react source code analysis 12 Status update process

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setState&forceUpdate

Several ways to trigger status update in react:

• ReactDOM.render
• this.setState
• this.forceUpdate
• useState
• useReducer

Let's focus on this Setstate and this Forceupdate, hook is discussed in Chapter 13

1. this. Call this in setstate updater. Enqueuesetstate is mainly used to add update to updateQueue

   //ReactBaseClasses.js
   Component.prototype.setState = function (partialState, callback) {
     if (!(typeof partialState === 'object' || typeof partialState === 'function' || partialState == null)) {
       {
         throw Error( "setState(...): takes an object of state variables to update or a function which returns an object of state variables." );
       }
     }
     this.updater.enqueueSetState(this, partialState, callback, 'setState');
   };
   

//ReactFiberClassComponent.old.js
enqueueSetState(inst, payload, callback) {
  const fiber = getInstance(inst);//fiber instance

  const eventTime = requestEventTime();
  const suspenseConfig = requestCurrentSuspenseConfig();
  
  const lane = requestUpdateLane(fiber, suspenseConfig);//priority

  const update = createUpdate(eventTime, lane, suspenseConfig);//Create update

  update.payload = payload;

  if (callback !== undefined && callback !== null) {  //Assignment callback
    update.callback = callback;
  }

  enqueueUpdate(fiber, update);//update join updateQueue
  scheduleUpdateOnFiber(fiber, lane, eventTime);//Schedule update
}

enqueueUpdate is used to add update to the updateQueue queue

//ReactUpdateQueue.old.js
export function enqueueUpdate<State>(fiber: Fiber, update: Update<State>) {
  const updateQueue = fiber.updateQueue;
  if (updateQueue === null) {
    return;
  }

  const sharedQueue: SharedQueue<State> = (updateQueue: any).shared;
  const pending = sharedQueue.pending;
  if (pending === null) {
    update.next = update;//Form a circular linked list with yourself
  } else {
    update.next = pending.next;//Add to the end of the linked list
    pending.next = update;
  }
  sharedQueue.pending = update;
}

1. this.forceUpdate and this It is the same as setstate, but it will assign the tag ForceUpdate

   //ReactBaseClasses.js
   Component.prototype.forceUpdate = function(callback) {
     this.updater.enqueueForceUpdate(this, callback, 'forceUpdate');
   };
   

//ReactFiberClassComponent.old.js
enqueueForceUpdate(inst, callback) {
    const fiber = getInstance(inst);
    const eventTime = requestEventTime();
    const suspenseConfig = requestCurrentSuspenseConfig();
    const lane = requestUpdateLane(fiber, suspenseConfig);

    const update = createUpdate(eventTime, lane, suspenseConfig);
    
    //tag assignment ForceUpdate
    update.tag = ForceUpdate;
    
    if (callback !== undefined && callback !== null) {
      update.callback = callback;
    }
    
    enqueueUpdate(fiber, update);
    scheduleUpdateOnFiber(fiber, lane, eventTime);

  },
};

If ForceUpdate is marked, the component Update in the render phase will be judged according to checkHasForceUpdateAfterProcessing and checkShouldComponentUpdate. If the Update tag is ForceUpdate, checkHasForceUpdateAfterProcessing is true. When the component is PureComponent, checkShouldComponentUpdate will shallow compare state and props, so when using this ForceUpdate will be updated

//ReactFiberClassComponent.old.js
const shouldUpdate =
  checkHasForceUpdateAfterProcessing() ||
  checkShouldComponentUpdate(
    workInProgress,
    ctor,
    oldProps,
    newProps,
    oldState,
    newState,
    nextContext,
  );

Overall process of status update

Update&updateQueue

After the update is triggered by HostRoot or ClassComponent, an update will be created in the function createUpdate, and the update will be calculated in beginWork in the later render stage. The update corresponding to functional component is described in Chapter 11. It is somewhat different from the update structure of HostRoot or ClassComponent

//ReactUpdateQueue.old.js
export function createUpdate(eventTime: number, lane: Lane): Update<*> {//Create update
  const update: Update<*> = {
    eventTime,
    lane,

    tag: UpdateState,
    payload: null,
    callback: null,

    next: null,
  };
  return update;
}

We mainly focus on these parameters:

• lane: priority (Chapter 12)
• tag: type of update, such as UpdateState and ReplaceState
• Payload: the payload of ClassComponent is the first parameter of setState, and the payload of HostRoot is reactdom The first parameter of render
• callback: second parameter of setState
• Next: connect the next Update to form a linked list. For example, when multiple setstates are triggered at the same time, multiple updates will be formed, and then connect with next

For HostRoot or ClassComponent, an updateQueue will be created using initializeUpdateQueue during mount, and then the updateQueue will be mounted on the fiber node

