Angular 4.x 事件管理器及自定义EventManagerPlugin

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>复制代码

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>复制代码

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;}
}复制代码

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NgZonegetZone(): NgZone { return this._zone; }/** @internal */_findPluginFor(eventName: string): EventManagerPlugin {// 优先从_eventNameToPlugin对象中获取eventName对应的EventManagerPluginconst 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}`);}
}复制代码

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);};
}复制代码

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>复制代码

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
}复制代码

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 changeDetectionlet 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);}复制代码

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);})});
}复制代码

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 { }复制代码

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 changeDetectionlet 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);});});}
}复制代码

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');}
}复制代码

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 { }复制代码

参考资源

Hacking Angular2: Binding Multiple DOM Events