今天突然发现同步异步和我想的不太一样，特地记录一下。
在主线程执行：
dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t concurrentQueue = dispatch_queue_create("concurrentQueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"主线程执行同步任务到主队列, 挂起当前任务，主线程死锁");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"主线程执行异步任务到主队列, 相当于delay执行");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
dispatch_sync(serialQueue, ^{
NSLog(@"主线程执行同步任务到串行队列, 挂起当前任务，不开启线程，在主线程执行不同任务");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
dispatch_sync(concurrentQueue, ^{
NSLog(@"主线程执行同步任务到并行队列, 挂起当前任务，不开启线程，在主线程执行同步任务");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
dispatch_async(serialQueue, ^{
NSLog(@"主线程执行异步任务到串行队列，开启新线程，在新线程执行任务");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
NSLog(@"主线程执行异步任务到并行队列，开启新线程，在新线程执行任务");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
在子线程执行：
dispatch_async(concurrentQueue, ^{
NSLog(@"当前是子线程");
NSLog(@"current thread is %@", [NSThread currentThread]);
for (int i = 0; i < 10; i++) {
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"在子线程中执行同步任务到组队列, 挂起当前任务，到主线程执行");
NSLog(@"在子线程中执行同步任务到主队列, 挂起当前任务，到主线程执行 current thread is %@", [NSThread currentThread]);
});
}
for (int i = 0; i < 10; i++) {
dispatch_sync(serialQueue, ^{
NSLog(@"在子线程中执行同步任务到串行队列，不开启新线程，在当前当先线程执行");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
}
for (int i = 0; i < 10; i++) {
dispatch_sync(concurrentQueue, ^{
NSLog(@"在子线程执行同步任务到并行队列，不开启新线程，在当前线程执行");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
}
for (int i = 0; i < 10; i ++) {
dispatch_async(serialQueue, ^{
NSLog(@"在子线程异步执行到串行队列，在另外一个子线程10个任务依次执行");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
}
for (int i = 0; i < 10; i ++) {
dispatch_async(concurrentQueue, ^{
NSLog(@"在子线程执行异步任务到并行队列，可能会开启子线程，也有可能在原来线程执行");
NSLog(@"current thread is %@", [NSThread currentThread]);
});
}
});
总结：
- 在当前线程中执行同步任务到其他队列，不开启子线程，在当前线程执行；除非队列为组队列，会在主线程执行；
- 在当前线程中执行异步任务到其他队列，一般会开启子线程，也有可能在当前线程执行
- 同步和异步的唯一区别是，会不会暂停当前任务