//ReactUpdateQueue.old.js
export function initializeUpdateQueue<State>(fiber: Fiber): void {
  const queue: UpdateQueue<State> = {
    baseState: fiber.memoizedState,
    firstBaseUpdate: null,
    lastBaseUpdate: null,
  shared: {
      pending: null,
    },
    effects: null,
  };
fiber.updateQueue = queue;
}

• baseState: initial state. Based on this state, a new state will be calculated according to Update
• firstBaseUpdate and lastBaseUpdate: the head and tail of the linked list formed by Update
• shared.pending: the newly generated update will be saved in shared. Net as a one-way circular linked list On pending, when calculating the state, the circular linked list will be cut off and linked after lastBaseUpdate
• effects: update whose callback is not null

Traverse upward from the fiber node that triggered the update to rootFiber

In the markUpdateLaneFromFiberToRoot function, the node triggering the update will be traversed upward to rootFiber. The traversal process will deal with the priority of the node (described in Chapter 15)

//ReactFiberWorkLoop.old.js
function markUpdateLaneFromFiberToRoot(
    sourceFiber: Fiber,
    lane: Lane,
  ): FiberRoot | null {
    sourceFiber.lanes = mergeLanes(sourceFiber.lanes, lane);
    let alternate = sourceFiber.alternate;
    if (alternate !== null) {
      alternate.lanes = mergeLanes(alternate.lanes, lane);
    }
    let node = sourceFiber;
    let parent = sourceFiber.return;
    while (parent !== null) {//Start from the node triggering the update and traverse upward to rootFiber
      parent.childLanes = mergeLanes(parent.childLanes, lane);//Merge childLanes priority
      alternate = parent.alternate;
      if (alternate !== null) {
        alternate.childLanes = mergeLanes(alternate.childLanes, lane);
      } else {
      }
      node = parent;
      parent = parent.return;
    }
    if (node.tag === HostRoot) {
      const root: FiberRoot = node.stateNode;
      return root;
    } else {
      return null;
    }
  }

For example, node B triggers an update. Node B is marked as normal update, that is, u1 in the figure, and then traverses upward to the root node, marking a normal update on the root node. If node B triggers another userBlocking update, it will also traverse upward to the root node, marking a userBlocking update on the root node.

If the update priority of the current root node is normal, both u1 and u2 participate in the status calculation. If the update priority of the current root node is userBlocking, only u2 participates in the calculation

dispatch

In ensureroot isscheduled, scheduleCallback will schedule the start function performSyncWorkOnRoot or performcurrentworkonroot of the render phase with a priority

//ReactFiberWorkLoop.old.js
if (newCallbackPriority === SyncLanePriority) {
  // The task has expired and the render phase needs to be executed synchronously
  newCallbackNode = scheduleSyncCallback(
    performSyncWorkOnRoot.bind(null, root)
  );
} else {
  // The render phase is executed asynchronously according to the task priority
  var schedulerPriorityLevel = lanePriorityToSchedulerPriority(
    newCallbackPriority
  );
  newCallbackNode = scheduleCallback(
    schedulerPriorityLevel,
    performConcurrentWorkOnRoot.bind(null, root)
  );
}

Status update

The classComponent status calculation occurs in the processUpdateQueue function and involves many linked list operations. It is more straightforward to see the figure

• Initial fiber The updatequeue single linked list has firstBaseUpdate (update1) and lastBaseUpdate (update2), which are connected by next
• fiber. updateQueue. There are update3 and update4 on the shared circular linked list, which are connected with each other by next connection
• When calculating state, first set fiber updateQueue. The shared circular linked list is' cut open 'to form a single linked list, which is connected to fiber baseUpdate is formed after updatequeue

• Then traverse the linked list and calculate the memoizedState according to the baseState

  

Status update with priority

Similar to git submission, c3 here means high priority tasks, such as user departure events, data requests, synchronously executed code, etc.

• Through reactdom The application created by render does not have the concept of priority. Compared with git submission, it is equivalent to commit ting first and then submitting c3
  

• In the concurrent mode, similar to git rebase, the previous code is temporarily stored, developed on the master, and then rebased to the previous branch

  The priority is scheduled by the Scheduler. Here, we only care about the priority ranking during status calculation, that is, the calculation in the function processUpdateQueue. For example, there are four updates c1-c4 at the beginning, of which c1 and c3 are high priority

  1. In the first render, low priority update s will be skipped, so only c1 and c3 are added to the state calculation
  2. In the second render, the update (c1) before the skipped update (c2) in the first will be used as the baseState, and the skipped update and subsequent updates (c2, c3, c4) will be recalculated as the baseUpdate

  In concurrent mode, componentWillMount may execute multiple times, which is inconsistent with the previous version

