05-2-performUnitOfWork

performUnitOfWork 作用

创建下一个 Fiber 节点，并赋值给 workInProgress，同时把 workInProgress 与已创建的 Fiber 节点连接起来构成 Fiber 树。

  function performUnitOfWork(unitOfWork) {
    // The current, flushed, state of this fiber is the alternate. Ideally
    // nothing should rely on this, but relying on it here means that we don't
    // need an additional field on the work in progress.
    var current = unitOfWork.alternate;
    setCurrentFiber(unitOfWork);
    var next;

    if ((unitOfWork.mode & ProfileMode) !== NoMode) {
      startProfilerTimer(unitOfWork);
      //对当前节点进行协调，如果存在子节点，则返回子节点的引用
      next = beginWork$1(current, unitOfWork, subtreeRenderLanes);
      stopProfilerTimerIfRunningAndRecordDelta(unitOfWork, true);
    } else {
      next = beginWork$1(current, unitOfWork, subtreeRenderLanes);
    }

    resetCurrentFiber();
    unitOfWork.memoizedProps = unitOfWork.pendingProps;

    //如果无子节点，则代表当前的child链表已经遍历完
    if (next === null) {
      // If this doesn't spawn new work, complete the current work.
      //此函数内部会帮我们找到下一个可执行的节点
      completeUnitOfWork(unitOfWork);
    } else {
      workInProgress = next;
    }

    ReactCurrentOwner$2.current = null;
  }