React深入分析useEffect源码

var hook = mountWorkInProgressHook();

function mountEffectImpl(fiberFlags, hookFlags, create, deps) {
  // hook初始化
  var hook = mountWorkInProgressHook();
  // 判断是否有传入deps，如果有会作为下次更新的deps
  var nextDeps = deps === undefined ? null : deps;
  // 给hook所在的fiber打上有副作用的更新的标记
  currentlyRenderingFiber$1.flags |= fiberFlags;
  // 将副作用操作存放到fiber.memoizedState.hook.memoizedState中
  hook.memoizedState = pushEffect(HasEffect | hookFlags, create, undefined, nextDeps);
}

function pushEffect(tag, create, destroy, deps) {
  // 初始化副作用结构，
  var effect = {
    tag: tag,
    create: create,   // 回调函数
    destroy: destroy,  // 回调函数里的return（mount时是undefined）
    deps: deps,    // 依赖数组
    // 闭环链表
    next: null
  };
  // 下面的一大段代码看着复杂，但是有没有很熟悉的感觉？
  var componentUpdateQueue = currentlyRenderingFiber$1.updateQueue;
  if (componentUpdateQueue === null) {
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber$1.updateQueue = componentUpdateQueue;
    // effect.next = effect形成环形链表
    componentUpdateQueue.lastEffect = effect.next = effect;   
  } else {
    var lastEffect = componentUpdateQueue.lastEffect;
    if (lastEffect === null) {
      componentUpdateQueue.lastEffect = effect.next = effect;
    } else {
      var firstEffect = lastEffect.next;
      lastEffect.next = effect;
      effect.next = firstEffect;
      componentUpdateQueue.lastEffect = effect;
    }
  }
  return effect;
}

function updateEffectImpl(fiberFlags, hookFlags, create, deps) {
  var hook = updateWorkInProgressHook();
  var nextDeps = deps === undefined ? null : deps;
  var destroy = undefined;
  if (currentHook !== null) {
    var prevEffect = currentHook.memoizedState;
    destroy = prevEffect.destroy;
    if (nextDeps !== null) {
      var prevDeps = prevEffect.deps;
      // 比较两次依赖数组中的值是否有变化
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        // 和之前初始化时一样
        pushEffect(hookFlags, create, destroy, nextDeps);
        return;
      }
    }
  }
  // 和之前初始化时一样
  currentlyRenderingFiber$1.flags |= fiberFlags;
  hook.memoizedState = pushEffect(HasEffect | hookFlags, create, destroy, nextDeps);
}

function areHookInputsEqual(nextDeps, prevDeps) {
  // 没有传deps的情况返回false
  if (prevDeps === null) {
    return false;
  }
  // deps不是[]，且其中的值有变动才会返回false
  for (var i = 0; i < prevDeps.length && i < nextDeps.length; i++) {
    if (objectIs(nextDeps[i], prevDeps[i])) {
      continue;
    }
    return false;
  }
  // deps = []，或者deps里面的值没有变化会返回true
  return true;
}

// if内部的，第一个参数是hookFlags = 4
pushEffect(hookFlags, create, destroy, nextDeps);
// if外部的，第一个参数是HasEffect | hookFlags = 5
hook.memoizedState = pushEffect(HasEffect | hookFlags, create, destroy, nextDeps);

function schedulePassiveEffects(finishedWork) {
  var updateQueue = finishedWork.updateQueue;
  var lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;
  if (lastEffect !== null) {
    var firstEffect = lastEffect.next;
    var effect = firstEffect;
    // 遍历effect链表
    do {
      var _effect = effect,
          next = _effect.next,
          tag = _effect.tag;
      // 基于effect.tag决定是否添加到副作用执行队列
      if ((tag & Passive$1) !== NoFlags$1 && (tag & HasEffect) !== NoFlags$1) {
        enqueuePendingPassiveHookEffectUnmount(finishedWork, effect);
        enqueuePendingPassiveHookEffectMount(finishedWork, effect);
      }
      effect = next;
    } while (effect !== firstEffect);
  }
}

function flushPassiveEffectsImpl() {
  // 执行上次更新动作的销毁函数
  var unmountEffects = pendingPassiveHookEffectsUnmount;
  pendingPassiveHookEffectsUnmount = [];
  for (var i = 0; i < unmountEffects.length; i += 2) {
    ...destroy()
  }
  // 执行本次更新动作的回调函数
  var mountEffects = pendingPassiveHookEffectsMount;
  pendingPassiveHookEffectsMount = [];
  for (var _i = 0; _i < mountEffects.length; _i += 2) {
    ...create()
  }
}

var unmountEffects = pendingPassiveHookEffectsUnmount;
var mountEffects = pendingPassiveHookEffectsMount;