FastClick的一个bug

300ms延时问题

近来没有什么时间写博客了，再次想写的时候就想简短点，算是做个记录吧。最近遇到一个问题，webapp在ios滑动的时候会出现误触click的情况。基本可以想到是fastclick导致的，因为习惯性为了减去300ms延时（https://developers.google.com/web/updates/2013/12/300ms-tap-delay-gone-away），就会引入fastclick，之前也没去仔细研究过这是一个机会。
click 300ms延时在mobile webview中基本是众所周知的事情，在chrome和firefox中通过meta标签设置width:device-width，明确告诉浏览器禁止缩放，可以去掉这个延时；但在ios中却没有什么很好的方案，所以还是需要一些click库（比如fastclick）来解决这个问题，心塞。

fastclick代码

fastclick代码也不长，核心代码如下：

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

		if (event.targetTouches.length > 1) {
			return true;
		}

		targetElement = this.getTargetElementFromEventTarget(event.target);
		touch = event.targetTouches[0];

		if (deviceIsIOS) {
			selection = window.getSelection();
			if (selection.rangeCount && !selection.isCollapsed) {
				return true;
			}
			if (!deviceIsIOS4) {
				if (touch.identifier && touch.identifier === this.lastTouchIdentifier) {
					event.preventDefault();
					return false;
				}

				this.lastTouchIdentifier = touch.identifier;
				this.updateScrollParent(targetElement);
			}
		}

		this.trackingClick = true;
		this.trackingClickStart = event.timeStamp;
		this.targetElement = targetElement;

		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();
		}

		return true;
	};


	/**
	 * Based on a touchmove event object, check whether the touch has moved past a boundary since it started.
	 * ios涉及到transform形式的快速滑动，fastclick会误认为是click，因为touchHasMoved会返回false,认为没有移动位置
	 * @param {Event} event
	 * @returns {boolean}
	 */
	FastClick.prototype.touchHasMoved = function(event) {
		var touch = event.changedTouches[0], boundary = this.touchBoundary;

		if (Math.abs(touch.pageX - this.touchStartX) > boundary || Math.abs(touch.pageY - this.touchStartY) > boundary) {
			return true;
		}

		return false;
	};


	/**
	 * Update the last position.
	 *
	 * @param {Event} event
	 * @returns {boolean}
	 */
	FastClick.prototype.onTouchMove = function(event) {
		if (!this.trackingClick) {
			return true;
		}

		// If the touch has moved, cancel the click tracking
		if (this.targetElement !== this.getTargetElementFromEventTarget(event.target) || this.touchHasMoved(event)) {
			this.trackingClick = false;
			this.targetElement = null;
		}

		return true;
	};


	/**
	 * Attempt to find the labelled control for the given label element.
	 *
	 * @param {EventTarget|HTMLLabelElement} labelElement
	 * @returns {Element|null}
	 */
	FastClick.prototype.findControl = function(labelElement) {

		// Fast path for newer browsers supporting the HTML5 control attribute
		if (labelElement.control !== undefined) {
			return labelElement.control;
		}

		// All browsers under test that support touch events also support the HTML5 htmlFor attribute
		if (labelElement.htmlFor) {
			return document.getElementById(labelElement.htmlFor);
		}

		// If no for attribute exists, attempt to retrieve the first labellable descendant element
		// the list of which is defined here: http://www.w3.org/TR/html5/forms.html#category-label
		return labelElement.querySelector('button, input:not([type=hidden]), keygen, meter, output, progress, select, textarea');
	};


	/**
	 * On touch end, determine whether to send a click event at once.
	 *
	 * @param {Event} event
	 * @returns {boolean}
	 */
	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)
		if ((event.timeStamp - this.lastClickTime) < this.tapDelay) {
			this.cancelNextClick = true;
			return true;
		}

		if ((event.timeStamp - this.trackingClickStart) > this.tapTimeout) {
			return true;
		}

		// Reset to prevent wrong click cancel on input (issue #156).
		this.cancelNextClick = false;

		this.lastClickTime = event.timeStamp;

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

		// On some iOS devices, the targetElement supplied with the event is invalid if the layer
		// is performing a transition or scroll, and has to be re-detected manually. Note that
		// for this to function correctly, it must be called *after* the event target is checked!
		// See issue #57; also filed as rdar://13048589 .
		if (deviceIsIOSWithBadTarget) {
			touch = event.changedTouches[0];

			// In certain cases arguments of elementFromPoint can be negative, so prevent setting targetElement to null
			targetElement = document.elementFromPoint(touch.pageX - window.pageXOffset, touch.pageY - window.pageYOffset) || targetElement;
			targetElement.fastClickScrollParent = this.targetElement.fastClickScrollParent;
		}

		targetTagName = targetElement.tagName.toLowerCase();
		if (targetTagName === 'label') {
			forElement = this.findControl(targetElement);
			if (forElement) {
				this.focus(targetElement);
				if (deviceIsAndroid) {
					return false;
				}

				targetElement = forElement;
			}
		} else if (this.needsFocus(targetElement)) {

			// Case 1: If the touch started a while ago (best guess is 100ms based on tests for issue #36) then focus will be triggered anyway. Return early and unset the target element reference so that the subsequent click will be allowed through.
			// Case 2: Without this exception for input elements tapped when the document is contained in an iframe, then any inputted text won't be visible even though the value attribute is updated as the user types (issue #37).
			if ((event.timeStamp - trackingClickStart) > 100 || (deviceIsIOS && window.top !== window && targetTagName === 'input')) {
				this.targetElement = null;
				return false;
			}

			this.focus(targetElement);
			this.sendClick(targetElement, event);

			// Select elements need the event to go through on iOS 4, otherwise the selector menu won't open.
			// Also this breaks opening selects when VoiceOver is active on iOS6, iOS7 (and possibly others)
			if (!deviceIsIOS || targetTagName !== 'select') {
				this.targetElement = null;
				event.preventDefault();
			}

			return false;
		}

		if (deviceIsIOS && !deviceIsIOS4) {

			// Don't send a synthetic click event if the target element is contained within a parent layer that was scrolled
			// and this tap is being used to stop the scrolling (usually initiated by a fling - issue #42).
			scrollParent = targetElement.fastClickScrollParent;
			if (scrollParent && scrollParent.fastClickLastScrollTop !== scrollParent.scrollTop) {
				return true;
			}
		}

		// 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();
			this.sendClick(targetElement, event);
		}

		return false;
	};

三个touch监听事件，在onTouchMove中调用touchHasMoved判断滑动距离是否超出设定误差范围。如果超出，则认为是滑动，不是点击。问题就出在这里，如果在ios中快速短促地滑动。在onTouchMove中的判断可能trackingClick依然是true。导致在onTouchEnd中sendClick,dispatch出点击事件。

解决方案：
在onTouchEnd头部 if(!trackingClick) 判断之前，再次为trackingClick校验一次，方法与onTouchMove中相同。