How to disable caching from NSURLSessionTask

问题:

In my iOS app, I am using NSURLSessionTask to download json data to my app. I discovered that when I call the url directly from the browser, I get an up to date json and when it's called from within the app, I get an older version of the json.

Is this due to caching? How can I tell NSURLSessionTask to not use caching.

This is the call I use:

NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

Thanks!

回答1:

If your read the links from @runmad you can see in the flow chart that if the HEAD of the file is unchanged it will still used the cached version when you set the cachePolicy.

In Swift3 I had to do this to get it to work:

let config = URLSessionConfiguration.default
config.requestCachePolicy = .reloadIgnoringLocalCacheData
config.urlCache = nil

let session = URLSession.init(configuration: config)

That got a truly non-cached version of the file, which I needed for bandwidth estimation calculations.

回答2:

Rather than using the sharedSession, you also can create your own NSURLSession using a NSURLSessionConfiguration that specifies a default cache policy. So, define a property for your session:

@property (nonatomic, strong) NSURLSession *session;

And then:

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
configuration.requestCachePolicy = NSURLRequestReloadIgnoringLocalCacheData;
self.session = [NSURLSession sessionWithConfiguration:configuration];

Then requests using that session will use that requestCachePolicy.

回答3:

Swift 3, Xcode 8

extension UIImageView {
func donloadImage(fromUrl url: URL) {
    let request = URLRequest(url: url, cachePolicy: URLRequest.CachePolicy.reloadIgnoringLocalCacheData, timeoutInterval: 60.0)
    URLSession.shared.dataTask(with: request) { (data, response, error) in
        guard
            let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
            let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
            let data = data, error == nil,
            let image = UIImage(data: data)
            else { return }
        DispatchQueue.main.async() { () -> Void in
            self.image = image
        }
    }.resume()
}

回答4:

The below code worked for me, the catch is setting URLCache to nil.

 NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
    config.requestCachePolicy = NSURLRequestReloadIgnoringLocalCacheData;
    config.URLCache = nil;

回答5:

You need to set the cachePolicy on NSURLRequest, here's the documentation.

Here's some insight as to how caching works in general.

You can read about the specific enums you can use for specifying the cachePolicy here in particular:

enum
{
   NSURLRequestUseProtocolCachePolicy = 0,
   NSURLRequestReloadIgnoringLocalCacheData = 1,
   NSURLRequestReturnCacheDataElseLoad = 2,
   NSURLRequestReturnCacheDataDontLoad = 3,
};
typedef NSUInteger NSURLRequestCachePolicy;

For example, you would do:

NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:60.0];

回答6:

I was getting cache image for same url .. so i have done this

imageView.imageFromUrl(self.shareData.userResponseData["photo"] as! String)

extension UIImageView {

public func imageFromUrl(urlString: String) {

    if let url = NSURL(string: urlString) {

        let request = NSURLRequest(URL: url, cachePolicy: NSURLRequestCachePolicy.ReloadIgnoringLocalCacheData, timeoutInterval: 60.0)

        NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue()) {

            (response: NSURLResponse!, data: NSData!, error: NSError!) -> Void in
            self.image = UIImage(data: data)

        }
    }
}

}