只处理一次事件

¥Handling events only once

当使用 eventEmitter.on() 方法注册监听器时，每次触发命名事件时都会调用该监听器。

¥When a listener is registered using the eventEmitter.on() method, that listener is invoked every time the named event is emitted.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2 拷贝

使用 eventEmitter.once() 方法，可以注册一个监听器，该监听器最多为特定事件调用一次。事件触发后，监听器将被取消注册，然后被调用。

¥Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored 拷贝