细数iOS中的线程同步方案(二) 细数iOS中的线程同步方案(二

细数iOS中的线程同步方案(一)
细数iOS中的线程同步方案(二)

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NSLock 这个其实就是对pthread_mutex普通互斥锁的封装；面向对象，使用起来更方便；

- (void)lock; - (void)unlock; - (BOOL)tryLock; - (BOOL)lockBeforeDate:(NSDate *)limit;

NSRecursiveLock 递归锁对pthread_mutex递归锁的封装，方法和上面的一样；
NSCondition 对pthread_cond条件锁的封装，使用pthread_cond需要配合pthread_mutex互斥锁使用，NSCondition封装好了,一把锁就能实现：

NSCondition *lock = [[NSCondition alloc] init]; NSOperationQueue *queue = [[NSOperationQueue alloc] init]; [queue addOperationWithBlock:^{ [lock lock]; while (self.condition_value <= 0) { // 条件成立则暂时解锁并等待 [lock wait]; }NSLog(@"%@===read===start",[NSThread currentThread]); sleep(2); NSLog(@"%@===read===end",[NSThread currentThread]); [lock unlock]; }]; [queue addOperationWithBlock:^{ [lock lock]; NSLog(@"%@===write===start",[NSThread currentThread]); sleep(3); self.condition_value = https://www.it610.com/article/1; // 一定要更改条件否则上面read线程条件成立又会wait NSLog(@"%@===write===end",[NSThread currentThread]); [lock signal]; // 传递信号给等待的线程而且是在解锁前 //[lock broadcast] // 通知所有线程[lock unlock]; }];

NSConditionLock 对NSCondition的进一步封装，在NSCondition基础上，加了可控制的条件condition；通过条件变量，控制通知哪条线程；

@property (readonly) NSInteger condition;

NSConditionLock *lock = [[NSConditionLock alloc] initWithCondition:1]; // 初始化，设置condition=1NSOperationQueue *queue = [[NSOperationQueue alloc] init]; [queue addOperationWithBlock:^{ [lock lockWhenCondition:1]; // 当condition=1时获取锁成功否则等待（但是首次使用lockWhenCondition时condition不对时也能获取锁成功）NSLog(@"%@===A===start",[NSThread currentThread]); sleep(2); NSLog(@"%@===A===end",[NSThread currentThread]); // unlock根据不同的条件控制对应的线程 [lock unlockWithCondition:2]; // 解锁，同时设置condition=2并signal； //[lock unlockWithCondition:3]; }]; [queue addOperationWithBlock:^{ [lock lockWhenCondition:2]; NSLog(@"%@===B===start",[NSThread currentThread]); sleep(1); NSLog(@"%@===B===end",[NSThread currentThread]); [lock unlock]; }]; [queue addOperationWithBlock:^{ [lock lockWhenCondition:3]; NSLog(@"%@===C===start",[NSThread currentThread]); sleep(1); NSLog(@"%@===C===end",[NSThread currentThread]); [lock unlock]; }];

线程A解锁时可以传不同条件值，对应条件值的其他等待线程就会被唤醒；这里条件值为2，则执行线程B任务；条件设置为3，则执行线程C任务；如果是其他值则线程B，C继续一直等待；

NSThread: 0x282b66340>{number = 6, name = (null)}===A===start NSThread: 0x282b66340>{number = 6, name = (null)}===A===end NSThread: 0x282b68240>{number = 3, name = (null)}===B===start NSThread: 0x282b68240>{number = 3, name = (null)}===B===end

@synchronized 是对mutex递归锁的封装；
@synchronized(obj)内部会生成obj对应的递归锁，然后进行加锁、解锁操作；一个对象对应一把锁；

NSObject *obj = [[NSObject alloc] init]; @synchronized (obj) { // ... }

GCD相关 dispatch_semaphore信号量这个和上篇讲的semaphore差不多；

// 创建信号量 dispatch_semaphore_t sem = dispatch_semaphore_create(1); // 判断信号量，如果=0则等待，否则信号值-1往下执行 dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER); // 发送信号量，信号值+1 dispatch_semaphore_signal(sem);

DISPATCH_QUEUE_SERIAL 串行队列串行队列的任务就是同步执行的；

dispatch_queue_t queue = dispatch_queue_create("serial_queue", DISPATCH_QUEUE_SERIAL); dispatch_async(queue, ^{ // ThreadA dosomething.... }); dispatch_async(queue, ^{ // ThreadB dosomething.... });

dispatch_group 将任务分组，组内任务异步执行；当所有任务执行完后，可以通知其他线程执行任务：

// group必须使用自己创建的并发队列使用global全局队列无效 dispatch_queue_t queue = dispatch_queue_create("concurrent_queue", DISPATCH_QUEUE_CONCURRENT); //dispatch_queue_t queue = dispatch_get_global_queue(0, 0); xxxdispatch_group_t group = dispatch_group_create(); dispatch_group_async(group, queue, ^{ sleep(1); NSLog(@"%@===TaskA",[NSThread currentThread]); }); dispatch_group_async(group, queue, ^{ sleep(1); NSLog(@"%@===TaskB",[NSThread currentThread]); }); dispatch_group_notify(group, queue, ^{ NSLog(@"%@===TaskC",[NSThread currentThread]); }); //dispatch_async(queue, ^{ //dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC))); // 可以设置等待的超时时间 //NSLog(@"%@===TaskC",[NSThread currentThread]); //});

