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dispatch_group是干什么的？
在处理并发操作的时候，如果是几个block同时并发，我们无法知道最后执行完毕的时候是在哪一个Block中，那么就无法知道在哪一个位置写上结束时的代码？
dispatch_group_async 和 dispatch_group_notify可以实现这个功能。
dispatch_group
dispatch_group将提交到这个group的blocks关联起来，group需要等到所有的Block执行完成之后才能释放内存。
Creates new group with which blocks may be associated.
* @discussion
* This function creates a new group with which blocks may be associated.
* The dispatch group may be used to wait for the completion of the blocks it
* references. The group object memory is freed with dispatch_release().
dispatch_group_t dispatch_group_create(void);dispatch_group_async
Submits a block to a dispatch queue and associates the block with the given dispatch group.
*
* @discussion
* Submits a block to a dispatch queue and associates the block with the given
* dispatch group. The dispatch group may be used to wait for the completion
* of the blocks it references.
*
* @param block
* The block to perform asynchronously.
void dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block);dispatch_group_wait
Wait synchronously until all the blocks associated with a group have completed or until the specified timeout has elapsed.
*
* @discussion
* This function waits for the completion of the blocks associated with the
* given dispatch group, and returns after all blocks have completed or when
* the specified timeout has elapsed.
*
* This function will return immediately if there are no blocks associated
* with the dispatch group (i.e. the group is empty).
*
* The result of calling this function from multiple threads simultaneously
* with the same dispatch group is undefined.
*
* After the successful return of this function, the dispatch group is empty.
* It may either be released with dispatch_release() or re-used for additional
* blocks. See dispatch_group_async() for more information.
*
* @param timeout
* When to timeout (see dispatch_time). As a convenience, there are the
* DISPATCH_TIME_NOW and DISPATCH_TIME_FOREVER constants.
long dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);dispatch_group_notify
Schedule a block to be submitted to a queue when all the blocks associated with a group have completed.
*
* @discussion
* This function schedules a notification block to be submitted to the specified
* queue once all blocks associated with the dispatch group have completed.
*
* If no blocks are associated with the dispatch group (i.e. the group is empty)
* then the notification block will be submitted immediately.
*
* The group will be empty at the time the notification block is submitted to
* the target queue. The group may either be released with dispatch_release()
* or reused for additional operations.
* See dispatch_group_async() for more information.
void dispatch_group_notify(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block);dispatch_group_enter & dispatch_group_leave
Manually indicate a block has entered the group
*
* @discussion
* Calling this function indicates another block has joined the group through
* a means other than dispatch_group_async(). Calls to this function must be
* balanced with dispatch_group_leave().
void dispatch_group_enter(dispatch_group_t group);
Manually indicate a block in the group has completed
*
* @discussion
* Calling this function indicates block has completed and left the dispatch
* group by a means other than dispatch_group_async().
void dispatch_group_leave(dispatch_group_t group);demo
dispatch_group_notify的应用
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_queue_create("com.gcd-group.www",DISPATCH_QUEUE_CONCURRENT);
dispatch_group_async(group, queue, ^{
for (int i = 0; i < 1000; i++) {
if (i == 999) {
NSLog(@"11111111");
}
}
});
dispatch_group_async(group, queue, ^{
NSLog(@"22222222");
});
dispatch_group_async(group, queue, ^{
NSLog(@"33333333");
});
dispatch_group_notify(group, queue, ^{
NSLog(@"done");
});
会在最后输出done，这个地方是处理所有并操作之后的结束位置dispatch_group_wait的应用
- (NSArray *)fetchTheNetUserData{
//使用GCD创建一个group 组，这个其实不是很好理解，我们可以理解成，创建一个组，然后给这个组添加任务，等到所有组的任务都结束之后在进行一个颁奖典礼，这个就是我们要的效果
dispatch_group_t group = dispatch_group_create();
NSMutableArray * array = [NSMutableArray array];
for (int i=0; i<5; i++) {
//group 进入一个组 和dispatch_group_leave 这两个必须是成对出现，缺一不可，如果对应不上则会出现线程阻塞， dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
[array addObject:[NSNumber numberWithInt:i]];
//dispatch_group_leave 离开一个组 ，这个和排毒其实有点类似
dispatch_group_leave(group);
});
}
//dispatch_group_wait 这个函数返回 0则会继续执行，否则一直等待 group 组内的所有成员任务完毕，这个其实也是资源数增加的一个函数，等待当前线程结束
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
NSLog(@"array = %@",array); return array;
}
#========================================================
- (void)syncAction{
dispatch_group_t group =dispatch_group_create();
dispatch_queue_t globalQueue=dispatch_get_global_queue(0, 0);
dispatch_group_enter(group);
//模拟多线程耗时操作
dispatch_group_async(group, globalQueue, ^{
sleep(3);
NSLog(@"%@---block1结束。。。",[NSThread currentThread]);
dispatch_group_leave(group);
});
NSLog(@"%@---1结束。。。",[NSThread currentThread]);
dispatch_group_enter(group);
//模拟多线程耗时操作
dispatch_group_async(group, globalQueue, ^{
sleep(3);
NSLog(@"%@---block2结束。。。",[NSThread currentThread]);
dispatch_group_leave(group);
});
NSLog(@"%@---2结束。。。",[NSThread currentThread]);
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"%@---全部结束。。。",[NSThread currentThread]);
});
}