saving images in string format (to use in xml)..no

问题:

i am converting image from picker into nsdata(jpeg representation) and then converting it into nsstring using the following code

 NSData *data=UIImageJPEGRepresentation(image,1.0);
 NSString *imageString=[[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
[[NSUserDefaults standardUserDefaults] setObject:imageString forKey:@"image_name"];

and at the other end where i need to display the image the uiimage is formed as follows.

 NSString *imageString=[[NSString alloc] init];
 imageString=[[NSUserDefaults standardUserDefaults] objectForKey:@"image_name"];
 UIImage *image=[UIImage imageWithData:[imageString dataUsingEncoding:NSUTF8StringEncoding]];

the image variable used in the top code is not nil ,but the image formed from data is getting nil...when i nslogged userdefaults some string is present for the key mentioned above .can anyone explain why is this so..what is the right way to do this

回答1:

If it goes through a web server or the like, you could encapsulate it with base64 enc/decoding or some other plain encoder.

It removes "bad" char, ie that screw up the string during transformation, and change them to generic alphabetical chars and then back again.

if this is the reason to your issues, here is a short one i use (which I most probably stole and adapted, but do not remember from whom. Sorry! :-) )

base64helper.h

    #import <Foundation/Foundation.h>
    @interface NSData (MBBase64)

base64helper.m

#import "base64helper.h"

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] == '\0')
                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];
}

- (NSString *)base64Encoding;
{
    if ([self length] == 0)
        return @"";

    char *characters = malloc((([self length] + 2) / 3) * 4);
    if (characters == NULL)
        return nil;
    NSUInteger length = 0;

    NSUInteger i = 0;
    while (i < [self length])
    {
        char buffer[3] = {0,0,0};
        short bufferLength = 0;
        while (bufferLength < 3 && i < [self length])
            buffer[bufferLength++] = ((char *)[self bytes])[i++];

        //  Encode the bytes in the buffer to four characters, including padding "=" characters if necessary.
        characters[length++] = encodingTable[(buffer[0] & 0xFC) >> 2];
        characters[length++] = encodingTable[((buffer[0] & 0x03) << 4) | ((buffer[1] & 0xF0) >> 4)];
        if (bufferLength > 1)
            characters[length++] = encodingTable[((buffer[1] & 0x0F) << 2) | ((buffer[2] & 0xC0) >> 6)];
        else characters[length++] = '=';
        if (bufferLength > 2)
            characters[length++] = encodingTable[buffer[2] & 0x3F];
        else characters[length++] = '=';    
    }

    return [[[NSString alloc] initWithBytesNoCopy:characters length:length encoding:NSASCIIStringEncoding freeWhenDone:YES] autorelease];
}
@end