React Fiber源码分析第三篇（异步状态）

先附上流程图~

调用setState时，会调用classComponentUpdater的enqueueSetState方法，同时将新的state作为payload参数传进

enqueueSetState会先调用requestCurrentTime获取一个currentTime，

function requestCurrentTime() {
  // 维护两个时间 一个renderingTime 一个currentSechedulerTime
 //  rederingTime 可以随时更新  currentSechedulerTime只有在没有新任务的时候才更新
  if (isRendering) {
    return currentSchedulerTime;
  }
  findHighestPriorityRoot();
  if (nextFlushedExpirationTime === NoWork || nextFlushedExpirationTime === Never) {
    recomputeCurrentRendererTime();
    currentSchedulerTime = currentRendererTime;
    return currentSchedulerTime;
  }
  return currentSheculerTime

通过获取到的currentTime, 调用computeExpirationForFiber，计算该fiber的优先级，

if (fiber.mode & AsyncMode) {
      if (isBatchingInteractiveUpdates) {
        // This is an interactive update
        expirationTime = computeInteractiveExpiration(currentTime);
      } else {
        // This is an async update
        expirationTime = computeAsyncExpiration(currentTime);
      }
      ...
    }

这个函数其他点比较简单，里面主要有下面这个判断要说明一下，如果是属于异步更新的话，会根据是交互引起的更新还是其他更新来调用不同的函数computeInteractiveExpiration和computeAsyncExpiration，

可以看到这两个函数最后返回的都是computeExpirationBucket函数的结果，只是入参不同， computeInteractiveExpiration的参数是500， 100， computeAsyncExpiration的参数是5000， 250，然后看computeExpirationBucket函数可以看到，第二个参数（500和5000）越大，则返回的expirationTime越大，也就是说 computeInteractiveExpiration的更新优先级高于computeAsyncExpiration, 则交互的优先级高于其他

获得优先级后则和同步更新一样，创建update并放进队列，然后调用sheuduleWork

var classComponentUpdater = {
  isMounted: isMounted,
  enqueueSetState: function (inst, payload, callback) {
    var fiber = get(inst);
　　 // 获得优先级
    var currentTime = requestCurrentTime();
    var expirationTime = computeExpirationForFiber(currentTime, fiber);
　　 // 创建更新
    var update = createUpdate(expirationTime);
    update.payload = payload;
    if (callback !== undefined && callback !== null) {
      update.callback = callback;
    }

    enqueueUpdate(fiber, update);
    scheduleWork(fiber, expirationTime);
  },

接下来的步骤和同步一样，直到同步调用的是performSyncWork函数，而异步调用的是scheduleCallbackWithExpirationTime函数

scheduleCallbackWithExpirationTime函数首先判断是否存在callback正在进行中，判断现有expirationTime和其优先级，若优先级比较低则直接返回，否则设置现在的fiber任务为新的callback，并把原来的回调从列表中移除

function scheduleCallbackWithExpirationTime(root, expirationTime) {
  if (callbackExpirationTime !== NoWork) {
    //  判断优先级
    if (expirationTime > callbackExpirationTime) {
      // Existing callback has sufficient timeout. Exit.
      return;
    } else {
      if (callbackID !== null) {
        // 取消, 从回调列表中删除
        schedule.unstable_cancelScheduledWork(callbackID);
      }
    }
    // The request callback timer is already running. Don't start a new one.
  } 
  // 设置新的callback和callbackExiporationTime
  callbackExpirationTime = expirationTime;
  var currentMs = schedule.unstable_now() - originalStartTimeMs;
  var expirationTimeMs = expirationTimeToMs(expirationTime);
  // 计算是否超时 
  var timeout = expirationTimeMs - currentMs;
  callbackID = schedule.unstable_scheduleWork(performAsyncWork, { timeout: timeout });
}

