nextTick 实现原理 |
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nextTick 实现原理
先上源码vue2 var timerFunc; // The nextTick behavior leverages the microtask queue, which can be accessed // via either native Promise.then or MutationObserver. // MutationObserver has wider support, however it is seriously bugged in // UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It // completely stops working after triggering a few times... so, if native // Promise is available, we will use it: /* istanbul ignore next, $flow-disable-line */ if (typeof Promise !== 'undefined' && isNative(Promise)) { var p = Promise.resolve(); timerFunc = function () { p.then(flushCallbacks); // In problematic UIWebViews, Promise.then doesn't completely break, but // it can get stuck in a weird state where callbacks are pushed into the // microtask queue but the queue isn't being flushed, until the browser // needs to do some other work, e.g. handle a timer. Therefore we can // "force" the microtask queue to be flushed by adding an empty timer. if (isIOS) { setTimeout(noop); } }; isUsingMicroTask = true; } else if (!isIE && typeof MutationObserver !== 'undefined' && ( isNative(MutationObserver) || // PhantomJS and iOS 7.x MutationObserver.toString() === '[object MutationObserverConstructor]' )) { // Use MutationObserver where native Promise is not available, // e.g. PhantomJS, iOS7, Android 4.4 // (#6466 MutationObserver is unreliable in IE11) var counter = 1; var observer = new MutationObserver(flushCallbacks); var textNode = document.createTextNode(String(counter)); observer.observe(textNode, { characterData: true }); timerFunc = function () { counter = (counter + 1) % 2; textNode.data = String(counter); }; isUsingMicroTask = true; } else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) { // Fallback to setImmediate. // Technically it leverages the (macro) task queue, // but it is still a better choice than setTimeout. timerFunc = function () { setImmediate(flushCallbacks); }; } else { // Fallback to setTimeout. timerFunc = function () { setTimeout(flushCallbacks, 0); }; } function nextTick (cb, ctx) { debugger var _resolve; callbacks.push(function () { if (cb) { try { cb.call(ctx); } catch (e) { handleError(e, ctx, 'nextTick'); } } else if (_resolve) { _resolve(ctx); } }); if (!pending) { pending = true; timerFunc(); } // $flow-disable-line if (!cb && typeof Promise !== 'undefined') { return new Promise(function (resolve) { _resolve = resolve; }) } } 复制代码 nextTick实现原理 第一步: 创建flushCallbacks回调函数执行器callbacks用于存储$nextTick传入的函数 flushCallbacks用于遍历执行回调函数 const callbacks = [] let pending = false function flushCallbacks () { pending = false const copies = callbacks.slice(0) callbacks.length = 0 for (let i = 0; i < copies.length; i++) { copies[i]() } } 复制代码 第二步: 创建微任务包装器，并判断浏览器支持的微任务API赋予timerFunc 先判断浏览器是否支持Promise，如果支持，则使用Promise来实现 在判断浏览器是否支持MutationObserver，如果支持，则使用MutationObserver来实现 setImmediate/setTimeout function nextTick (cb, ctx) { debugger var _resolve; callbacks.push(function () { if (cb) { try { cb.call(ctx); } catch (e) { handleError(e, ctx, 'nextTick'); } } else if (_resolve) { _resolve(ctx); } }); if (!pending) { pending = true; timerFunc(); } // $flow-disable-line if (!cb && typeof Promise !== 'undefined') { return new Promise(function (resolve) { _resolve = resolve; }) } } 复制代码导出nextTick方法，可接受两个参数cb是回调函数，ctx执行上下文 首先判断cb是否存在，存在则将绑定了ctx的回调函数push入callbacks中 pending默认值false，然后调用timerFunc微任务包装器来执行我们传入的回调 $nextTick微任务包装器的shim降级顺序： Promise > MutationObserver > setImmediate > setTimeout |
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