React源码分析（一）=> scheduler分析

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文章目录

• 1. 前言
• 2. getCurrentTime
• 3. unstable_scheduleCallback函数
• 4. scheduleHostCallbackIfNeeded
• 4.1. flushWork
• 4.2. flushFirstCallback
• 5. requestHostCallback
• 5.1. requestAnimationFrameWithTimeout
• 5.2. animationTick

1. 前言

为了读代码更加有效率，提前看了一篇如何阅读源码的文章：https://zxc0328.github.io/2018/05/01/react-source-reading-howto/
因此此次本人阅读源码主要想看懂以下6个问题：

1. ReactDOM.render()是如何挂载到真实DOM上的。
2. setState实现原理，为什么是异步的
3. 生命周期结合2号问题一起看
4. react16的fiber架构是什么
5. jsx是如何解析的
6. react hook是如何做到的

分析代码基于React V16.8.6，react源码目录截图如下图所示：

所需要看的代码一个库就够了，https://github.com/facebook/react/

---

先看的react-dom代码, 一点点单步到了scheduler, 这个包的代码看起来不多(可能是我第一次看框架源码, 看的有点恶心… 一堆全局变量, 各个函数来回调用, 看了好几天, 如果下面哪里有问题, 还请各位同行指点), 那就先来梳理下这个包吧.

scheduler这个包主要是在react做diff做任务分配机制, 核心机制类似于requestidlecallback,

window.requestIdleCallback()会在浏览器空闲时期依次调用函数， 这就可以让开发者在主事件循环中执行后台或低优先级的任务，而且不会对像动画和用户交互这样延迟敏感的事件产生影响。

但这个函数支持度太惨

react则使用requestAnimationFrame和postMessage来模拟实现的requestidlecallback. 工作原理是调度requestAnimationFrame，存储帧开始的时间，然后调度postMessage，后者在绘制后进行调度。

该包主要流程是把所有任务通过双向链表连接起来, 通过requestAnimationFrame来在浏览器每帧的空闲时间循环处理所有任务, 直到链表为空为止.

---

2. getCurrentTime

这个函数后面会经常用到的, 先到前面来说下, 先看代码:

// packages\scheduler\src\forks\SchedulerHostConfig.default.js
const hasNativePerformanceNow =
  typeof performance === 'object' && typeof performance.now === 'function';
const localDate = Date;

if (hasNativePerformanceNow) {
  const Performance = performance;
  getCurrentTime = function() {
    return Performance.now();
  };
} else {
  getCurrentTime = function() {
    // 该方法在 ECMA-262 第五版中被标准化, Date.now() === new Date().getTime();
    // 出处 https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Date/now#Compatibility
    return localDate.now();
  };
}

performance.now() 与 Date.now() 不同的是，返回了以微秒（百万分之一秒）为单位的时间，更加精准。

并且与 Date.now() 会受系统程序执行阻塞的影响不同，performance.now() 的时间是以恒定速率递增的，不受系统时间的影响（系统时间可被人为或软件调整）。

注意Date.now()输出的是 UNIX 时间，即距离 1970 的时间，而performance.now()输出的是相对于 time origin(页面初始化: https://developer.mozilla.org/en-US/docs/Web/API/DOMHighResTimeStamp#The_time_origin) 的时间。

使用 Date.now() 的差值并非绝对精确，因为计算时间时受系统限制（可能阻塞）。但使用 performance.now() 的差值，并不影响我们计算程序执行的精确时间。

3. unstable_scheduleCallback函数

• 函数前面的unstable表示不稳定的意思, 之后还会有变动.
• 这个方法主要就是将任务组成双向链表, 并按照过期时间作为优先级.

先定义优先级, 代码如下:

// packages\scheduler\src\Scheduler.js

// Max 31 bit integer. The max integer size in V8 for 32-bit systems.
// 这是32位系统V8引擎里最大的整数
// Math.pow(2, 30) - 1
// 0b111111111111111111111111111111
var maxSigned31BitInt = 1073741823;
// Times out immediately 立即过期
var IMMEDIATE_PRIORITY_TIMEOUT = -1;
// Eventually times out
var USER_BLOCKING_PRIORITY = 250;
var NORMAL_PRIORITY_TIMEOUT = 5000;
var LOW_PRIORITY_TIMEOUT = 10000;
// Never times out
var IDLE_PRIORITY = maxSigned31BitInt;

主函数代码如下:

// packages\scheduler\src\Scheduler.js

// 组成双向链表, 开始安排任务
function unstable_scheduleCallback(priorityLevel,  callback,  deprecated_options) {
  var startTime =
    currentEventStartTime !== -1 ? currentEventStartTime : getCurrentTime();