  Attention, fiber updateQueue. Shared exists in workInprogress Fiber and current Fiber at the same time. The purpose is to prevent high priority from interrupting the ongoing calculation and causing state loss. This code also occurs in processUpdateQueue

Look at the demo_ Priority of 8

Now let's look at the function that calculates the state

//ReactUpdateQueue.old.js
export function processUpdateQueue<State>(
  workInProgress: Fiber,
  props: any,
  instance: any,
  renderLanes: Lanes,
): void {
  const queue: UpdateQueue<State> = (workInProgress.updateQueue: any);
  hasForceUpdate = false;

  let firstBaseUpdate = queue.firstBaseUpdate;//First Update of updateQueue
  let lastBaseUpdate = queue.lastBaseUpdate;//Last Update of updateQueue
  let pendingQueue = queue.shared.pending;//Uncomputed pendingQueue

  if (pendingQueue !== null) {
    queue.shared.pending = null;
    const lastPendingUpdate = pendingQueue;//The last update of an uncomputed ppendingQueue
    const firstPendingUpdate = lastPendingUpdate.next;//The first update of an uncomputed pendingQueue
    lastPendingUpdate.next = null;//Cut open the circular linked list
    if (lastBaseUpdate === null) {//Add pendingQueue to updateQueue
      firstBaseUpdate = firstPendingUpdate;
    } else {
      lastBaseUpdate.next = firstPendingUpdate;
    }
    lastBaseUpdate = lastPendingUpdate;

    const current = workInProgress.alternate;//Do the same on current
    if (current !== null) {
      const currentQueue: UpdateQueue<State> = (current.updateQueue: any);
      const currentLastBaseUpdate = currentQueue.lastBaseUpdate;
      if (currentLastBaseUpdate !== lastBaseUpdate) {
        if (currentLastBaseUpdate === null) {
          currentQueue.firstBaseUpdate = firstPendingUpdate;
        } else {
          currentLastBaseUpdate.next = firstPendingUpdate;
        }
        currentQueue.lastBaseUpdate = lastPendingUpdate;
      }
    }
  }

  if (firstBaseUpdate !== null) {
    let newState = queue.baseState;

    let newLanes = NoLanes;

    let newBaseState = null;
    let newFirstBaseUpdate = null;
    let newLastBaseUpdate = null;

    let update = firstBaseUpdate;
    do {
      const updateLane = update.lane;
      const updateEventTime = update.eventTime;
      if (!isSubsetOfLanes(renderLanes, updateLane)) {//Determine whether priority is enough
        const clone: Update<State> = {//The priority is not enough. Skip the current update
          eventTime: updateEventTime,
          lane: updateLane,

          tag: update.tag,
          payload: update.payload,
          callback: update.callback,

          next: null,
        };
        if (newLastBaseUpdate === null) {//Save skipped update
          newFirstBaseUpdate = newLastBaseUpdate = clone;
          newBaseState = newState;
        } else {
          newLastBaseUpdate = newLastBaseUpdate.next = clone;
        }
        newLanes = mergeLanes(newLanes, updateLane);
      } else {
        //It will not be calculated until newLastBaseUpdate is null, so as to prevent the updateQueue from not completing the calculation
        if (newLastBaseUpdate !== null) {
          const clone: Update<State> = {
            eventTime: updateEventTime,
            lane: NoLane,

            tag: update.tag,
            payload: update.payload,
            callback: update.callback,

            next: null,
          };
          newLastBaseUpdate = newLastBaseUpdate.next = clone;
        }

        newState = getStateFromUpdate(//Calculate state based on updateQueue
          workInProgress,
          queue,
          update,
          newState,
          props,
          instance,
        );
        const callback = update.callback;
        if (callback !== null) {
          workInProgress.flags |= Callback;//Callback flag
          const effects = queue.effects;
          if (effects === null) {
            queue.effects = [update];
          } else {
            effects.push(update);
          }
        }
      }
      update = update.next;//Next update
      if (update === null) {//Reset updateQueue
        pendingQueue = queue.shared.pending;
        if (pendingQueue === null) {
          break;
        } else {
          const lastPendingUpdate = pendingQueue;

          const firstPendingUpdate = ((lastPendingUpdate.next: any): Update<State>);
          lastPendingUpdate.next = null;
          update = firstPendingUpdate;
          queue.lastBaseUpdate = lastPendingUpdate;
          queue.shared.pending = null;
        }
      }
    } while (true);

    if (newLastBaseUpdate === null) {
      newBaseState = newState;
    }

    queue.baseState = ((newBaseState: any): State);//New state
    queue.firstBaseUpdate = newFirstBaseUpdate;//New first update
    queue.lastBaseUpdate = newLastBaseUpdate;//New last update

    markSkippedUpdateLanes(newLanes);
    workInProgress.lanes = newLanes;
    workInProgress.memoizedState = newState;
  }

    //...
}