Grand Central Dispatch 并行任务管理

例：
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
    // 耗时的操作 
    dispatch_async(dispatch_get_main_queue(), ^{ 
        // 更新界面 
    }); 
});

参考：
https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html