ios - 如何使用同一 session 和不同的下载任务下载多个图像

Question

我正在尝试使用与问题所述相同的 session 和不同的下载任务来下载多个图像。我可以下载第一张图片，但不能下载第二张。在 didFinishDownloadingToURL 中，我使用 if 条件来识别 downloadTask 并将某个 downloadTask 设置为某个 imageView。

这是我的代码，请耐心等待:

@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");

    }
}

提前致谢!

Answer 1

我看到你已经解决了你自己的问题，但我想补充几点。

1. 下面的实例不需要那么多属性就足够了。

@property NSURLSession * session;

也可以通过taskIdentifier来识别任务

NSNumber * key = @(firstDownloadTask.taskIdentifier);

1. 在delegate中进行ui操作时需要小心，如果不是从主线程调用可能会导致卡顿。为了安全起见，应该使用:

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