以上代码对应的场景就是：A,B线程可以并发执行，但C线程一定要在AB线程执行完后再执行；
dispatch_group_notify也可以使用dispatch_group_wait替代，一样是阻塞的作用，而dispatch_group_wait能设置等待超时时间；超过时间将不再阻塞，继续任务；
还有一点需要注意的是，dispatch_group必须使用自己创建的并发队列，使用global全局队列无效，使用串行队列没有意义；
dispatch_barrier 如同它的名字一样，dispatch_barrier就是起到一个栅栏的作用；栅栏两边的任务可以并发执行，栅栏里的任务必须等到栅栏上边的任务执行完才执行，栅栏下边的任务必须等栅栏里的任务执行完后才执行；
dispatch_barrier其实就是阻塞队列的作用；
这个其实也可以通过dispatch_group实现，但dispatch_barrier更加方便；

dispatch_queue_t queue = dispatch_queue_create("concurrent_queue", DISPATCH_QUEUE_CONCURRENT); dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===TaskA",[NSThread currentThread]); }); dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===TaskB",[NSThread currentThread]); }); // async不会阻塞当前线程（主线程） dispatch_barrier_async(queue, ^{ NSLog(@"%@===Barrier",[NSThread currentThread]); }); // sync会阻塞当前队列（主队列） //dispatch_barrier_sync(queue, ^{ //NSLog(@"%@===Barrier",[NSThread currentThread]); //}); dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===TaskC",[NSThread currentThread]); }); dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===TaskD",[NSThread currentThread]); }); NSLog(@"%@===MainTask",[NSThread currentThread]);

{number = 1, name = main}===MainTask {number = 3, name = (null)}===TaskB {number = 4, name = (null)}===TaskA {number = 4, name = (null)}===Barrier {number = 3, name = (null)}===TaskD {number = 4, name = (null)}===TaskC

dispatch_barrier的使用有两种方式

dispatch_barrier_async
dispatch_barrier_sync

async不会阻塞当前队列，sync同时会阻塞当前队列；如果以上代码换成dispatch_barrier_sync，最终的结果将是MainTask会在Barrier任务后；
基于barrier的这种特性，很容易实现一个读写锁；栅栏内为write，栅栏外为read；这样同样能实现读任务能异步执行，写任务只能同步执行；同时在写操作时，不允许读操作；

dispatch_queue_t queue = dispatch_queue_create("concurrent_queue", DISPATCH_QUEUE_CONCURRENT); for (int i = 0; i < 3; i ++) { dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===read",[NSThread currentThread]); }); }for (int i = 0; i < 3; i ++) { dispatch_barrier_async(queue, ^{ sleep(1); NSLog(@"%@===write",[NSThread currentThread]); }); }for (int i = 0; i < 3; i ++) { dispatch_async(queue, ^{ sleep(1); NSLog(@"%@===read",[NSThread currentThread]); }); }

{number = 4, name = (null)}===read {number = 6, name = (null)}===read {number = 5, name = (null)}===read {number = 1, name = main}===write {number = 1, name = main}===write {number = 1, name = main}===write {number = 5, name = (null)}===read {number = 7, name = (null)}===read {number = 4, name = (null)}===read

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NSOperation相关 NSOperation是对GCD的封装
最大并发数

NSOperationQueue *queue = [[NSOperationQueue alloc] init]; // 最大并发数设置为1，queue内任务同步执行 queue.maxConcurrentOperationCount = 1;

设置栅栏

// similarly to the `dispatch_barrier_async` function. [queue addBarrierBlock:^{}];

设置依赖关系使用场景：线程B必须要等线程A任务执行完后才执行，即线程A依赖线程B:

NSOperationQueue *queue = [[NSOperationQueue alloc] init]; NSBlockOperation *taskA = [NSBlockOperation blockOperationWithBlock:^{ sleep(2); NSLog(@"%@===TaskA",[NSThread currentThread]); }]; NSBlockOperation *taskB = [NSBlockOperation blockOperationWithBlock:^{ sleep(.5); NSLog(@"%@===TaskB",[NSThread currentThread]); }]; [taskB addDependency:taskA]; [queue addOperation:taskA]; [queue addOperation:taskB];