EventTarget的简单实现

mdn中EventTarget的简单实现代码

let EventTarget = function () {
    this.listeners = {};
}

EventTarget.prototype.listeners = null;

EventTarget.prototype.addEventListener = function (type, callback) {
    if (!(type in this.listeners)) {
        this.listeners[type] = [];
    }
    this.listeners[type].push(callback);
};

EventTarget.prototype.removeEventListener = function (type, callback) {
    if (!(type in this.listeners)) {
        return;
    }
    let stack = this.listeners[type];
    for (let i = 0, l = stack.length; i < l; i++) {
        if (stack[i] === callback) {
            stack.splice(i, 1);
            return this.removeEventListener(type, callback);
        }
    }
};

EventTarget.prototype.dispatchEvent = function (event) {
    if (!(event.type in this.listeners)) {
        return;
    }
    let stack = this.listeners[event.type];
    event.target = this;
    for (let i = 0, l = stack.length; i < l; i++) {
        stack[i].call(this, event);
    }
};