就需要在 supports() 方法中进行相应处理。

  • DomEventsPlugin 插件作为列表最后一项,因为它能够处理所有的事件。

  • KeyEventsPlugin、HammerGesturesPlugin、DomEventsPlugin 插件类都继承于 EventManagerPlugin 抽象类。

EventManagerPlugin 抽象类

export abstract class EventManagerPlugin {
  constructor(private _doc: any) {}

  manager: EventManager;

  // 判断是否支持eventName对应的事件
  abstract supports(eventName: string): boolean;

  // 添加事件监听
  abstract addEventListener(element: HTMLElement, eventName: string, 
    handler: Function): Function;

  // 添加全局的事件监听
  addGlobalEventListener(element: string, eventName: string, 
    handler: Function): Function {
      const target: HTMLElement = getDOM().getGlobalEventTarget(this._doc, element);
       if (!target) {
           throw new Error(`Unsupported event target ${target} for event 
            ${eventName}`);
       }
       return this.addEventListener(target, eventName, handler);
  };
}

图片描述

时机已成熟,接下来我们开始实现上述的功能。

自定义插件

Step 1: Creating a new plugin

正如上面提到的,我们希望在我们的 Angular 模板上有多个事件绑定到同一个表达式:

<div>
   <button (click, mouseover)="onClick()">Click me</button>
</div>

如果是这样,我们的 supports() 函数的内部规则应该很清楚。我们需要一个字符串,其中有一个或多个逗号,分隔事件名称。当人们把一些愚蠢的东西放在(,click)中时,我们也应该处理。所以我们的 supports() 函数如下:

getMultiEventArray(eventName: string): string[] {
  return eventName.split(",")
    .filter((item, index): boolean => { return item && item != '' })
}

supports(eventName: string): boolean {
  return this.getMultiEventArray(eventName).length > 1
}

这将允许 EventManager 将事件字符串如 (click, mouseover) 委派给此插件。

Step 2: Implementing the eventListeners

现在我们已经实现了supports() 方法,EventManager 将调用 plugin.addEventListener() 方法,因此插件需要实现 addEventListener() 方法,从而实现我们的自定义行为。我们的自定义行为很简单 - 为我们解析的eventArray 中的所有事件添加事件侦听器。

addEventListener

addEventListener(element: HTMLElement, eventName: string, handler: Function): Function {
        let zone = this.manager.getZone();
        let eventsArray = this.getMultiEventArray(eventName);

        // Entering back into angular to trigger changeDetection
        let outsideHandler = (event: any) => {
            zone.runGuarded(() => handler(event));
        };

        // Executed outside of angular so that change detection is not 
        // constantly triggered.
        let addAndRemoveHostListenersForOutsideEvents = () => {
            eventsArray.forEach((singleEventName: string) => {
                this.manager.addEventListener(element, singleEventName, outsideHandler);
            });
        }

        return this.manager.getZone()
                   .runOutsideAngular(addAndRemoveHostListenersForOutsideEvents);
    }

addGlobalEventListener

 addGlobalEventListener(target: string, eventName: string, handler: Function): Function {
        let zone = this.manager.getZone();
        let eventsArray = this.getMultiEventArray(eventName);
        let outsideHandler = (event: any) => zone.runGuarded(() => handler(event));

        return this.manager.getZone().runOutsideAngular(() => {
            eventsArray.forEach((singleEventName: string) => {
                this.manager.addGlobalEventListener(target, singleEventName, 
                    outsideHandler);
            })
        });
}

Step 3: Register plugin

import { EVENT_MANAGER_PLUGINS } from '@angular/platform-browser';

@NgModule({
  ...
  providers: [
    { provide: EVENT_MANAGER_PLUGINS, useClass: MultiEventPlugin, multi: true }
  ]
})
export class AppModule { }

完整示例

multi-event.plugin.ts

import { Injectable, Inject } from '@angular/core';
import { EventManager, DOCUMENT, ɵd as EventManagerPlugin } from '@angular/platform-browser';

/**
 * Support Multi Event
 */
@Injectable()
export class MultiEventPlugin extends EventManagerPlugin {
    manager: EventManager;

    constructor( @Inject(DOCUMENT) doc: any) { super(doc); }

    getMultiEventArray(eventName: string): string[] { 
        return eventName.split(",")   // click,mouseover => [click,mouseover]
            .filter((item, index): boolean => { return item && item != '' })
    }

    supports(eventName: string): boolean {
        return this.getMultiEventArray(eventName).length > 1;
    }

    addEventListener(element: HTMLElement, eventName: string, 
      handler: Function): Function {
        let zone = this.manager.getZone();
        let eventsArray = this.getMultiEventArray(eventName);

        // Entering back into angular to trigger changeDetection
        let outsideHandler = (event: any) => {
            zone.runGuarded(() => handler(event));
        };

        // Executed outside of angular so that change detection is
        // not constantly triggered.
        let addAndRemoveHostListenersForOutsideEvents = () => {
            eventsArray.forEach((singleEventName: string) => {
                this.manager.addEventListener(element, singleEventName, outsideHandler);
            });
        }

        return this.manager.getZone()
                .runOutsideAngular(addAndRemoveHostListenersForOutsideEvents);
    }

    addGlobalEventListener(target: string, eventName: string, 
      handler: Function): Function {
        let zone = this.manager.getZone();
        let eventsArray = this.getMultiEventArray(eventName);
        let outsideHandler = (event: any) => zone.runGuarded(() => handler(event));

        return this.manager.getZone().runOutsideAngular(() => {
            eventsArray.forEach((singleEventName: string) => {
                this.manager.addGlobalEventListener(target, singleEventName, 
                    outsideHandler);
            });
        });
    }
}

app.component.ts

import { Component } from '@angular/core';

@Component({
  selector: 'exe-app',
  template: `
    <div>
      <button (click,mouseover)="onClick()">Click me</button>
    </div>
  `
})
export class AppComponent {
  onClick() {
    console.log('Click');
  }
}

app.module.ts

import { NgModule, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
import { EVENT_MANAGER_PLUGINS } from '@angular/platform-browser';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule } from '@angular/forms';

import { AppComponent } from './app.component';
import { MultiEventPlugin } from './plugins/multi-event.plugin';

@NgModule({
  imports: [BrowserModule],
  declarations: [AppComponent],
  bootstrap: [AppComponent],
  providers: [
    { provide: EVENT_MANAGER_PLUGINS, useClass: MultiEventPlugin, multi: true }
  ],
  schemas: [CUSTOM_ELEMENTS_SCHEMA]
})
export class AppModule { }

参考资源


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