移动端触摸、点击事件优化（fastclick源码学习）

摘要：移动端触摸点击事件优化源码学习最近在做一些微信移动端的页面，在此记录关于移动端触摸和点击事件的学习优化过程，主要内容围绕展开。当指针设备通常指鼠标在元素上移动时事件被触发。移动端有延迟问题，可没有哦。

最近在做一些微信移动端的页面，在此记录关于移动端触摸和点击事件的学习优化过程，主要内容围绕fastclick展开。
fastclick github

移动端浏览器一般在用户点击屏幕之后会延迟大约300ms才触发click event
——GOOGLE

手机打开此链接查看延迟demo
（现在许多浏览器已经不存在延迟问题了，详见fastclick github，但笔者的手机浏览器还是出现了三百毫秒延迟的问题）
截图如下

为什么会300ms延迟呢，主要是有一个双击缩放功能，浏览器需要判断用户点击是否为双击缩放。这个问题不解决，
1、用户体验就会很差，很不流畅，尤其是在密集操作场景下，比如计算器,不解决300ms延迟问题，感觉反应很慢；
2、点击穿透问题

在了解fastclick的思路之前，我们先看一下事件触发顺序是怎样的

touchstart

touchmove

touchend

mouseover ：当指针设备移动到存在监听器的元素或其子元素的时候，mouseover事件就会被触发。

mouseenter：当指针设备( 通常指鼠标 )在元素上移动时, mousemove 事件被触发。

mousedown

click

移动端click有300ms延迟问题，touch可没有哦。

fastclick的思路就是利用touch来模拟tap（触碰），如果认为是一次有效的tap，则在touchend时立即模拟一个click事件，分发到事件源（相当于主动触发一次click），同时阻止掉浏览器300ms后产生的click。

先看使用示例，很简单，我们的思路就一直跟着attach走。

if ("addEventListener" in document) {
    document.addEventListener("DOMContentLoaded", function() {
        FastClick.attach(document.body);
    }, false);
}

直接给body绑定fastlick就行了- -。
看源代码结构（注：以下所有代码均去掉了一些不影响理解思路的部分，大部分思路写在注释中）

//构造函数
    function FastClick(layer, options)
//判断是否需要浏览器原生的click事件（针对一些特殊元素比如表单）
    FastClick.prototype.needsClick = function(target)
//发送模拟的click event
    FastClick.prototype.sendClick = function(targetElement, event)
// touchstart eventhandler
    FastClick.prototype.onTouchStart = function(event)
// touchmove eventhandler
    FastClick.prototype.onTouchMove = function(event)
// touchend eventhandler
    FastClick.prototype.onTouchEnd = function(event)
// 判断这次tap是否有效
    FastClick.prototype.onMouse = function(event) 
//click handler 捕获阶段监听
    FastClick.prototype.onClick = function(event)
//销毁fastlick，移除事件绑定
    FastClick.prototype.destroy = function()
//绑定接口
    FastClick.attach = function(layer, options) {
        return new FastClick(layer, options);
    };

attach实际就执行了构造函数进行初始化，接下来我们来看构造函数发生了什么

    function FastClick(layer,options){
        //一些属性初始化
        //安卓一些老版本浏览器不支持bind， poly fill
        function bind (method, context) {
          return function () {
            return method.apply(context, arguments);
          };
        }
        var methods = ["onMouse", "onClick", "onTouchStart", "onTouchMove", 
        "onTouchEnd", "onTouchCancel"];
        var context = this;
        //将所有handler的this绑定到fastclick实例
        for (var i = 0, l = methods.length; i < l; i++) {
            context[methods[i]] = bind(context[methods[i]], context);
        }
        //为当前fast click对象绑定的layer（我们的示例中时document.body）加监听
        layer.addEventListener("click", this.onClick, true);//true 捕获阶段触发 
        layer.addEventListener("touchstart", this.onTouchStart, false);
        layer.addEventListener("touchmove", this.onTouchMove, false);
        layer.addEventListener("touchend", this.onTouchEnd, false);
        layer.addEventListener("touchcancel", this.onTouchCancel, false);
    }

构造函数主要是初始化一些属性，polyfill，和添加监听，
下面开始看一下重头戏，touchstart，touchend是如何判断tap是否有效、如何模拟click事件、如何阻止300ms后的click
touchstart

  FastClick.prototype.onTouchStart = function (event) {
    var targetElement, touch, selection;

    // Ignore multiple touches, otherwise pinch-to-zoom is prevented if both fingers are on the FastClick element (issue #111).
    // 如果多触点可能是在缩放，不对targetElement初始化，在此提前终止避免误模拟产生click
    if (event.targetTouches.length > 1) {
      return true;
    }

    //获取发生事件源元素（目标阶段的元素）
    targetElement = this.getTargetElementFromEventTarget(event.target);
    touch = event.targetTouches[0];
    
    this.trackingClick = true;//标记开始跟踪click
    this.trackingClickStart = event.timeStamp;//开始跟踪时间
    this.targetElement = targetElement;//事件源元素

    //触摸坐标，接下来判断是否越界用到
    this.touchStartX = touch.pageX;
    this.touchStartY = touch.pageY;

    // Prevent phantom clicks on fast double-tap (issue #36)
    if ((event.timeStamp - this.lastClickTime) < this.tapDelay) {
      event.preventDefault();//阻止之后的click
    }

    return true;
  };

touchstart主要是初始化跟踪的tap相关的一些属性，用于之后的判断‘
接下来touchmove

 FastClick.prototype.onTouchMove = function (event) {
    if (!this.trackingClick) {
      return true;
    }

