for (int i=0; i<[array count]; i++)
{
    NSError *error;
    NSArray *ipaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *idocumentsDir = [ipaths objectAtIndex:0];
    NSString *idataPath = [idocumentsDir stringByAppendingPathComponent:@"File"];
    NSLog(@"idataPath:%@",idataPath);

    //Create folder here
    if (![[NSFileManager defaultManager] fileExistsAtPath:idataPath])
    {
        [[NSFileManager defaultManager] createDirectoryAtPath:idataPath withIntermediateDirectories:NO attributes:nil error:&error];
    }
    // Image Download here
    NSString *fileName = [idataPath stringByAppendingFormat:@".jpg"];
    NSLog(@"imagePathDOWNLOAD:%@",fileName);

    _imgData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:[array objectAtIndex:i]]];
    [_imgData writeToFile:fileName atomically:YES];

    tempImg.image = [UIImage imageWithData:_imgData];   
}

如何为这个循环设置进度 View ，我想为下载数据设置进度 View 。 以及进度标签(即 %)我想要百分比小数。

最佳答案

简单的解决方案是异步执行此操作，同时更新进度 View :

1. 创建进度 View 并将其添加到您的 View

2. 将您的代码分派(dispatch)到后台队列

3. 每次下载完成后，将进度 View 的更新分派(dispatch)回主队列

在伪代码中，它看起来像

UIProgressView *progressView = [[UIProgressView alloc] init];
// configure the progress view and add it to your UI

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    for (int i=0; i<[array count]; i++)
    {
        NSError *error;
        NSArray *ipaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *idocumentsDir = [ipaths objectAtIndex:0];
        NSString *idataPath = [idocumentsDir stringByAppendingPathComponent:@"File"];
        NSLog(@"idataPath:%@",idataPath);

        //Create folder here
        if (![[NSFileManager defaultManager] fileExistsAtPath:idataPath])
        {
            [[NSFileManager defaultManager] createDirectoryAtPath:idataPath withIntermediateDirectories:NO attributes:nil error:&error];
        }
        // Image Download here
        NSString *fileName = [idataPath stringByAppendingFormat:@".jpg"];
        NSLog(@"imagePathDOWNLOAD:%@",fileName);

        _imgData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:[array objectAtIndex:i]]];
        [_imgData writeToFile:fileName atomically:YES];

        // now dispatch any UI updates back to the main queue
        dispatch_async(dispatch_get_main_queue(), ^{

            [progressView setProgress: (CGFloat) (i + 1.0) / [array count] animated:YES];
            tempImg.image = [UIImage imageWithData:_imgData];
        });
    }
});

还有一系列越来越优雅的方法:

1. 使用并发队列(而不是上面那种串行下载图片的方式)来下载图片，速度会明显加快。我可能建议使用 5 的 maxConcurrentCount 操作队列，以享受并发性，但请确保您没有超过 iOS 的并发请求数限制。

2. 使用基于 NSURLConnectionDataDelegate 的下载而不是 NSData 方法 initWithContentsOfURL，后者可以在单个下载期间提供临时进度。参见 download manager或 download operation例如。

3. 使用 AFNetworking它还提供了基于下载进度 block 的接口(interface)。

上面，在第 1 点中，我建议您考虑使用并发队列，因此我决定对其进行基准测试。对我来说，下面的这个 GCD 实现比它后面的 NSOperationQueue 实现慢 3-4 倍。

这是 GCD 实现:

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

UIProgressView *progressView = [self addProgressView];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    NSInteger downloadSuccessCount = 0;
    NSInteger downloadFailureCount = 0;

    NSString *idataPath = [self createDownloadPath];

    for (int i = 0; i < [array count]; i++)
    {
        // Image Download here
        NSString *filename = [self pathForItem:i array:array folder:idataPath];
        NSURL *url = [self urlForItem:i array:array];
        NSData *data = [[NSData alloc] initWithContentsOfURL:url];
        UIImage *image = nil;
        if (data)
            image = [UIImage imageWithData:data];
        if (image) {
            downloadSuccessCount++;
            [data writeToFile:filename atomically:YES];
        } else {
            downloadFailureCount++;
        }

        // now dispatch any UI updates back to the main queue
        dispatch_async(dispatch_get_main_queue(), ^{

            [progressView setProgress: (CGFloat) (downloadSuccessCount + downloadFailureCount) / [array count] animated:YES];

            // update the image in the UI if you want

            [UIView transitionWithView:self.imageView duration:0.25 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
                tempImg.image = image;
            } completion:nil];
        });
    }

    NSLog(@"Completed in %.1f seconds", CFAbsoluteTimeGetCurrent() - start);
});

对于这个 NSOperationQueue 实现:

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

UIProgressView *progressView = [self addProgressView];

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 5;

NSString *idataPath = [self createDownloadPath];
self.downloadSuccessCount = 0;
self.downloadFailureCount = 0;

NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"Completed in %.1f seconds", CFAbsoluteTimeGetCurrent() - start);
}];

for (int i = 0; i < [array count]; i++)
{
    NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        // Image Download here
        NSString *filename = [self pathForItem:i array:array folder:idataPath];
        NSURL *url = [self urlForItem:i array:array];
        NSData *data = [NSData dataWithContentsOfURL:url];
        UIImage *image = nil;
        if (data)
            image = [UIImage imageWithData:data];
        if (image)
            [data writeToFile:filename atomically:YES];

        // now dispatch any UI updates back to the main queue
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            if (image) {
                self.downloadSuccessCount++;

                // update the image in the UI if you want, though this slows it down

                [UIView transitionWithView:self.imageView duration:0.25 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
                    tempImg.image = image;
                } completion:nil];
            }
            else
                self.downloadFailureCount++;

            [progressView setProgress: (CGFloat) (self.downloadSuccessCount + self.downloadFailureCount) / [array count] animated:YES];
        }];
    }];

    [queue addOperation:operation];
    [completionOperation addDependency:operation];
}

[queue addOperation:completionOperation];

最重要的是，如果您使用 NSOperationQueue(它不仅提供并发性，您也可以在 GCD 并发队列中执行此操作，而且还可以让您轻松控制并发操作的数量(您对于网络操作应限制为五个或更少))，您将享受显着的性能优势。

如我所建议的，更好的方法是使用 AFNetworking，在其中您不仅可以享受操作队列并发优势，还可以享受其他优势。

关于ios - 在循环中设置进度 View ，或者更新进度 View ，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19834016/