dispatch_group

通常我们执行耗时操作会放到子线程中并发执行,这个过程中我们可能想知道各个任务全部执行完毕的时刻,当然我们可以通过记录一些标志位等手段来达到要求,但是可能步骤会比较复杂.强大的GCD中的dispatch_group_t可以轻松帮我们达到目的.

我们先来看group中的api

• dispatch_group_async

//将block(任务)提交到指定的队列中,并且将次任务放到(关联)指定的group,block将异步执行.
void dispatch_group_async(dispatch_group_t group,dispatch_queue_t queue,dispatch_block_t block);
//上面方法的函数版本.接受一个函数指针.
void dispatch_group_async_f(dispatch_group_t group,dispatch_queue_t queue,void *_Nullable context,dispatch_function_t work);

• dispatch_group_wait

//这个方法会同步的等,接受一个timeout,group的任务全部完成或者超时以后会返回,返回值为0代表任务完成,非0代表超时.
long dispatch_group_wait(dispatch_group_t group,dispatch_time_t timeout);

• dispatch_group_notify

//当group中的任务都完成以后会执行block.注意这句代码要加到所有任务提交之后才管用.参数queue代表block会提交到哪个队列中.
void dispatch_group_notify(dispatch_group_t group,dispatch_queue_t queue,dispatch_block_t block);
//上面方法的函数版本
void dispatch_group_notify_f(dispatch_group_t group,dispatch_queue_t queue,void *_Nullable context,dispatch_function_t work);

• dispatch_group_enter

//用这个方法指定一个操作将要加到group中,用来替代`dispatch_group_async`,注意它只能和`dispatch_group_leave`配对使用.
void dispatch_group_enter(dispatch_group_t group);

• dispatch_group_leave

//指定一个任务完成了.只能和`dispatch_group_enter`配对使用.
void dispatch_group_leave(dispatch_group_t group);

使用案例

dispatch_group_async

dispatch_queue_t concurrentQueue = dispatch_queue_create("com.GCDDemo.queue",DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, concurrentQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"first download task success! %@",[NSThread currentThread]);
});
dispatch_group_async(group, concurrentQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"second download task success! %@",[NSThread currentThread]);
});
dispatch_group_notify(group, concurrentQueue, ^{
NSLog(@"begin task three! %@",[NSThread currentThread]);
});
输出台:
second download task success! <NSThread: 0x600000279c80>{number = 4, name = (null)}
first download task success! <NSThread: 0x6080002674c0>{number = 3, name = (null)}
begin task three! <NSThread: 0x6080002674c0>{number = 3, name = (null)}

dispatch_group_enter & dispatch_group_leave

• 个人感觉这种方式比dispatch_group_async更加灵活.比如我们可以在任务的完成回调里面写dispatch_group_leave().

dispatch_queue_t concurrentQueue = dispatch_queue_create("com.GCDDemo.concurrentQueue",DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t serialQueue = dispatch_queue_create("com.GCDDemo.serialQueue", DISPATCH_QUEUE_SERIAL);
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"first download task success! %@",[NSThread currentThread]);
dispatch_group_leave(group);
});
dispatch_group_enter(group);
dispatch_async(serialQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"second download task success! %@",[NSThread currentThread]);
dispatch_group_leave(group);
});
dispatch_group_enter(group);
dispatch_async(serialQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"three download task success! %@",[NSThread currentThread]);
dispatch_group_leave(group);
});
//超时处理.
long result = dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC));
if (result == 0) {
NSLog(@"success");
}else {
NSLog(@"time out!");
}
dispatch_group_notify(group, concurrentQueue, ^{
NSLog(@"begin task three! %@",[NSThread currentThread]);
});
//输出台:
time out!
first download task success! <NSThread: 0x608000277200>{number = 4, name = (null)}
second download task success! <NSThread: 0x608000270c40>{number = 3, name = (null)}
three download task success! <NSThread: 0x608000270c40>{number = 3, name = (null)}
begin task three! <NSThread: 0x608000277200>{number = 4, name = (null)}
注意我的任务1放在了并发队列中,任务2,和任务3放在了串行队列中,所以你看到的打印顺序是任务1和任务2几乎同时打印,任务3等2秒后打印,(在任务2之后).