Question

我有一个iPad应用程序，我可以下载图像列表，并在下载图像时更新进度条。我似乎无法获得进度条和图像下载以匹配。进度条始终在图像下载完成之前完成。我有一个方法UpdateProgressBar，每次下载图像时都应该增加进度条。

-(void)DownloadPhoto{
    NSMutableArray *failedDownloads = [[NSMutableArray alloc]init];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        listPhoto = [CoreDataRead GetPhotoList:[self selectedAccountGuid]];
        dispatch_group_t downloadGroup = dispatch_group_create();
        for (Photo *item in listPhoto) {
            NSString *imageName = item.photoName;
            NSString *myURL = [NSString stringWithFormat:@"%@%@", @"http://acimagedownload.com/photos/", imageName];
            NSURL        *url       = [NSURL URLWithString:myURL];
            NSURLRequest *request   = [NSURLRequest requestWithURL:url];

            dispatch_group_enter(downloadGroup);
            [NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {

            if (connectionError == nil && data != nil)
            {
                if (data != nil)
                {
                    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
                    NSString *documentsDirectory = [paths objectAtIndex:0];
                    NSString* path = [documentsDirectory stringByAppendingPathComponent: [NSString stringWithFormat:@"%@%@",item.guid, @".png"]];

                        [data writeToFile:path atomically:YES];
                        NSLog(@"Photo Downloaded %@!", @"");
                    }
                    else
                    {
                        NSLog(@"image is not downloaded");
                    }
                }
                else if (connectionError != nil)
                {
                    [failedDownloads addObject:myURL];
                    NSLog(@"Error %@",[connectionError description]);
                }
                else
                {
                    [failedDownloads addObject:myURL];
                    NSLog(@"Image Download Failed %@!", @"");
                }
                dispatch_group_leave(downloadGroup);
                dispatch_async(dispatch_get_main_queue(), ^{
                    [self UpdateProgressBar];
                });
            }];
            dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER);
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [self DownloadVideo];
        });
    });
}

-(void)UpdateProgressBar{
    currentTask = currentTask + 1;
    NSLog(@"Current Task %@!", [@(currentTask) stringValue]);
    float progressPercentage = (float)currentTask/(float)taskCount;
    [self.progressBar setProgress:progressPercentage animated:YES];
    if(currentTask == taskCount){
        [self ShowDoneAlert];
    }
}

Answer 1

如果没有看到updateProgressBar方法，为什么你会得到这种行为很难说，但我注意到的一件事是你的dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER);在你的循环中。

所以会发生什么，你进入循环的第一次迭代，在它结束时，线程执行停止并等待第一个dispatch_group_leave。之后继续下一次迭代，依此类推。

如果这是期望的行为（但我怀疑它，我认为您实际上希望您的下载并行运行），您应该使用dispatch_semaphore_t甚至是串行队列。

进度条更新与图像下载不同步

1 个答案: