Javascript/原型范围混淆

我正在创建一个JavaScript类(使用Prototype),如果在指定的秒数内没有鼠标移动,它会将页面状态设置为空闲。当鼠标移动时,类将通过向侦听器列表发送消息来“唤醒”页面。

this.handlers setIdle ),但不是另一个( setActive )。下面的注释代码说明了我的问题:

var IM2 = Class.create({

handlers: null,

initialize: function(callback, frequency) {
    this.handlers = [];
    Event.observe(document, "mousemove", this.sendActiveSignal);
    Event.observe(document, "keypress", this.sendActiveSignal);
    setInterval(this.sendIdleSignal.bind(this), 5000);
},

addListener: function(h) {
    console.log(this.handlers.size());  // it's 0 here, as expected
    this.handlers.push(h);
    console.log(this.handlers.size());  // it's 1 here, as expected
},

sendIdleSignal: function(args) {
    console.log("IDLE");
    this.handlers.each(function(i){
        i.setIdle();
    })
},

sendActiveSignal: function() {
                                    // this.handlers is undefined here. Why?
    this.handlers.each(function(r) {
        r.setActive();
    })
}

});