+[NSURL URLWithString:] returning nil

问题:

I use this code to load a UIImage from a URL:

  NSURL *imageURL = [NSURL URLWithString:@"http://testwebsite.com/image.png"];
  NSData *imageData = [[NSData alloc] initWithContentsOfURL:imageURL];
  imageView.image = [UIImage imageWithData: imageData];

but I'm stuck at a URL that looks like this one :
http://www.testwebsite.com/getFile?userID=123
this works fine in the browser but returns nil in the imageData variable above.

how can I load the image from such an obscured URL (i.e. from a URL that does not show off the file name but rather redirects to the image file) ?

thank you in advance.

回答1:

for some weird reason I have to use :
[NSURL URLWithString:[urlStr stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];

when I create the URL and this solved the issue.

回答2:

NSData initWithContentsOfURL: returns nil if the data cannot be loaded (i.e. if you get a HTTP error code).

Chances are the URL you point to has some referrer blocker or cookie requirement that prevents its use from within your app (just one suggestion of the problem).

I recommend you look at using the following variant instead:

- (id)initWithContentsOfURL:(NSURL *)aURL options:(NSDataReadingOptions)mask error:(NSError **)errorPtr

And pay attention to the value of errorPtr in case the return value is nil.

For example:

NSError *error;
NSData* imageData = [[NSData alloc] initWithContentsOfURL:
    [NSURL URLWithString:@"http://example.com/notfound"]
    options:0 error:&error];
if(imageData == nil) {
    NSLog(@"Error: %@", error);
} else {
    NSLog(@"Got %u bytes", imageData.length);
}

You can see the following error:

Error: Error Domain=NSCocoaErrorDomain Code=256 "The operation couldn’t be completed. (Cocoa error 256.)" UserInfo=0xa881c20 {NSURL=http://example.com/notfound}

回答3:

If you load the image like that it's blocking the main thread and leaving your application unresponsive and I'm not sure if the initWithContentsOfURL: method is handling redirects at all if that is the issue... You can create another class (ImageFetcher for example) that uses NSURLConnection and implements the NSURLConnectionDataDelegate. Using the connection:willSendRequest:redirectResponse: method from the NSURLConnectionDataDelegate you can handle redirects and in the connection:didReceiveData: method you can append the received data to a NSMutableData (it's not guaranteed that you receive all the data in one go). Furthermore you don't have to wait for the connection to finish or download any data if you implement the connection:didReceiveResponse: method and check if the response is indicating an error and in the connectionDidFinishLoading: method you can call another method to update your image view (or call the delegate of the ImageFetcher class with the appropriate data to update the image view).

回答4:

NSURL URLWithString:@"" will return nil if the URL does not conform to RFC 2396 and they must be escaped

If you read through rfc2396 in the link you will get loads of details

A great site I found for checking where the offending character is, choose the path option for URL

http://www.websitedev.de/temp/rfc2396-check.html.gz