  // 过期时间 = 加入时间 + 优先级时间
  var expirationTime;
  if (
    typeof deprecated_options === 'object' &&
    deprecated_options !== null &&
    typeof deprecated_options.timeout === 'number'
  ) {
    expirationTime = startTime + deprecated_options.timeout;
  } else {
    // 根据不同的优先级, 赋予不同的过期时间
    switch (priorityLevel) {
      case ImmediatePriority:
        expirationTime = startTime + IMMEDIATE_PRIORITY_TIMEOUT;
        break;
      case UserBlockingPriority:
        expirationTime = startTime + USER_BLOCKING_PRIORITY;
        break;
      case IdlePriority:
        expirationTime = startTime + IDLE_PRIORITY;
        break;
      case LowPriority:
        expirationTime = startTime + LOW_PRIORITY_TIMEOUT;
        break;
      case NormalPriority:
      default:
        expirationTime = startTime + NORMAL_PRIORITY_TIMEOUT;
    }
  }
  
// 未完

上面先计算一下callback的过期时间, 接下来创建链表节点, 并组成链表, 代码如下:

// packages\scheduler\src\Scheduler.js

// 续上
// 基于上面的优先级和过期时间创建一个节点
var newNode = {
    callback,
    priorityLevel: priorityLevel,
    expirationTime,
    next: null,
    previous: null,
  };

  if (firstCallbackNode === null) {
    // This is the first callback in the list. 如果firstCallbackNode没有, 说明是第一个节点
    firstCallbackNode = newNode.next = newNode.previous = newNode;
    scheduleHostCallbackIfNeeded(); // 之后再说, 先忽略
  } else {
    var next = null;
    var node = firstCallbackNode;
    do {
      if (node.expirationTime > expirationTime) {
        // The new callback expires before this one.
        next = node;
        break;
      }
      node = node.next;
    } while (node !== firstCallbackNode);

    if (next === null) {
      // No callback with a later expiration was found, which means the new
      // callback has the latest expiration in the list.
      // 列表中新插入的节点具有最大的到期时间, 插入最后
      next = firstCallbackNode;
    } else if (next === firstCallbackNode) {
      // The new callback has the earliest expiration in the entire list.
      // 新插入的节点具有最小的到期时间, 插在最前面
      firstCallbackNode = newNode;
      scheduleHostCallbackIfNeeded();
    }

    var previous = next.previous;
    previous.next = next.previous = newNode;
    newNode.next = next;
    newNode.previous = previous;
  }

  return newNode;
}

这部分当初第一次看的时候也看了很久, 看明白之后才发现原来创建双向链表居然这么简单, 就是有点绕, 双向链表的定义:

双向链表也叫双链表，是链表的一种，它的每个数据结点中都有两个指针，分别指向直接后继和直接前驱。所以，从双向链表中的任意一个结点开始，都可以很方便地访问它的前驱结点和后继结点。一般我们都构造双向循环链表。
https://baike.baidu.com/item/双向链表/2968731?fr=aladdin

这部分创建初始双链表其实就只有用了一句话firstCallbackNode = newNode.next = newNode.previous = newNode;, 从右向左赋值, 也就是说这段代码中用的变量全都指向newNode这个对象的地址.

---

4. scheduleHostCallbackIfNeeded

// packages\scheduler\src\Scheduler.js
// This is set while performing work, to prevent re-entrancy.
var isPerformingWork = false;
var isHostCallbackScheduled = false;

function scheduleHostCallbackIfNeeded() {
  if (isPerformingWork) {
    // Don't schedule work yet; wait until the next time we yield.
    // 有个work正在执行
    return;
  }

  if (firstCallbackNode !== null) {
    // Schedule the host callback using the earliest expiration in the list.
    // firstCallbackNode的过期时间是最早的
    var expirationTime = firstCallbackNode.expirationTime;
    if (isHostCallbackScheduled) {
      // Cancel the existing host callback.
      // 取消存在的回调函数
      cancelHostCallback();
    } else {
      isHostCallbackScheduled = true;
    }
    requestHostCallback(flushWork, expirationTime);
  }
}

requestHostCallback这个函数内部主要是通过本文开头所讲的requestAnimationFrame和postMessage来模拟实现的requestidlecallback调度任务执行这个flushWork, 那么我们先来看下这个flushWork.

