Question

I’ve written this bit of code to process a URL in string form to try and fetch retina image of possible. The intent is that is turns

scheme://host/path/name.extension

into

scheme://host/path/[e-mail]

The URL is predictable so assuming, for example, there is just one ‘.’ in the final filename is safe. But I have that iOS feeling, where I’m wondering if this is too much code to get this done; is there a better, maybe API-provided way to do this?

This is just the beginning of a method that kicks off the request.

- (NSData *)fetchResourceWithURLLocation:(NSString *)location
                             requestedBy:(id<DataRequestDelegate>)requester
                          withIdentifier:(id)requestID {

    NSData *resultData = nil;

    // some disassembly to get rid of the double '/' in scheme
    NSURL *locationURL = [NSURL URLWithString:location];
    NSString *host = [locationURL host];
    NSString *scheme = [locationURL scheme];
    NSString *dataPath = [locationURL relativePath];

    // if this is a retina display, rewrite the dataPath to use @2x
    if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] && ([UIScreen mainScreen].scale == 2.0)) {

        NSArray *pathComponents = [dataPath componentsSeparatedByString:@"/"];
        NSString *nameComponent = [pathComponents lastObject];
        NSArray *nameComponents = [nameComponent componentsSeparatedByString:@"."];
        NSAssert(([nameComponents count] == 2), @"nameComponent contains more than one \".\"");
        nameComponent = [NSString stringWithFormat:@"%@@2x.%@", [nameComponents objectAtIndex:0], [nameComponents lastObject]];
        dataPath = @"";
        for(NSString *pathComponent in pathComponents) {
            if(pathComponent != [pathComponents lastObject])
                dataPath = [dataPath stringByAppendingString:[NSString stringWithFormat:@"%@/", pathComponent]];
        }
        dataPath = [dataPath stringByAppendingString:nameComponent];
    }

    // reassembly to check existence
    NSString *processedLocation = [NSString stringWithFormat:@"%@://%@%@", scheme, host, dataPath];

Answer 1

You are right to think that is a dramatic amount of code to insert a string into your address string. What you want to do is fairly straightforward, so here is an example method that just pops the "@2x" into a string just before the last ".". It returns nil if there is no ".". Further explanation in code comments.

-(NSString *)retinaURLStringForString:(NSString *)nonRetinaAddress{
    // Find the range (location and length of ".")
    // Use options parameter to start from the back.
    NSRange extDotRange = [nonRetinaAddress rangeOfString:@"." options:NSBackwardsSearch];
    // You can check whether the "." is there or not like this:
    if (extDotRange.location == NSNotFound){
        // Handle trouble
        return nil;
    }

    // We can use NSString's stringByReplacingCharactersInRange:withString: method to insert the "@2x".
    // To do this we first calculate the range to 'replace'.
    // For location we use the location of the ".".
    // We use 0 for length since we do not want to replace anything.
    NSRange insertRange = NSMakeRange(extDotRange.location, 0);

    // Lastly simply use the stringByReplacingCharactersInRange:withString: method to insert "@2x" in the insert range.
    NSString *retinaAddress = [nonRetinaAddress stringByReplacingCharactersInRange:insertRange withString:@"@2x"];
    return retinaAddress;
}

When tested like:

NSString *nonRetinaAddress = @"scheme://host/path/name.extension";
NSString *retinaAddress = [self retinaURLStringForString:nonRetinaAddress];

NSLog(@"%@",nonRetinaAddress);
NSLog(@"%@",retinaAddress);

It logs:

scheme://host/path/name.extension
scheme://host/path/[e-mail]

Small side note:

If the original string was a mutable string, you could use [mutableAddressString insertString:@"@2x" atIndex:extDotRange.location] instead of calculating a replacement range. Not worth creating a mutable for though.