接下来调用schedule.unstable_scheduleWork(performAsyncWork, { timeout: timeout })函数，并生成一个节点，存储回调函数和超时时间，插入到回调列表，并根据超时排序, 调用ensureHostCallBackIsScheduled函数，最后返回该节点

function unstable_scheduleWork(callback, options) {
  var currentTime = exports.unstable_now();

  var timesOutAt; 
  // 获取超时时间
  if (options !== undefined && options !== null && options.timeout !== null && options.timeout !== undefined) {
    // Check for an explicit timeout 
    timesOutAt = currentTime + options.timeout;
  } else {
    // Compute an absolute timeout using the default constant.
    timesOutAt = currentTime + DEFERRED_TIMEOUT;
  }
　// 生成一个节点， 存储回调函数和超时时间
  var newNode = {
    callback: callback,
    timesOutAt: timesOutAt,
    next: null,
    previous: null
  };

  // 插入到回调列表， 并根据超时排序, 最后返回该节点
  if (firstCallbackNode === null) {
    // This is the first callback in the list.
    firstCallbackNode = newNode.next = newNode.previous = newNode;
    ensureHostCallbackIsScheduled(firstCallbackNode);
  } else {
    ...var previous = next.previous;
    previous.next = next.previous = newNode;
    newNode.next = next;
    newNode.previous = previous;
  }

  return newNode;
}

ensureHostCallBackIsScheduled函数如名，相对比较简单

function ensureHostCallbackIsScheduled() {
  if (isPerformingWork) {
    // Don't schedule work yet; wait until the next time we yield.
    return;
  }
  // Schedule the host callback using the earliest timeout in the list.
  var timesOutAt = firstCallbackNode.timesOutAt;
  if (!isHostCallbackScheduled) {
    isHostCallbackScheduled = true;
  } else {
    // Cancel the existing host callback.
    cancelCallback();
  }
  requestCallback(flushWork, timesOutAt);
}

往下看requestCallback，这里说的如果已经在执行任务的话，就必须有一个错误被抛出（抛出的错误是啥？？），同时不要等待下一帧，尽快开始新事件

如果如果当前没有调度帧回调函数，我们需要进行一个调度帧回调函数, 并设置isAnimationFrameScheduled为true,
接着执行requestAnimationFrameWithTimeout;函数

requestCallback = function (callback, absoluteTimeout) {
    scheduledCallback = callback;
    timeoutTime = absoluteTimeout;
    if (isPerformingIdleWork) {
      // 如果已经在执行任务的话， 就必须有一个错误被抛出（抛出的错误是啥？？），同时不要等待下一帧， 尽快开始新事件
      window.postMessage(messageKey, '*');
    } else if (!isAnimationFrameScheduled) {
      isAnimationFrameScheduled = true;
      requestAnimationFrameWithTimeout(animationTick);
    }
  };

requestAnimationFrameWithTimeout函数就是执行一个异步操作，执行完毕后，假设此时又有N个回调任务进入，同时原来的回调还没有进行，则回到scheduleCallbackWithExpirationTime函数上，

分为两个分支： 1. 假设优先级低于目前的回调任务，则直接返回（已经把root加到root队列中）
2. 优先级高于目前的回调任务，将目前的回调任务从列表中移除，并将callBackID设为传入的回调，接下来的路线与上面一致，假设该传入的回调超时最早，则会进入到cancelCallback函数，重置各变量，并进入到requestCallback函数，此时除了赋值操作，没有其他动作

到了这时候，已经把新的回调替换正在进行的回调到回调列表。
函数正常执行，调用callback, 即animationTick函数

cancelCallback = function () {
    scheduledCallback = null;
    isIdleScheduled = false;
    timeoutTime = -1;
  };

var ANIMATION_FRAME_TIMEOUT = 100;
var rAFID;
var rAFTimeoutID;
var requestAnimationFrameWithTimeout = function (callback) {
  // schedule rAF and also a setTimeout
  rAFID = localRequestAnimationFrame(function (timestamp) {
    // cancel the setTimeout
    localClearTimeout(rAFTimeoutID);
    callback(timestamp);
  });
  rAFTimeoutID = localSetTimeout(function () {
    // cancel the requestAnimationFrame
    localCancelAnimationFrame(rAFID);
    callback(exports.unstable_now());
  }, ANIMATION_FRAME_TIMEOUT);
};

animationTick一个是把isAnimationFrameScheduled状态设为false，即不在调度帧回调的状态，同时计算帧到期时间frameDeadline , 判断是否在帧回调的状态，否的话调用window.postMessage ,并设置isIdleScheduled状态为true

