vue的使用中有時會用到nextTick方法，以下是nexttick中的部分源碼（稍微增加了幾句說明與吐槽~）

// 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:
// 上面解釋了為什么在native不使用MutationObserver骄瓣，居然是因為ios的一個bug~
// 在其他情況下 優(yōu)先使用MutationObserver
/* istanbul ignore next, $flow-disable-line */
// 首先判定 Promise可用 并且是native端
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    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.
    // UIWebview是ios的webview組件（舊）蒋畜。特殊處理了一下
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) { // ie...... 如果MutationObserver 可用的話 使用MutationObserver
  // Use MutationObserver where native Promise is not available,
  // e.g. PhantomJS, iOS7, Android 4.4
  // (#6466 MutationObserver is unreliable in IE11)
  // 這里就是大家都在說的 隱形的text 
  // 通過監(jiān)聽這個節(jié)點的變化,保證nexitTick執(zhí)行時dom已經(jīng)變更成功
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2  // 0 1 0 1 0 比 += 1 好
    textNode.data = String(counter)
  }
  isUsingMicroTask = true
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // 然后檢測是否支持setImmediate 
  // Fallback to setImmediate.
  // Techinically it leverages the (macro) task queue,
  // but it is still a better choice than setTimeout.
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // 最后的方案 setTimeout
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

最頂部有一句解釋

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.

這里說到了microtask（微任務）朵耕，與之對應的是macrotasks（微任務）浊服。他們包含的api如下

  macrotasks: setTimeout, setInterval, setImmediate, I/O, UI rendering,requestAnimationFrame(瀏覽器)
  microtasks: process.nextTick(node), Promises, Object.observe(廢棄), MutationObserver(瀏覽器)

js的任務隊列與之對應，microtask queues（微任務隊列）與macrotasks queues（宏任務隊列）驱证。js執(zhí)行時步驟如下
【侵刪】

image

event loop中一個有意思的地方是每次執(zhí)行完一個宏任務后將執(zhí)行微任務隊列中所有（劃重點）任務
所以會出現(xiàn)下面的輸出結果

console.log('0');
setTimeout(() => console.log('4'), 0);
Promise.resolve().then(()=> console.log('2'));
Promise.resolve().then(()=> console.log('3'));
console.log('1');
// 0
// 1
// 2
// 3

然后我就想到一個問題扫沼，當正在執(zhí)行微任務隊列中的任務時，添加新的微任務庄吼。會立刻增加到當前微任務隊列中嗎缎除？于是有了下面的嘗試

console.log('0');
setTimeout(() => {
    console.log('4'); 
}, 0);
Promise.resolve().then(()=>{
    console.log('2');
    Promise.resolve().then(()=>{
        console.log('3');
    })
})
console.log('1');
// 0
// 1
// 2
// 3
// 4

最后，下面的輸出順序～捋一捋～

console.log('0');
setTimeout(() => {
    Promise.resolve().then(()=>{
        console.log('2');
        Promise.resolve().then(()=>{
            console.log('4');
        }).then(()=>{
            console.log('7');
        })
    }).then(()=>{
        console.log('5');
    })
    Promise.resolve().then(()=>{
        console.log('3');
    }).then(()=>{
        console.log('6');
    })
}, 0);
setTimeout(() => {
    console.log('8'); 
}, 0);
console.log('1');