react浅析4 hook之setState

与以往一样， 以一个简单的示例入手

import React, {useState} from 'react';
 
function HelloWord() {
    const [msg, setMsg] = useState('hello word');
    const [name, setName] = useState('ali');

    const chgValue = () => {
        let dateTmp =  Date.now();
        setMsg('hello' + dateTmp);
        setMsg("hi" + dateTmp);
        setName('ali' + dateTmp);
    }
    return (
        <>
            <div>{msg}</div>
            <div>{name}</div>
            <span onClick={chgValue}> click me</span>
        </>
    );
}

export default HelloWord

useState调用

我们知道不管是初建一个function组件还是update更新function组件时， 最终都要触发function方法来生成虚拟DOM。
这也就是说初建和update更新调用useState会走不同的处理。其调用代码如下所示， 初建主要执行mountState方法， 而更新则使用updateState方法

HooksDispatcherOnMountInDEV = {
    ...
    useState: function (initialState) {
      currentHookNameInDev = 'useState';
      mountHookTypesDev();
      var prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;

      try {
        return mountState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    }
    ...
  };

HooksDispatcherOnUpdateInDEV = {
    useState: function (initialState) {
      currentHookNameInDev = 'useState';
      updateHookTypesDev();
      var prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;

      try {
        return updateState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    }
  };

mountState

1. mount的时候会新生成一个hook对象并将其挂载到hook链上, 同时将当前fiber的memoizedState与hook链建立连接
2. 将初始值赋值给memoizedState以供渲染使用
3. dispatch为一个闭包函数，它会将当前fiber及queue作为参数传入， 在调用该方法时可获取当前的fiber以供使用

function mountState(initialState) {
  var hook = mountWorkInProgressHook();

  if (typeof initialState === 'function') {
    // $FlowFixMe: Flow doesn't like mixed types
    initialState = initialState();
  }

  hook.memoizedState = hook.baseState = initialState;
  var queue = hook.queue = {
    pending: null,
    dispatch: null,
    lastRenderedReducer: basicStateReducer,
    lastRenderedState: initialState
  };
  var dispatch = queue.dispatch = dispatchAction.bind(null, currentlyRenderingFiber$1, queue);
  return [hook.memoizedState, dispatch];
}

function mountWorkInProgressHook() {
  var hook = {
    memoizedState: null,
    baseState: null,
    baseQueue: null,
    queue: null,
    next: null
  };

  if (workInProgressHook === null) {
    // This is the first hook in the list
    currentlyRenderingFiber$1.memoizedState = workInProgressHook = hook;
  } else {
    // Append to the end of the list
    workInProgressHook = workInProgressHook.next = hook;
  }

  return workInProgressHook;
}

hook链如下图所示
useState_memoizedState.png

dispatchAction

该方法用来触发action, 实现hook state的更新。 其主要功能如下

1. 创建update对象，并将其挂载到hook的queue属性上
2. 将Action的变更值放到update的eagerState属性上
3. 将fiber推入scheduleWork任务调度中， 接下来会进入workLoop中进行调和、commit 最终实现view渲染。

function dispatchAction(fiber, queue, action) {
  ...
  var update = {
    expirationTime: expirationTime,
    suspenseConfig: suspenseConfig,
    action: action,
    eagerReducer: null,
    eagerState: null,
    next: null
  };

  {
    update.priority = getCurrentPriorityLevel();
  } // Append the update to the end of the list.


  var pending = queue.pending;

  if (pending === null) {
    // This is the first update. Create a circular list.
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }

  queue.pending = update;
  var alternate = fiber.alternate;

  if (fiber === currentlyRenderingFiber$1 || alternate !== null && alternate === currentlyRenderingFiber$1) {
    // This is a render phase update. Stash it in a lazily-created map of
    // queue -> linked list of updates. After this render pass, we'll restart
    // and apply the stashed updates on top of the work-in-progress hook.
    didScheduleRenderPhaseUpdate = true;
    update.expirationTime = renderExpirationTime;
    currentlyRenderingFiber$1.expirationTime = renderExpirationTime;
  } else {
    if (fiber.expirationTime === NoWork && (alternate === null || alternate.expirationTime === NoWork)) {
      // The queue is currently empty, which means we can eagerly compute the
      // next state before entering the render phase. If the new state is the
      // same as the current state, we may be able to bail out entirely.
      var lastRenderedReducer = queue.lastRenderedReducer;

      if (lastRenderedReducer !== null) {
        var prevDispatcher;

        {
          prevDispatcher = ReactCurrentDispatcher.current;
          ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
        }

        try {
          var currentState = queue.lastRenderedState;
          var eagerState = lastRenderedReducer(currentState, action); // Stash the eagerly computed state, and the reducer used to compute
          // it, on the update object. If the reducer hasn't changed by the
          // time we enter the render phase, then the eager state can be used
          // without calling the reducer again.

