iOS UIImage 和 NSString的相互转化

首先需要定义一个由NSString类转化为NSData类的方法

+ (NSData *)dataWithBase64EncodedString:(NSString *)string
{
static const char encodingTable[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
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];
}

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


相互转化：
UIImage* pic;
NSData* pictureData = UIImagePNGRepresentation(pic);
NSString* pictureDataString = [pictureData base64Encoding];
UIImage* image = [UIImage imageWithData:[PlantListViewController dataWithBase64EncodedString:pictureDataString]];