percent escapes removed in NSData writeToURL - nsdata

Question

I am trying to cache an image retrieved from Flickr. In order to create a unique filename for the cached image, I use CFURLCreateStringByAddingPercentEscapes to percent escape the URL. Appending that to the cache directory, I get a URL with the embedded Flickr URL properly percent escaped; but when I try to cache the image using NSData writeToURL:options:error: I get "The operation couldn’t be completed. No such file or directory" - and it shows the file URL with the original, unescaped Flickr URL where the file name should be.
For example, I NSLog the URL as:
file://localhost/Users/rick/Library/Application%20Support/iPhone%20Simulator/6.1/Applications/77C4A7AA-C386-4575-AD21-B4027D080408/Library/Caches/http%3A%2F%2Ffarm3.static.flickr.com%2F2887%2F9391679341_26643bcafa_b.jpg
but the error message shows
NSFilePath=/Users/rick/Library/Application Support/iPhone Simulator/6.1/Applications/77C4A7AA-C386-4575-AD21-B4027D080408/Library/Caches/http://farm3.static.flickr.com/2887/9391679341_26643bcafa_b.jpg
It's as if in the process of converting the URL to a file path, writeToURL is removing the percent escapes.
Is there a way to prevent this from happening, or do I just need to come up with another way to generate unique names based on the url?
Here's the relevant code:
NSURL *cacheDirectoryURL=[[fileManager URLsForDirectory:NSCachesDirectory inDomains:NSUserDomainMask] lastObject];
NSString *photoURLString= (NSString *) CFBridgingRelease(CFURLCreateStringByAddingPercentEscapes(NULL,
(__bridge CFStringRef)([self.photoURL absoluteString]),
NULL,
(CFStringRef) #"!*'();:#&=+$,/?%#[]",
kCFStringEncodingUTF8));
if (photoURLString)
{
NSURL *cachedPhotoURL=[NSURL URLWithString:[[cacheDirectoryURL absoluteString] stringByAppendingString:photoURLString]];
NSData *photoData=[NSData dataWithContentsOfURL:cachedPhotoURL];
if (photoData)
{
UIImage *image=[UIImage imageWithData:photoData];
self.imageView.image=image;
[self setupScrollView]; // new image, need to adjust scroll view
} else {
dispatch_queue_t fetchQueue=dispatch_queue_create("photo downloader", NULL);
dispatch_async(fetchQueue, ^{
NSData *photoData=[NSData dataWithContentsOfURL:self.photoURL];
NSError *error;
if ([photoData writeToURL:cachedPhotoURL options:NSDataWritingAtomic error:&error])
{
NSLog(#"Cached photo");
} else {
NSLog(#"Failed to cache photo");
NSLog(#"%#",error);
}
});
}
}
Thanks in advance for your help!

Answer 1

The problem is that [NSURL URLWithString:...] parses the given string and interprets the
percent escapes. Generally, to create a URL for a file system path, fileURLWithPath:
should be used.
In your case, the following simple code should work:
NSURL *cachedPhotoURL = [cacheDirectoryURL URLByAppendingPathComponent:photoURLString]