// 有執(zhí)行任務(wù)
    if (scheduledHostCallback !== null) {
      const currentTime = getCurrentTime();
      // 計算一幀的過期時間點
      deadline = currentTime + yieldInterval;
      const hasTimeRemaining = true;
      try {
        // 執(zhí)行c回調(diào)
        const hasMoreWork = scheduledHostCallback(
          hasTimeRemaining,
          currentTime,
        );
        // 執(zhí)行完該回調(diào)后, 判斷后續(xù)是否還有其他任務(wù)
        if (!hasMoreWork) {
          isMessageLoopRunning = false;
          scheduledHostCallback = null;
        } else {
          // 還有其他任務(wù), 推進(jìn)進(jìn)入下一個宏任務(wù)隊列中
          port.postMessage(null);
        }
      } catch (error) {
        // If a scheduler task throws, exit the current browser task so the
        // error can be observed.
        port.postMessage(null);
        throw error;
      }
    } else {
      isMessageLoopRunning = false;
    }
    // Yielding to the browser will give it a chance to paint, so we can
    // reset this.
    // 重置狀態(tài)
    needsPaint = false;
  };

  const channel = new MessageChannel();
  // port2 發(fā)送
  const port = channel.port2;
  // port1 接收
  channel.port1.onmessage = performWorkUntilDeadline;
  // 在每一幀中執(zhí)行任務(wù)
  requestHostCallback = function(callback) {
    // 回調(diào)注冊
    scheduledHostCallback = callback;
    if (!isMessageLoopRunning) {
      isMessageLoopRunning = true;
      // 進(jìn)入宏任務(wù)隊列
      port.postMessage(null);
    }
  };
  // 取消回調(diào)
  cancelHostCallback = function() {
    scheduledHostCallback = null;
  };
  // 設(shè)置超時回調(diào)
  requestHostTimeout = function(callback, ms) {
    taskTimeoutID = setTimeout(() => {
      callback(getCurrentTime());
    }, ms);
  };
  // 取消超時
  cancelHostTimeout = function() {
    clearTimeout(taskTimeoutID);
    taskTimeoutID = -1;
  };