如何下载图片？

Question

使用此代码下载图片。但是图像没有出现在显示器上。

  NSURL *url = [NSURL URLWithString:
 @"http://upload.wikimedia.org/wikipedia/commons/7/7f/Williams_River-27527.jpg"];

NSURLSessionDownloadTask *downloadPhotoTask = [[NSURLSession sharedSession]
 downloadTaskWithURL:url completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
  _downloadedImage = [UIImage imageWithData:
    [NSData dataWithContentsOfURL:location]];
}];

// 4    
[downloadPhotoTask resume];

_downloadedImage = [UIImage imageNamed:@"Williams_River-27527.jpg"];
    [_book3 setBackgroundImage: _downloadedImage forState:UIControlStateNormal];
    [_scroller addSubview:_book3];

UPD

我在我的 xcode 项目中更新了我的代码，但它也不起作用......为什么它的代码在我的 Xcode 项目中也不起作用？

#import "ViewController.h"

@interface ViewController () <NSURLSessionDelegate, NSURLSessionDownloadDelegate>

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
    NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithURL:[NSURL URLWithString:@"http://cdn.tutsplus.com/mobile/uploads/2013/12/sample.jpg"]];
    [downloadTask resume];

}

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

    dispatch_async(dispatch_get_main_queue(), ^{
        [self.progressView setHidden:YES];
        [self.imageView setImage:[UIImage imageWithData:data]];
    });
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes {

}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float progress = (double)totalBytesWritten / (double)totalBytesExpectedToWrite;

    dispatch_async(dispatch_get_main_queue(), ^{
        [self.progressView setProgress:progress];
    });
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

为什么它的代码在我的 Xcode 项目中也不起作用？

Answer 1

如果您希望它按代码运行（不确定是否会），那么您需要运行此过程中的步骤：

//1     
NSURL *url = [NSURL URLWithString:
  @"http://upload.wikimedia.org/wikipedia/commons/7/7f/Williams_River-27527.jpg"];

// 2    
NSURLSessionDownloadTask *downloadPhotoTask = [[NSURLSession sharedSession]
 downloadTaskWithURL:url completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
    // 3 
    _downloadedImage = [UIImage imageWithData:
    [NSData dataWithContentsOfURL:location]];

    //5
    dispatch_async(dispatch_get_main_queue(), ^{
        [_book3 setBackgroundImage: _downloadedImage forState:UIControlStateNormal];
        [_scroller addSubview:_book3];
    });
}];

// 4    
[downloadPhotoTask resume];

Answer 2

您通过从包中加载图像来创建图像的第一个实例：_downloadedImage = [UIImage imageNamed:@"Williams_River-27527.jpg"];（希望有一个）

从您的 url 下载图像后，您使用下载的内容创建图像的 second 实例。但是您的按钮仍然链接到 first 实例（强引用）。按钮根本不知道新图片。

因此，如果您想更改显示的图像，则必须在将其分配给 _downloadedImage 后直接重新设置它：[_book3 setBackgroundImage: _downloadedImage forState:UIControlStateNormal];

之后可能需要刷新 ui。

Answer 3

这是 Swift

的答案

    let url = URL.init(string: "https:yourUrl.com") as! URL

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        if let error = error {
            print("Error needs to be handled")
            return
        }

        let image = UIImage.init(data: data!)
        DispatchQueue.main.async {
            self.yourImageView.image = image
        }
    }.resume()

Answer 4

Xamarin.iOS的简单而纯粹的答案：

var url = new NSUrl("https:yourUrl.com");

var task = await NSUrlSession.SharedSession.CreateDataTaskAsync(url);
var image = UIImage.LoadFromData(task.Data);

yourImageView.Image = image;

如何下载图片？

How to download image?

objective-c

nsurl

ios