4.1. flushWork

这个函数的参数didUserCallbackTimeout只会有两种情况:

1. 当前帧没过期;didUserCallbackTimeout = false
2. 当前帧过期且当前任务过期didUserCallbackTimeout = true

// packages\scheduler\src\Scheduler.js

function flushWork(didUserCallbackTimeout) {
  // Exit right away if we're currently paused
  if (enableSchedulerDebugging && isSchedulerPaused) {
    return;
  }

  // We'll need a new host callback the next time work is scheduled.
  // 安排callback完成了
  isHostCallbackScheduled = false;

  isPerformingWork = true;
  const previousDidTimeout = currentHostCallbackDidTimeout;
  currentHostCallbackDidTimeout = didUserCallbackTimeout;
  try {
    if (didUserCallbackTimeout) {
      // 当前帧过期且当前任务过期
      // Flush all the expired callbacks without yielding.
      while (
        firstCallbackNode !== null &&
        !(enableSchedulerDebugging && isSchedulerPaused)
      ) {
        // TODO Wrap in feature flag
        // Read the current time. Flush all the callbacks that expire at or
        // earlier than that time. Then read the current time again and repeat.
        // This optimizes for as few performance.now calls as possible.
        // 读取当前时间, 刷新在该时间之前过期的所有回调
        var currentTime = getCurrentTime();
        if (firstCallbackNode.expirationTime <= currentTime) {
          do {
            flushFirstCallback(); // 见4.2节
          } while (
            firstCallbackNode !== null &&
            firstCallbackNode.expirationTime <= currentTime &&
            !(enableSchedulerDebugging && isSchedulerPaused)
          );
          continue;
        }
        break;
      }
    } else {
      // Keep flushing callbacks until we run out of time in the frame.
      // 进入此循环说明, 帧没有过期
      // 继续刷新回调，直到我们在帧中耗尽时间。
      if (firstCallbackNode !== null) {
        do {
          if (enableSchedulerDebugging && isSchedulerPaused) {
            break;
          }
          flushFirstCallback(); // 见4.2节
        } while (firstCallbackNode !== null && !shouldYieldToHost());
      }
    }
  } finally {
    isPerformingWork = false;
    currentHostCallbackDidTimeout = previousDidTimeout;
    // There's still work remaining. Request another callback.
    scheduleHostCallbackIfNeeded();
  }
}

这部分主要基于当前帧以及当前任务过期时间来决定是否执行flushFirstCallback函数, 这个函数就是最终的执行任务函数.

4.2. flushFirstCallback

这个函数比较长, 主要功能为: 执行队首任务并把队首任务从链表移除，把第二个任务置为队首任务。执行任务可能产生新的任务，再把新任务插入到任务链表中. 见下方代码:

// packages\scheduler\src\Scheduler.js

// 执行任务, 并更新链表
function flushFirstCallback() {
  const currentlyFlushingCallback = firstCallbackNode;

  // Remove the node from the list before calling the callback. That way the
  // list is in a consistent state even if the callback throws.
  // 在执行callback之前, 最好先把这个节点从列表中删掉
  var next = firstCallbackNode.next;
  if (firstCallbackNode === next) {
    // This is the last callback in the list.
    // 由于是双向链表, 这种情况就是现在只剩一个节点了
    // 全部设置为null
    firstCallbackNode = null;
    next = null;
  } else {
    // 链表中删除firstCallbackNode
    var lastCallbackNode = firstCallbackNode.previous;
    firstCallbackNode = lastCallbackNode.next = next;
    next.previous = lastCallbackNode;
  }

  // 从链表中彻底剥离, 把原对象引用置空
  currentlyFlushingCallback.next = currentlyFlushingCallback.previous = null;

  // Now it's safe to call the callback.
  // 获取属性
  var callback = currentlyFlushingCallback.callback;
  var expirationTime = currentlyFlushingCallback.expirationTime;
  var priorityLevel = currentlyFlushingCallback.priorityLevel;

  // 临时保存
  var previousPriorityLevel = currentPriorityLevel;
  var previousExpirationTime = currentExpirationTime;
  currentPriorityLevel = priorityLevel;
  currentExpirationTime = expirationTime;
  var continuationCallback;
  try {
    const didUserCallbackTimeout =
      currentHostCallbackDidTimeout ||
      // Immediate priority callbacks are always called as if they timed out
      priorityLevel === ImmediatePriority;
    continuationCallback = callback(didUserCallbackTimeout);
  } catch (error) {
    throw error;
  } finally {
    // 还原
    currentPriorityLevel = previousPriorityLevel;
    currentExpirationTime = previousExpirationTime;
  }