假设此时，有N个回调进入，分为两个情况： 1.假设优先级低于目前的回调任务，则直接返回（已经把root加到root队列中）
2.优先级高于目前的回调任务，将目前的回调任务从列表中移除，并将callBackID设为传入的回调，接下来的路线与上面一致，一直到animationTick函数，因为 postMessage比setTImeout更快执行，所以此时isIdleScheduled为false，和之前一样正常执行。

var animationTick = function (rafTime) {
    isAnimationFrameScheduled = false;
    ...
    ...
    // 每帧到期时间为33ms
    frameDeadline = rafTime + activeFrameTime;
    if (!isIdleScheduled) {
      isIdleScheduled = true;
      window.postMessage(messageKey, '*');
    }
  };

postMessage会执行idleTick ，首先把isIdleScheduled\didTimeout置为false,

先判断帧到期时间和超时时间是否小于当前时间，如果是的话，则置didTimeout为true,
如果帧到期，但超时时间小于当前时间，则置isAnimationFrameScheduled 为false，并调用requestAnimationFrameWithTimeout, 即进入下一帧
如果帧未到期，则调用callbak函数，并把isPerformingIdleWork置为true

idleTick 会先执行callback，完成后才将isPerformingIdleWork 置为false，执行callback的时候会传入didTimeout作为参数， callback为flushWork

 var idleTick = function (event) {
    ...
    isIdleScheduled = false;

    var currentTime = exports.unstable_now();

    var didTimeout = false;
    if (frameDeadline - currentTime <= 0) {
      // 帧过期
      if (timeoutTime !== -1 && timeoutTime <= currentTime) {
        // 回调超时
        didTimeout = true;
      } else {
        // No timeout.
        if (!isAnimationFrameScheduled) {
          // 到下一帧继续任务
          isAnimationFrameScheduled = true;
          requestAnimationFrameWithTimeout(animationTick);
        }
        // Exit without invoking the callback.
        return;
      }
    }

    timeoutTime = -1;
    var callback = scheduledCallback;
    scheduledCallback = null;
    if (callback !== null) {
      isPerformingIdleWork = true;
      try {
        callback(didTimeout);
      } finally {
        isPerformingIdleWork = false;
      }
    }
  };

flushwork首先把isPerformingWork置为true，然后把didTimeout赋值给deallinObject对象，接下来进行判断
如果已经过了帧的结束期，则判断链表中有哪个节点已超时，并循环调用flushFirstCallback函数解决超时节点，
如果还没有过帧的结束期，则调用flushFirstCallback函数处理链表中的第一个节点，循环处理一直到该帧结束

最后， flushwork函数会将isPerformingWork置为false，并判断是否还有任务有则执行ensureHostCallbackIsScheduled函数

function flushWork(didTimeout) {
  isPerformingWork = true;
  deadlineObject.didTimeout = didTimeout;
  try {
    if (didTimeout) {
      while (firstCallbackNode !== null) {
        var currentTime = exports.unstable_now();
        if (firstCallbackNode.timesOutAt <= currentTime) {
          do {
            flushFirstCallback();
          } while (firstCallbackNode !== null && firstCallbackNode.timesOutAt <= currentTime);
          continue;
        }
        break;
      }
    } else {
      // Keep flushing callbacks until we run out of time in the frame.
      if (firstCallbackNode !== null) {
        do {
          flushFirstCallback();
        } while (firstCallbackNode !== null && getFrameDeadline() - exports.unstable_now() > 0);
      }
    }
  } finally {
    isPerformingWork = false;
    if (firstCallbackNode !== null) {
      // There's still work remaining. Request another callback.
      ensureHostCallbackIsScheduled(firstCallbackNode);
    } else {
      isHostCallbackScheduled = false;
    }
  }
}

继续往下看，则是flushFirstCallback函数，先把该节点从链表中清掉，然后调用callback函数，并带入deadlineObject作为参数

function flushFirstCallback(node) {
  var flushedNode = firstCallbackNode;

