iphone - Reading binary image data from a web service into UIImage

Question

I'm consuming a web service in my iPhone app. The web service method returns a response which has several fields (eg. ID, Description, etc..). One of these fields contains binary image data which I need to convert to UIImage in my iPhone application.

I'm using a NSXMLParser successfully to extract data from the XML response. In the parser:foundCharacters: selector of the XMLParser, it gives a NSString* pointing to the string within each field. Since this is a string, this is what I do to read image data when i encounter the image field:

UIImage *img = [[UIImage alloc] initWithData:[string dataUsingEncoding:NSUTF8StringEncoding]];

But img variable is still "nil" after this line. Seems like the data from XML string is not compatible with conversion. What am I doing wrong here? (I'm capable of reading other fields into my variables but not this image data field)

Thanks in advance..

Answer 1

Following fbrereto's answer, I managed to convert the string into NSData using a code sample at this link: http://www.cocoadev.com/index.pl?BaseSixtyFour (scroll to the code sample by MiloBird user). Now I can construct the image successfully.

It uses objective-c categories to add the selector + (id)dataWithBase64EncodedString:(NSString *)string; to the NSData class. Here's the necessary code sample I extracted from the above link:

//MBBase64.h

@interface NSData (MBBase64)

+ (id)dataWithBase64EncodedString:(NSString *)string;     //  Padding '=' characters are optional. Whitespace is ignored.

@end


//MBBase64.m

static const char encodingTable[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

@implementation NSData (MBBase64)

+ (id)dataWithBase64EncodedString:(NSString *)string;
{
    if (string == nil)
        [NSException raise:NSInvalidArgumentException format:nil];
    if ([string length] == 0)
        return [NSData data];

    static char *decodingTable = NULL;
    if (decodingTable == NULL)
    {
        decodingTable = malloc(256);
        if (decodingTable == NULL)
            return nil;
        memset(decodingTable, CHAR_MAX, 256);
        NSUInteger i;
        for (i = 0; i < 64; i++)
            decodingTable[(short)encodingTable[i]] = i;
    }

    const char *characters = [string cStringUsingEncoding:NSASCIIStringEncoding];
    if (characters == NULL)     //  Not an ASCII string!
        return nil;
    char *bytes = malloc((([string length] + 3) / 4) * 3);
    if (bytes == NULL)
        return nil;
    NSUInteger length = 0;

    NSUInteger i = 0;
    while (YES)
    {
        char buffer[4];
        short bufferLength;
        for (bufferLength = 0; bufferLength < 4; i++)
        {
            if (characters[i] == '')
                break;
            if (isspace(characters[i]) || characters[i] == '=')
                continue;
            buffer[bufferLength] = decodingTable[(short)characters[i]];
            if (buffer[bufferLength++] == CHAR_MAX)      //  Illegal character!
            {
                free(bytes);
                return nil;
            }
        }

        if (bufferLength == 0)
            break;
        if (bufferLength == 1)      //  At least two characters are needed to produce one byte!
        {
            free(bytes);
            return nil;
        }

        //  Decode the characters in the buffer to bytes.
        bytes[length++] = (buffer[0] << 2) | (buffer[1] >> 4);
        if (bufferLength > 2)
            bytes[length++] = (buffer[1] << 4) | (buffer[2] >> 2);
        if (bufferLength > 3)
            bytes[length++] = (buffer[2] << 6) | buffer[3];
    }

    realloc(bytes, length);
    return [NSData dataWithBytesNoCopy:bytes length:length];
}

@end

Thank you all!