react源码学习之Scheduler

简介

Scheduler主要用于调度执行Reconciler

组成

api

• unstable_scheduleCallback
• unstable_cancelCallback

调度回调有两种方式

• setTimeout
• 通过MessageChannel

Task

为任务调度单元

type Task={id:number,callback:Callback|null,priorityLevel:PriorityLevel,startTime:number,expirationTime:number,sortIndex:number,isQueued?:boolean,};

双堆

维护两个最小堆，按Task的sortIndex排序

• taskQueue：排序键为expirationTime，startTime小于等于当前时间currentTime的任务放入此堆中
• timerQueue：排序键为startTime，startTime大于当前时间currentTime的任务放入此堆中

vartaskQueue:Array<Task>=[];vartimerQueue:Array<Task>=[];

优先级 到超时时间 的映射

调度工作循环workLoop

functionworkLoop(initialTime:number){letcurrentTime=initialTime;advanceTimers(currentTime);currentTask=peek(taskQueue);while(currentTask!==null){if(!enableAlwaysYieldScheduler){if(currentTask.expirationTime>currentTime&&shouldYieldToHost()){// This currentTask hasn't expired, and we've reached the deadline.break;}}// $FlowFixMe[incompatible-use] found when upgrading Flowconstcallback=currentTask.callback;if(typeofcallback==='function'){// $FlowFixMe[incompatible-use] found when upgrading FlowcurrentTask.callback=null;// $FlowFixMe[incompatible-use] found when upgrading FlowcurrentPriorityLevel=currentTask.priorityLevel;// $FlowFixMe[incompatible-use] found when upgrading FlowconstdidUserCallbackTimeout=currentTask.expirationTime<=currentTime;if(enableProfiling){// $FlowFixMe[incompatible-call] found when upgrading FlowmarkTaskRun(currentTask,currentTime);}constcontinuationCallback=callback(didUserCallbackTimeout);currentTime=getCurrentTime();if(typeofcontinuationCallback==='function'){// If a continuation is returned, immediately yield to the main thread// regardless of how much time is left in the current time slice.// $FlowFixMe[incompatible-use] found when upgrading FlowcurrentTask.callback=continuationCallback;if(enableProfiling){// $FlowFixMe[incompatible-call] found when upgrading FlowmarkTaskYield(currentTask,currentTime);}advanceTimers(currentTime);returntrue;}else{if(enableProfiling){// $FlowFixMe[incompatible-call] found when upgrading FlowmarkTaskCompleted(currentTask,currentTime);// $FlowFixMe[incompatible-use] found when upgrading FlowcurrentTask.isQueued=false;}if(currentTask===peek(taskQueue)){pop(taskQueue);}advanceTimers(currentTime);}}else{pop(taskQueue);}currentTask=peek(taskQueue);if(enableAlwaysYieldScheduler){if(currentTask===null||currentTask.expirationTime>currentTime){// This currentTask hasn't expired we yield to the browser task.break;}}}// Return whether there's additional workif(currentTask!==null){returntrue;}else{constfirstTimer=peek(timerQueue);if(firstTimer!==null){requestHostTimeout(handleTimeout,firstTimer.startTime-currentTime);}returnfalse;}}

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