多线程中GCD(Grand Central Dispatch)是我们比较常用,今天对于其的常见用法做以总结.
-
Dispatch Queue的种类一共有两种:
- Serial Dispatch Queue
- Concurrent Dispatch Queue
即串行和并行队列.当为串行队列时，加入到队列的任务需要等待上一个执行完才进行下一个,而并行队列时无需等待即可紧接着执行下一个任务.
创建一个串行队列:
dispatch_queue_t mySerialDispatchQueue = dispatch_queue_create("io.github.junne.serialDispatchQueue", NULL);
dispatch_queue_t mySerialDispatchQueue = dispatch_queue_create("io.github.junne.serialDispatchQueue", DISPATCH_QUEUE_SERIAL);
这里创建队列的第二个参数,NULL等同于DISPATCH_QUEUE_SERIAL.
创建一个并行队列:
dispatch_queue_t myConcurrentDispatchQueue = dispatch_queue_create("io.github.junne.concurrentDispatchQueue", DISPATCH_QUEUE_CONCURRENT);
- 系统提供的队列
- Main Dispatch Queue(Serial Dispatch Queue)
- Global Dispatch Queue(Concurrent Dispatch Queue)
其中追加到Main Dispatch Queue的处理在主线程的Runloop执行,一般用于UI更新等必须在主线程进行的操作.而Global Dispatch Queue相当于系统为我们用dispatch_queue_create生成的一个Concurrent Dispatch Queue,我们可以将一些多线程操作追加到这个队列中.
Global Dispatch Queue有四个执行优先级. High Priority、Default Priority、Low Priority和Background Priority,创建方法为:
dispatch_queue_t globalDispatchQueueHigh = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_queue_t globalDispatchQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t globalDispatchQueueLow = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
dispatch_queue_t globalDispatchQueueBackgroungn = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
- dispatch_set_target_queue
dispatch_queue_create创建的不论是串行还是并行队列,都使用与默认优先级的Global Dispatch Queue相同的执行优先级的线程.
dispatch_queue_t mySerialDispatchQueue = dispatch_queue_create("io.github.junne.serialTestDispatchQueue", NULL);
dispatch_queue_t globalDispatchQueueBackgroud = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
dispatch_set_target_queue(mySerialDispatchQueue, globalDispatchQueueBackgroud);
dispatch_async(mySerialDispatchQueue, ^{
NSLog(@"mySerialDispatchQueue");
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"Default Priority");
});
执行顺序为先Default Priority后mySerialDiapatchQueue.
- dispatch_after
一般可用于延迟多少时间后将一些任务添加到队列,经常用于延后执行操作:
NSLog(@"dispatchAfter One");
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"dispatchAfter Two");
});
上述代码用来延迟三秒后执行打印dispatchAfter Two.
- Dispatch Group
当想要队列中追加的操作处理全部结束时再进行某项操作时,可以使用一个Serial Dispatch Queue将想要执行的操作追加到最后即可.但当使用并行队列或者多个Dispatch Queue时可以使用Dispatch Group:
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, globalQueue, ^{
NSLog(@"Group First");
});
dispatch_group_async(group, globalQueue, ^{
NSLog(@"Group Second");
});
dispatch_group_async(group, globalQueue, ^{
NSLog(@"Group Third");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"Group All Done");
});
当group里面的操作都完成时就会调用dispatch_group_notify里面的操作.
- dispatch_barrier_async
当访问数据库时可以用Serial Dispatch Queue来避免数据竞争.但当想要高效地并行访问数据库时可以使用dispatch_barrier_async:
dispatch_queue_t queue = dispatch_queue_create("io.guthub.junne.barrier", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"Reading First");
});
dispatch_async(queue, ^{
NSLog(@"Reading Second");
});
dispatch_async(queue, ^{
NSLog(@"Reading Third");
});
dispatch_barrier_async(queue, ^{
NSLog(@"Writing First");
});
dispatch_async(queue, ^{
NSLog(@"Reading Fourth");
});
dispatch_async(queue, ^{
NSLog(@"Reading Fifth");
}); dispatch_async(queue, ^{
NSLog(@"Reading Sixth");
});
程序会在执行dispatch_barrier_async之前,完成队列中的所有操作,继而执行barrier里面的内容,当barrier里面的内容执行完毕后再去执行之后的内容.
- dispatch_sync
dispatch_async中的async意味着asynchronous即非同步,就是将block非同步地追加到指定的Dispatch Queue中,不做任何等待.
dispatch_sync意味着synchronous即同步,就是将block同步地追加到指定的Dispatch Queue中,在追加结束之前,会一直等待.
dispatch_sync稍有不慎就会引发死锁:
dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_sync(mainQueue, ^{
NSLog(@"Ha ha ha ha...");
});
在主线程中执行指定的block,并等待其结束，而主线程中这在执行这些源代码,因此无法执行追加到Main Dispatch Queue的Block.
- dispatch_apply
dispatch_apply是dispatch_sync和Dispatch Group的关联API.此函数按指定的次数将block追加到制定的队列中,并等待全部处理结束:
NSArray *testArray = @[@"first",@"second",@"third",@"fourth",@"fifth"];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply([testArray count], queue, ^(size_t index) {
NSLog(@"%zu: %@", index, [testArray objectAtIndex:index]);
});
NSLog(@"TestApple Done");
- dispatch_suspend && dispatch_resume
当我们希望不执行已追加队列中的操作时,只要挂起Dispatch Queue即可,当需要时在恢复即可:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSLog(@"Suspend Start");
dispatch_suspend(queue);
NSLog(@"Susupend");
sleep(2);
dispatch_resume(queue);
NSLog(@"Resume");
});
- Dispatch Semaphore
直接上例子:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
NSMutableArray *mutableArray = [[NSMutableArray alloc] init];
for (int i = 0; i < 10; i++) {
dispatch_async(queue, ^{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
[mutableArray addObject:[NSNumber numberWithInt:i]];
dispatch_semaphore_signal(semaphore);
});
}
NSLog(@"mutableArray = %@", mutableArray);
- dispatch_once
用来保证在应用程序中只执行一次指定处理的API.常用来创建单例:
static ViewController *instance;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
instance = [[ViewController alloc] init];
});
return instance;
参考及拓展阅读:
- Objective-C高级编程(iOS与OS X多线程管理和内存管理)
- 菜鸟不要怕,看一眼,你就会用GCD,带你装逼带你飞
- 细说GCD（Grand Central Dispatch）如何用
- 谈iOS多线程（NSThread、NSOperation、GCD）编程
- Grand Central Dispatch (GCD) Reference
- iOS多线程编程总结
