多线程管理

    dispatch_async( 
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // 执行耗时的异步操作...
      dispatch_async(dispatch_get_main_queue(), ^{
        // 回到主线程，执行UI刷新操作
        });
});

dispatch_group_t group =  dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // 执行1个耗时的异步操作
});
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // 执行1个耗时的异步操作
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // 等前面的异步操作都执行完毕后，回到主线程...
});

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    // 只执行1次的代码(这里面默认是线程安全的)
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    NSLog(@"2秒后执行了.");
});

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t serialQueue = dispatch_queue_create("com.wzb.test.www", DISPATCH_QUEUE_SERIAL);
dispatch_group_enter(group);
dispatch_group_async(group, serialQueue, ^{
    // 网络请求一
    [WebClick getDataSuccess:^(ResponseModel *model) {
        dispatch_group_leave(group);
    } failure:^(NSString *err) {
        dispatch_group_leave(group);
    }];
});
dispatch_group_enter(group);
dispatch_group_async(group, serialQueue, ^{
    // 网络请求二
    [WebClick getDataSuccess:getBigTypeRM onSuccess:^(ResponseModel *model) {
        dispatch_group_leave(group);
    }                                  failure:^(NSString *errorString) {
        dispatch_group_leave(group);
    }];
});
dispatch_group_enter(group);
dispatch_group_async(group, serialQueue, ^{
    // 网络请求三
    [WebClick getDataSuccess:^{
        dispatch_group_leave(group);
    } failure:^(NSString *errorString) {
        dispatch_group_leave(group);
    }];
});

// 所有网络请求结束后会来到这个方法
dispatch_group_notify(group, serialQueue, ^{
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            // 刷新UI
        });
    });
});  

使用dispatch_get_global_queue函数获得全局的并发队列
dispatch_queue_t dispatch_get_global_queue(
dispatch_queue_priority_t priority, // 队列的优先级
unsigned long flags); // 此参数暂时无用，用0即可
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); // 获得全局并发队列
全局并发队列的优先级
p#define DISPATCH_QUEUE_PRIORITY_HIGH 2 // 高
p#define DISPATCH_QUEUE_PRIORITY_DEFAULT 0 // 默认（中）
p#define DISPATCH_QUEUE_PRIORITY_LOW (-2) // 低
p#define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN // 后台