          update.eagerReducer = lastRenderedReducer;
          update.eagerState = eagerState;

          if (objectIs(eagerState, currentState)) {
            // Fast path. We can bail out without scheduling React to re-render.
            // It's still possible that we'll need to rebase this update later,
            // if the component re-renders for a different reason and by that
            // time the reducer has changed.
            return;
          }
        } catch (error) {// Suppress the error. It will throw again in the render phase.
        } finally {
          {
            ReactCurrentDispatcher.current = prevDispatcher;
          }
        }
      }
    }

    {
      // $FlowExpectedError - jest isn't a global, and isn't recognized outside of tests
      if ('undefined' !== typeof jest) {
        warnIfNotScopedWithMatchingAct(fiber);
        warnIfNotCurrentlyActingUpdatesInDev(fiber);
      }
    }

    scheduleWork(fiber, expirationTime);
  }
}

updateState

updateState只是对updateReducer方法的调用

updateReducer

1. 通过updateWorkInProgresHook获取当前的hook对象， 访问时通过next在hook链上依次取出即可。
2. 遍历upate链更新, 完成后返回state数值， 和dispatch方法

function updateReducer(reducer, initialArg, init) {
  var hook = updateWorkInProgressHook();
  var queue = hook.queue;

  if (!(queue !== null)) {
    {
      throw Error( "Should have a queue. This is likely a bug in React. Please file an issue." );
    }
  }

  queue.lastRenderedReducer = reducer;
  var current = currentHook; // The last rebase update that is NOT part of the base state.

  var baseQueue = current.baseQueue; // The last pending update that hasn't been processed yet.

  var pendingQueue = queue.pending;

  if (pendingQueue !== null) {
    // We have new updates that haven't been processed yet.
    // We'll add them to the base queue.
    if (baseQueue !== null) {
      // Merge the pending queue and the base queue.
      var baseFirst = baseQueue.next;
      var pendingFirst = pendingQueue.next;
      baseQueue.next = pendingFirst;
      pendingQueue.next = baseFirst;
    }

    current.baseQueue = baseQueue = pendingQueue;
    queue.pending = null;
  }

  if (baseQueue !== null) {
    // We have a queue to process.
    var first = baseQueue.next;
    var newState = current.baseState;
    var newBaseState = null;
    var newBaseQueueFirst = null;
    var newBaseQueueLast = null;
    var update = first;

    do {
      var updateExpirationTime = update.expirationTime;

      if (updateExpirationTime < renderExpirationTime) {
        ...
      } else {
        ...

        if (update.eagerReducer === reducer) {
          // If this update was processed eagerly, and its reducer matches the
          // current reducer, we can use the eagerly computed state.
          newState = update.eagerState;
        } else {
          var action = update.action;
          newState = reducer(newState, action);
        }
      }

      update = update.next;
    } while (update !== null && update !== first);

    if (newBaseQueueLast === null) {
      newBaseState = newState;
    } else {
      newBaseQueueLast.next = newBaseQueueFirst;
    } // Mark that the fiber performed work, but only if the new state is
    // different from the current state.


    if (!objectIs(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }

    hook.memoizedState = newState;
    hook.baseState = newBaseState;
    hook.baseQueue = newBaseQueueLast;
    queue.lastRenderedState = newState;
  }

  var dispatch = queue.dispatch;
  return [hook.memoizedState, dispatch];
}

updateWorkInProgressHook

1. 我们知道在初次创建时会将hook链挂载到fiber上， 因此再次更新时只需从fiber上取得即可
2. 当组件update再次进入到function方法时，第一次调用useSate currentHook无值， 通过alternate进行获取； 而再次调用useSate通过next获取即可。

function updateWorkInProgressHook() {
  var nextCurrentHook;

  if (currentHook === null) {  
    var current = currentlyRenderingFiber$1.alternate;

    if (current !== null) {
      nextCurrentHook = current.memoizedState;
    } else {
      nextCurrentHook = null;
    }
  } else {
    nextCurrentHook = currentHook.next;
  }

  var nextWorkInProgressHook;

  if (workInProgressHook === null) {
    nextWorkInProgressHook = currentlyRenderingFiber$1.memoizedState;
  } else {
    nextWorkInProgressHook = workInProgressHook.next;
  }

  if (nextWorkInProgressHook !== null) {
    // There's already a work-in-progress. Reuse it.
    workInProgressHook = nextWorkInProgressHook;
    nextWorkInProgressHook = workInProgressHook.next;
    currentHook = nextCurrentHook;
  } else {
    // Clone from the current hook.
    if (!(nextCurrentHook !== null)) {
      {
        throw Error( "Rendered more hooks than during the previous render." );
      }
    }

    currentHook = nextCurrentHook;
    var newHook = {
      memoizedState: currentHook.memoizedState,
      baseState: currentHook.baseState,
      baseQueue: currentHook.baseQueue,
      queue: currentHook.queue,
      next: null
    };

    if (workInProgressHook === null) {
      // This is the first hook in the list.
      currentlyRenderingFiber$1.memoizedState = workInProgressHook = newHook;
    } else {
      // Append to the end of the list.
      workInProgressHook = workInProgressHook.next = newHook;
    }
  }

  return workInProgressHook;
}