摘要：移動端觸摸點擊事件優化源碼學習最近在做一些微信移動端的頁面，在此記錄關于移動端觸摸和點擊事件的學習優化過程，主要內容圍繞展開。當指針設備通常指鼠標在元素上移動時事件被觸發。移動端有延遲問題，可沒有哦。

最近在做一些微信移動端的頁面，在此記錄關于移動端觸摸和點擊事件的學習優化過程，主要內容圍繞fastclick展開。
fastclick github

移動端瀏覽器一般在用戶點擊屏幕之后會延遲大約300ms才觸發click event
——GOOGLE

手機打開此鏈接查看延遲demo
（現在許多瀏覽器已經不存在延遲問題了，詳見fastclick github，但筆者的手機瀏覽器還是出現了三百毫秒延遲的問題）
截圖如下

為什么會300ms延遲呢，主要是有一個雙擊縮放功能，瀏覽器需要判斷用戶點擊是否為雙擊縮放。這個問題不解決，
1、用戶體驗就會很差，很不流暢，尤其是在密集操作場景下，比如計算器,不解決300ms延遲問題，感覺反應很慢；
2、點擊穿透問題

在了解fastclick的思路之前，我們先看一下事件觸發順序是怎樣的

touchstart

touchmove

touchend

mouseover ：當指針設備移動到存在監聽器的元素或其子元素的時候，mouseover事件就會被觸發。

mouseenter：當指針設備( 通常指鼠標 )在元素上移動時, mousemove 事件被觸發。

mousedown

click

移動端click有300ms延遲問題，touch可沒有哦。

fastclick的思路就是利用touch來模擬tap（觸碰），如果認為是一次有效的tap，則在touchend時立即模擬一個click事件，分發到事件源（相當于主動觸發一次click），同時阻止掉瀏覽器300ms后產生的click。

先看使用示例，很簡單，我們的思路就一直跟著attach走。

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

直接給body綁定fastlick就行了- -。
看源代碼結構（注：以下所有代碼均去掉了一些不影響理解思路的部分，大部分思路寫在注釋中）

//構造函數
    function FastClick(layer, options)
//判斷是否需要瀏覽器原生的click事件（針對一些特殊元素比如表單）
    FastClick.prototype.needsClick = function(target)
//發送模擬的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)
// 判斷這次tap是否有效
    FastClick.prototype.onMouse = function(event) 
//click handler 捕獲階段監聽
    FastClick.prototype.onClick = function(event)
//銷毀fastlick，移除事件綁定
    FastClick.prototype.destroy = function()
//綁定接口
    FastClick.attach = function(layer, options) {
        return new FastClick(layer, options);
    };

attach實際就執行了構造函數進行初始化，接下來我們來看構造函數發生了什么

    function FastClick(layer,options){
        //一些屬性初始化
        //安卓一些老版本瀏覽器不支持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;
        //將所有handler的this綁定到fastclick實例
        for (var i = 0, l = methods.length; i < l; i++) {
            context[methods[i]] = bind(context[methods[i]], context);
        }
        //為當前fast click對象綁定的layer（我們的示例中時document.body）加監聽
        layer.addEventListener("click", this.onClick, true);//true 捕獲階段觸發 
        layer.addEventListener("touchstart", this.onTouchStart, false);
        layer.addEventListener("touchmove", this.onTouchMove, false);
        layer.addEventListener("touchend", this.onTouchEnd, false);
        layer.addEventListener("touchcancel", this.onTouchCancel, false);
    }

構造函數主要是初始化一些屬性，polyfill，和添加監聽，
下面開始看一下重頭戲，touchstart，touchend是如何判斷tap是否有效、如何模擬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).
    // 如果多觸點可能是在縮放，不對targetElement初始化，在此提前終止避免誤模擬產生click
    if (event.targetTouches.length > 1) {
      return true;
    }

    //獲取發生事件源元素（目標階段的元素）
    targetElement = this.getTargetElementFromEventTarget(event.target);
    touch = event.targetTouches[0];
    
    this.trackingClick = true;//標記開始跟蹤click
    this.trackingClickStart = event.timeStamp;//開始跟蹤時間
    this.targetElement = targetElement;//事件源元素

    //觸摸坐標，接下來判斷是否越界用到
    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主要是初始化跟蹤的tap相關的一些屬性，用于之后的判斷‘
接下來touchmove

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

    // If the touch has moved, cancel the click tracking 移動到了其他元素
    if (this.targetElement !== this.getTargetElementFromEventTarget(event.target) || this.touchHasMoved(event)) {//移動越界了，取消本次click模擬處理，走原生流程
      this.trackingClick = false;
      this.targetElement = null;
    }

    return true;
  };

touchmove比較簡單，主要是兼容滑動tap（swiper）等等，滑動越界則不模擬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)
    //阻止快速雙擊
    if ((event.timeStamp - this.lastClickTime) < this.tapDelay) {
      this.cancelNextClick = true;
      return true;
    }
    //超時就不算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);//發送模擬click
    }

    return false;
  };
 //發送模擬的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];

    //模擬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分發模擬的click
    targetElement.dispatchEvent(clickEvent);
  };

最后，還會在layer的click捕獲階段監聽

  //click handler 捕獲階段監聽
  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、出界會置為false，2成功模擬了一次完成tap并阻止click也會置為false，3、避免三方庫影響
      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;
  };

 // 判斷這次鼠標是否有效
  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;
    }

    // 標記fastclick模擬產生的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 阻止事件捕獲和冒泡
      event.stopPropagation();
      event.preventDefault();

      return false;
    }

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

這里主要是判斷這次click是否有效（如無效，則阻止捕獲和冒泡）
至此基本流程已經結束。
其中有1個注意的點，筆者在chrome(Version 64.0.3282.119 (Official Build) (64-bit))已測試
stopPropagation，stopImmediatePropagation不僅會阻止冒泡還會阻止捕獲過程哦。

推薦閱讀源碼，源碼中有許多關于focus、不同瀏覽器兼容和特殊表單元素的處理fastclick github。
這里是筆者帶有中文注釋的代碼中文注釋代碼。
如有紕漏，歡迎批評指正。

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