  // A callback may return a continuation. The continuation should be scheduled
  // with the same priority and expiration as the just-finished callback.
  // 如果返回的是函数, 则和刚刚完成的回调函数具有相同的过期时间和优先级
  if (typeof continuationCallback === 'function') {
    var continuationNode: CallbackNode = {
      callback: continuationCallback,
      priorityLevel,
      expirationTime,
      next: null,
      previous: null,
    };

    // Insert the new callback into the list, sorted by its expiration. This is
    // almost the same as the code in `scheduleCallback`, except the callback
    // is inserted into the list *before* callbacks of equal expiration instead
    // of after.
    // 下面和scheduleCallback函数一个逻辑, 只有一点不同, `callback`在等于到期时间的回调之前插入到列表中而不是插入到之后
    if (firstCallbackNode === null) {
      // This is the first callback in the list.
      firstCallbackNode = continuationNode.next = continuationNode.previous = continuationNode;
    } else {
      var nextAfterContinuation = null;
      var node = firstCallbackNode;
      do {
        if (node.expirationTime >= expirationTime) {
          // This callback expires at or after the continuation. We will insert
          // the continuation *before* this callback.
          nextAfterContinuation = node;
          break;
        }
        node = node.next;
      } while (node !== firstCallbackNode);

      if (nextAfterContinuation === null) {
        // No equal or lower priority callback was found, which means the new
        // callback is the lowest priority callback in the list.
        // 没有找到相等或者低优先级的callback, 因此放到第一个
        nextAfterContinuation = firstCallbackNode;
      } else if (nextAfterContinuation === firstCallbackNode) {
        // The new callback is the highest priority callback in the list.
        firstCallbackNode = continuationNode;
        scheduleHostCallbackIfNeeded();
      }

      // 插入操作
      var previous = nextAfterContinuation.previous;
      previous.next = nextAfterContinuation.previous = continuationNode;
      continuationNode.next = nextAfterContinuation;
      continuationNode.previous = previous;
    }
  }
}

---

5. requestHostCallback

接下来我们最后再看下这个函数, 代码如下:

// packages\scheduler\src\forks\SchedulerHostConfig.default.js

  // absoluteTimeout => 链表node的过期时间(expirationTime)
  // callback => flushWork函数
  requestHostCallback = function(callback, absoluteTimeout) {
    scheduledHostCallback = callback;
    timeoutTime = absoluteTimeout;
    if (isFlushingHostCallback || absoluteTimeout < 0) {
      // Don't wait for the next frame. Continue working ASAP, in a new event.
      // 如果absoluteTimeout时间小于1, 则此次work为ImmediatePriority优先级
      // 应该立即执行
      port.postMessage(undefined);
    } else if (!isAnimationFrameScheduled) {
      // If rAF didn't already schedule one, we need to schedule a frame.
      // TODO: If this rAF doesn't materialize because the browser throttles, we
      // might want to still have setTimeout trigger rIC as a backup to ensure
      // that we keep performing work.
      isAnimationFrameScheduled = true;
      requestAnimationFrameWithTimeout(animationTick); // animationTick为一个函数, 见5.2节
    }
  };

这部分没多少代码

5.1. requestAnimationFrameWithTimeout

我们接下来看下requestAnimationFrameWithTimeout函数, 代码如下:

// packages\scheduler\src\forks\SchedulerHostConfig.default.js

// requestAnimationFrame does not run when the tab is in the background. If
// we're backgrounded we prefer for that work to happen so that the page
// continues to load in the background. So we also schedule a 'setTimeout' as
// a fallback.
// TODO: Need a better heuristic for backgrounded work.

// requestAnimationFrame在切换tab之后不再运行，如果切换tab之后
// 我们还想让他在后台继续运行，应使用setTimeout作为兜底操作
const ANIMATION_FRAME_TIMEOUT = 100;
let rAFID;
let rAFTimeoutID;
const requestAnimationFrameWithTimeout = function(callback) {
  // schedule rAF and also a setTimeout
  rAFID = localRequestAnimationFrame(function(timestamp) {
    // timestamp 实际上就是performance.now()
    // cancel the setTimeout, 如果RAF好使就不使用Timeout
    // callback实际就是下面的animationTick函数, 见5.2节
    localClearTimeout(rAFTimeoutID);
    callback(timestamp);
  });
  rAFTimeoutID = localSetTimeout(function() {
    // cancel the requestAnimationFrame
    localCancelAnimationFrame(rAFID);
    callback(getCurrentTime());
  }, ANIMATION_FRAME_TIMEOUT);
};

5.2. animationTick

我要回家, 赶火车了今天先写到这里, 明天继续更

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