ios – 如何使用GCD说明后台任务？

- (IBACtion)signInBttn:(id)sender {

    MBProgressHUD *hudd = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    hudd.mode = MBProgressHUDModeAnnularDeterminate;
    hudd.labelText = @"Loading";

    __block float value = 0;
    for (int j = 0; j<2000; j++) {
        dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{

            for (int i = 0; i<20000 ; i++) {

            }
            value += 0.001;
            dispatch_async( dispatch_get_main_queue(),^{
                hudd.progress = value;
            });

        });
    }        
}

在这种情况下,您可以通过将计数器的更新与UI中的HUD更新分离来解决问题. Apple将此称为WWDC 2012视频 Asynchronous Design Patterns with Blocks,GCD,and XPC中的“异步更新状态”.

通常这不是必需的(大部分时间我们异步进行的操作都很慢,以至于我们没有问题),但如果做的事情比UI运行得更快,那么你可以创建这是一个“调度源”.我将用UIProgressview来说明它,但同样适用于任何UI：

// create source for which we'll be incremenTing a counter,// and tell it to run the event handler in the main loop
// (because we're going to be updating the UI)

dispatch_source_t source = dispatch_source_create(DISPATCH_sourcE_TYPE_DATA_ADD,dispatch_get_main_queue());

// specify what you want the even handler to do (i.e. update the HUD or progress bar)

dispatch_source_set_event_handler(source,^{
    self.iterations += dispatch_source_get_data(sourcE);
    [self.progressview setProgress: (float) self.iterations / kMaxIterations];
});

// start the dispatch source

dispatch_resume(sourcE);

// Now,initiate the process that will update the source

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,^{

    for (long i = 0; i < kMaxIterations; i++)
    {
        // presumably,do something meaningful here

        // Now increment counter (and the event handler will take care of the UI)

        dispatch_source_merge_data(source,1);
    }

    // when all done,cancel the dispatch source

    dispatch_source_cancel(sourcE);
});

在我的例子中,迭代只是一个很长的属性：

@property (nonatomiC) long iterations;

我将kMaxIterations常量定义如下：

static long const kMaxIterations = 10000000l;