  //从链表中清理掉该节点， 这样哪怕出错了， 也能保留原链表状态
  var next = firstCallbackNode.next;
  if (firstCallbackNode === next) {
    // This is the last callback in the list.
    firstCallbackNode = null;
    next = null;
  } else {
    var previous = firstCallbackNode.previous;
    firstCallbackNode = previous.next = next;
    next.previous = previous;
  }

  flushedNode.next = flushedNode.previous = null;

  // Now it's safe to call the callback.
  var callback = flushedNode.callback;
  callback(deadlineObject);
}

接下来的就是performAsyncWork函数，如果didTimeout为true，则表明至少有一个更新已过期，迭代所有root任务，把已过期的root的nextExpirationTimeToWorkOn重置为当前时间currentTime.
然后调用performWork(Nowork, dl)函数

function performAsyncWork(dl) {
  if (dl.didTimeout) {
    // 刷新所有root的nextEpirationTimeToWorkOn
    if (firstScheduledRoot !== null) {
      recomputeCurrentRendererTime();
      var root = firstScheduledRoot;
      do {
        didExpireAtExpirationTime(root, currentRendererTime);
        // The root schedule is circular, so this is never null.
        root = root.nextScheduledRoot;
      } while (root !== firstScheduledRoot);
    }
  }
  performWork(NoWork, dl);
}

performWork函数在之前已经分析过了，这里主要看存在deadline时的操作，在帧未到期或者当前渲染时间大于等于nextFlushedExpirationTime时才执行 performWorkOnRoot, 并将currentRendererTime >= nextFlushedExpirationTime作为第三个参数传入，一直循环处理任务，
最后清除callbackExpirationTime, callBackId, 同时，如果还有任务的话，则继续调用scheduleCallbackWithExpirationTime(nextFlushedRoot, nextFlushedExpirationTime);函数进入到回调

function performWork(minExpirationTime, dl) {
  deadline = dl;

  // Keep working on roots until there's no more work, or until we reach
  // the deadline.
  findHighestPriorityRoot();

  if (deadline !== null) {
    recomputeCurrentRendererTime();
    currentSchedulerTime = currentRendererTime;while (nextFlushedRoot !== null && nextFlushedExpirationTime !== NoWork && (minExpirationTime === NoWork || minExpirationTime >= nextFlushedExpirationTime) && (!deadlineDidExpire || currentRendererTime >= nextFlushedExpirationTime)) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, currentRendererTime >= nextFlushedExpirationTime);
      findHighestPriorityRoot();
      recomputeCurrentRendererTime();
      currentSchedulerTime = currentRendererTime;
    }
  } 
  if (deadline !== null) {
    callbackExpirationTime = NoWork;
    callbackID = null;
  }
  // If there's work left over, schedule a new callback.
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(nextFlushedRoot, nextFlushedExpirationTime);
  }

  // Clean-up.
  deadline = null;
  deadlineDidExpire = false;

  finishRendering();
}

接下来看异步状态下的performWorkOnRoot函数。基本操作和同步一样，在进入到renderRoot(root, _isYieldy, isExpired);函数时，会根据是否已超时将isYieldy置为true或者false, 异步状态下未超时为false,
renderRoot和同步一样，最后执行workLoop(isYieldy)
workLoop在未过期的情况下，会执行shouldYield()函数来判断是否执行nextUnitOfWork, 和同步一样，这里只需要关注shouldYied函数

function workLoop(isYieldy) {
  if (!isYieldy) {
    // Flush work without yielding
    while (nextUnitOfWork !== null) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  } else {
    // Flush asynchronous work until the deadline runs out of time.
    while (nextUnitOfWork !== null && !shouldYield()) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  }
}

shouldYield函数，如果deadlineDidExpire为true, 即帧已到期，直接返回true,
如果deadline不存在，并且帧未到期，则返回false, 可以执行单元
否则将deadlineDidExpire置为true

function shouldYield() {
  if (deadlineDidExpire) {
    return true;
  }
  if (deadline === null || deadline.timeRemaining() > timeHeuristicForUnitOfWork) {
    // Disregard deadline.didTimeout. Only expired work should be flushed
    // during a timeout. This path is only hit for non-expired work.
    return false;
  }
  deadlineDidExpire = true;
  return true;
}


完结~撒花

posted on 2018-11-10 11:47 菜的黑人牙膏阅读(1442) 评论(0) 编辑收藏举报

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