Question

我正在尝试使用相同的会话和不同的下载任务下载多个图像，如问题所示。我可以下载第一张图片而不是第二张图片。在didFinishDownloadingToURL中我使用if条件来识别downloadTask并且对于某个downloadTask将其设置为某个imageView。

这是我的代码，请耐心等待我：

@interface ViewController ()
{
    NSURLSessionConfiguration *sessionConfiguration;
    NSURLSessionDownloadTask *firstDownloadTask;
    NSURLSessionDownloadTask *secondDownloadTask;
    NSURLSession *session;
    UIImageView *firstImageHolder;
    UIImageView *secondImageHolder;
}
@end

- (void)viewDidLoad
{
            NSString *firstDownloadLink = @"http://letiarts.com/letiarts2014/wp-content/uploads/2014/04/icon_game.png";
            sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
            session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
            firstImageHolder = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 45, 45)];
            [_viewImages addSubview: firstImageHolder];
            firstDownloadTask = [session downloadTaskWithURL:[NSURL URLWithString:firstDownloadLink]];
            [firstDownloadTask resume];

            //2
            NSString *secondDownloadLink = @"http://downloads.bbc.co.uk/skillswise/images/promo/prefab-maths-game-336x189.jpg";
            secondImageHolder = [[UIImageView alloc] initWithFrame:CGRectMake(50, 0, 45, 45)];
            [_viewImages addSubview: secondImageHolder];
            secondDownloadTask = [session downloadTaskWithURL:[NSURL URLWithString:secondDownloadLink]];
            [secondDownloadTask resume];
}

在didFinishDownloadingToURL中：

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSData *data = [NSData dataWithContentsOfURL:location];

    if (downloadTask == firstDownloadTask) {

            UIImage *theImage1 = [UIImage imageWithData:data];

            [firstImageHolder setImage:theImage1];

        NSLog(@"DOWNLOAD FIRST IMAGE FINISHED");

    }
    //download finished
    if (downloadTask == secondDownloadTask) {

            UIImage *theImage2 = [UIImage imageWithData:data];

            [secondImageHolder setImage:theImage2];

        NSLog(@"DOWNLOAD SECOND IMAGE FINISHED");

    }
}

提前谢谢！

Answer 1

我看到你已经解决了自己的问题，但我想补充几点。

下面的实例不需要这么多属性就足够了。

@property NSURLSession * session;

您还可以通过taskIdentifier

NSNumber * key = @（firstDownloadTask.taskIdentifier）;

在委托中执行ui操作时需要小心，如果没有从主线程调用它可能会导致冻结。为安全起见，应使用：

dispatch_async（dispatch_get_main_queue（），^ {
[firstImageHolder setImage:theImage1];  
}）;

示例工作代码

@interface ViewController: ...<...>
@property (nonatomic, strong) NSURLSession *session;
@end

@implementation ViewController

- (void)viewDidLoad
{

        NSURLSessionConfiguration *sessionConfiguration;

        //NSURLSessionDownloadTask *downloadTask; //can use same task if cancelling is not intended.

        NSURLSessionDownloadTask *firstDownloadTask;
        NSURLSessionDownloadTask *secondDownloadTask;

        sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
        session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];

        //1
        NSString *firstDownloadLink = @"http://letiarts.com/letiarts2014/wp-content/uploads/2014/04/icon_game.png";
        firstImageHolder = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 45, 45)];
        [_viewImages addSubview: firstImageHolder];
        /*downloadTask*/firstDownloadTask = [session downloadTaskWithURL:[NSURL URLWithString:firstDownloadLink]];
        [/*downloadTask*/firstDownloadTask resume];

        //2
        NSString *secondDownloadLink = @"http://downloads.bbc.co.uk/skillswise/images/promo/prefab-maths-game-336x189.jpg";
        secondImageHolder = [[UIImageView alloc] initWithFrame:CGRectMake(50, 0, 45, 45)];
        [_viewImages addSubview: secondImageHolder];
        /*downloadTask*/secondDownloadTask = [session downloadTaskWithURL:[NSURL URLWithString:secondDownloadLink]];
        [/*downloadTask*/secondDownloadTask resume];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSData *data = [NSData dataWithContentsOfURL:location];

    if (downloadTask.taskIdentifier == 1) {
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *theImage1 = [UIImage imageWithData:data];
            [firstImageHolder setImage:theImage1];
        });

        NSLog(@"DOWNLOAD FIRST IMAGE FINISHED");

    }
    //download finished
    if (downloadTask.taskIdentifier == 2) {
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *theImage2 = [UIImage imageWithData:data];
            [secondImageHolder setImage:theImage2];
        });

        NSLog(@"DOWNLOAD SECOND IMAGE FINISHED");

    }
}

@end

Answer 2

代码很好，因为问题是在我的实际代码中我犯了一个错误，在第二个简历中我称之为第一个任务。

如何使用相同的会话和不同的下载任务下载多个图像

2 个答案: