调度组,一次性执行操作的使用

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event{    

//创建一个调度组   

dispatch_group_t group = dispatch_group_create();    

//把任务添加到调度组中    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{       

 NSLog(@"下载A%@",[NSThread currentThread]);   });    

dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{ 

  NSLog(@"下载B%@",[NSThread currentThread]);   });    

dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{        

NSLog(@"下载C%@",[NSThread currentThread]);   });    

dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{        

NSLog(@"下载D%@",[NSThread currentThread]);    });

//dispatch_group_notify 当调度组里的所有的任务执行完成后, 执行dispatch_group_notify这个函数里写的任务.    

dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{        dispatch_group_async(group, dispatch_get_main_queue(), ^{            

NSLog(@"在%@中更新UI,提示已下载完成",[NSThread currentThread]);        });    });}

打印如下:

2016-07-03 15:59:49.564 调度组的使用[4577:320369] 下载A{number = 3, name = (null)}

2016-07-03 15:59:49.564 调度组的使用[4577:320385] 下载C{number = 5, name = (null)}

2016-07-03 15:59:49.564 调度组的使用[4577:320379] 下载B{number = 2, name = (null)}

2016-07-03 15:59:49.564 调度组的使用[4577:320384] 下载D{number = 4, name = (null)}

2016-07-03 15:59:49.566 调度组的使用[4577:320225] 在{number = 1, name = main}中更新UI,提示已下载完成

一次性执行操作:

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event{

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self once];

});

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self once];

});

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self once];

});

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self once];

});

}

-(void)once{

static dispatch_once_t onceToken;

/**

dispatch_once : 保证block 里面的任务代码只执行一次

参数一  dispatch_once_t  token  标识

参数二  要执行的代码任务

*/

dispatch_once(&onceToken, ^{

NSLog(@"一次执行%@",[NSThread currentThread]);

});

NSLog(@"end");

}