    // If the touch has moved, cancel the click tracking 移动到了其他元素
    if (this.targetElement !== this.getTargetElementFromEventTarget(event.target) || this.touchHasMoved(event)) {//移动越界了，取消本次click模拟处理，走原生流程
      this.trackingClick = false;
      this.targetElement = null;
    }

    return true;
  };

touchmove比较简单，主要是兼容滑动tap（swiper）等等，滑动越界则不模拟click
下面是touchend

FastClick.prototype.onTouchEnd = function (event) {
    var forElement, trackingClickStart, targetTagName, scrollParent, touch, targetElement = this.targetElement;

    if (!this.trackingClick) {
      return true;
    }

    // Prevent phantom clicks on fast double-tap (issue #36)
    //阻止快速双击
    if ((event.timeStamp - this.lastClickTime) < this.tapDelay) {
      this.cancelNextClick = true;
      return true;
    }
    //超时就不算click了，走原生流程，不阻止click
    if ((event.timeStamp - this.trackingClickStart) > this.tapTimeout) {
      return true;
    }

    this.lastClickTime = event.timeStamp;

    this.trackingClick = false;
    this.trackingClickStart = 0;



    // Prevent the actual click from going though - unless the target node is marked as requiring
    // real clicks or if it is in the whitelist in which case only non-programmatic clicks are permitted.
    if (!this.needsClick(targetElement)) {
      event.preventDefault();//阻止之后的click
      this.sendClick(targetElement, event);//发送模拟click
    }

    return false;
  };
 //发送模拟的click event
  FastClick.prototype.sendClick = function (targetElement, event) {
    var clickEvent, touch;

    // On some Android devices activeElement needs to be blurred otherwise the synthetic click will have no effect (#24)
    if (document.activeElement && document.activeElement !== targetElement) {
      document.activeElement.blur();
    }

    touch = event.changedTouches[0];

    //模拟click
    // Synthesise a click event, with an extra attribute so it can be tracked
    clickEvent = document.createEvent("MouseEvents");
    clickEvent.initMouseEvent(this.determineEventType(targetElement), true, true, window, 1, touch.screenX, touch.screenY, touch.clientX, touch.clientY, false, false, false, false, 0, null);
    clickEvent.forwardedTouchEvent = true;
    //向targetElement分发模拟的click
    targetElement.dispatchEvent(clickEvent);
  };

最后，还会在layer的click捕获阶段监听

  //click handler 捕获阶段监听
  FastClick.prototype.onClick = function (event) {
    var permitted;
    // It"s possible for another FastClick-like library delivered with third-party code to fire a click event before FastClick does (issue #44). In that case, set the click-tracking flag back to false and return early. This will cause onTouchEnd to return early.
    if (this.trackingClick) {//1、出界会置为false，2成功模拟了一次完成tap并阻止click也会置为false，3、避免三方库影响
      this.targetElement = null;
      this.trackingClick = false;
      return true;
    }

    // Very odd behaviour on iOS (issue #18): if a submit element is present inside a form and the user hits enter in the iOS simulator or clicks the Go button on the pop-up OS keyboard the a kind of "fake" click event will be triggered with the submit-type input element as the target.
    if (event.target.type === "submit" && event.detail === 0) {
      return true;
    }

    permitted = this.onMouse(event);

    // Only unset targetElement if the click is not permitted. This will ensure that the check for !targetElement in onMouse fails and the browser"s click doesn"t go through.
    if (!permitted) {
      this.targetElement = null;
    }

    // If clicks are permitted, return true for the action to go through.
    return permitted;
  };

 // 判断这次鼠标是否有效
  FastClick.prototype.onMouse = function (event) {

    // If a target element was never set (because a touch event was never fired) allow the event
    if (!this.targetElement) {
      return true;
    }

    // 标记fastclick模拟产生的event
    if (event.forwardedTouchEvent) {
      return true;
    }

    // Programmatically generated events targeting a specific element should be permitted
    if (!event.cancelable) {
      return true;
    }

    // Derive and check the target element to see whether the mouse event needs to be permitted;
    // unless explicitly enabled, prevent non-touch click events from triggering actions,
    // to prevent ghost/doubleclicks.
    // 是否需要原生的click
    if (!this.needsClick(this.targetElement) || this.cancelNextClick) {

      // Prevent any user-added listeners declared on FastClick element from being fired.
      if (event.stopImmediatePropagation) {
        event.stopImmediatePropagation();
      } else {

        // Part of the hack for browsers that don"t support Event#stopImmediatePropagation (e.g. Android 2)
        event.propagationStopped = true;
      }

      // Cancel the event 阻止事件捕获和冒泡
      event.stopPropagation();
      event.preventDefault();

      return false;
    }

    // If the mouse event is permitted, return true for the action to go through.
    return true;
  };

这里主要是判断这次click是否有效（如无效，则阻止捕获和冒泡）
至此基本流程已经结束。
其中有1个注意的点，笔者在chrome(Version 64.0.3282.119 (Official Build) (64-bit))已测试
stopPropagation，stopImmediatePropagation不仅会阻止冒泡还会阻止捕获过程哦。

推荐阅读源码，源码中有许多关于focus、不同浏览器兼容和特殊表单元素的处理fastclick github。
这里是笔者带有中文注释的代码中文注释代码。
如有纰漏，欢迎批评指正。

MDN
https://juejin.im/entry/55d73...