不要使用 dispatch_get_current_queue()

当然这个方法在iOS6.0就废弃了，这里主要是重温一下同步、异步以及死锁。

- (NSString *)name
{__block NSString *name;dispatch_sync(_syncQueue, ^{name = @"不服不行";});return name;
}

上面的这种写法可能会出现死锁，假如调用获取方法的队列恰好是同步操作所在的队列。那么dispatch_sync就会一直不返回，需要等待块执行完毕。
这里就想到使用dispatch_get_current_queue来解决问题。

- (NSString *)name
{__block NSString *name;void (^block)() = ^{name = @"iOS";};if (dispatch_get_current_queue() == _syncQueue) {block();}else {dispatch_sync(_syncQueue, block);}return name;
}

但是 dispatch_get_current_queue 仍然可能造成死锁

  dispatch_sync(_syncQueueA, ^{dispatch_sync(_syncQueueB, ^{void (^block)() = ^{};if (dispatch_get_current_queue() == _syncQueueA) {block();}else {dispatch_sync(_syncQueueA, block);}});});

dispatch_get_current_queue 由于返回的是当前队列，上面返回的是_syncQueueB。因此，针对_syncQueueA的同步派发依然会执行，所以还是会造成死锁。
其实要解决这个问题，可以使用队列特有数据 dispatch_queue_set_specific。只要去看源码就会发现 GPUImage(https://github.com/BradLarson/GPUImage) 等多数框架都是这么解决的。

GPUImage中如何做到 统一在某个队列执行 并防止死锁的

@implementation GPUImageContextstatic void *openGLESContextQueueKey;- (id)init;
{if (!(self = [super init])){return nil;}openGLESContextQueueKey = &openGLESContextQueueKey;_contextQueue = dispatch_queue_create("com.sunsetlakesoftware.GPUImage.openGLESContextQueue", NULL);#if OS_OBJECT_USE_OBJCdispatch_queue_set_specific(_contextQueue, openGLESContextQueueKey, (__bridge void *)self, NULL);
#endifshaderProgramCache = [[NSMutableDictionary alloc] init];shaderProgramUsageHistory = [[NSMutableArray alloc] init];return self;
}+ (void *)contextKey {return openGLESContextQueueKey;
}// GPUImage/Output/GPUImageOutput.mvoid runSynchronouslyOnVideoProcessingQueue(void (^block)(void))
{dispatch_queue_t videoProcessingQueue = [GPUImageContext sharedContextQueue];
#if !OS_OBJECT_USE_OBJC
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"if (dispatch_get_current_queue() == videoProcessingQueue)
#pragma clang diagnostic pop
#elseif (dispatch_get_specific([GPUImageContext contextKey]))
#endif{block();}else{if(videoProcessingQueue)dispatch_sync(videoProcessingQueue, block);else{
//            NSLog(@"error: videoProcessingQueue should not be null");}}
}void runAsynchronouslyOnVideoProcessingQueue(void (^block)(void))
{dispatch_queue_t videoProcessingQueue = [GPUImageContext sharedContextQueue];#if !OS_OBJECT_USE_OBJC
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"if (dispatch_get_current_queue() == videoProcessingQueue)
#pragma clang diagnostic pop
#elseif (dispatch_get_specific([GPUImageContext contextKey]))
#endif{block();}else{dispatch_async(videoProcessingQueue, block);}
}

参考：

https://www.jianshu.com/p/a9066dde1f2b/

https://juejin.cn/post/7133204939475517477#heading-2