iOS 如何正确的从iPhone相册中导出和保存照片？

//reading out the orginal images
for (int j=0; j<[assetArray count]; j++) {

ALAssetRepresentation *representation = [[assetArray objectAtIndex:j] defaultRepresentation];
NSString* filename = [documentPath stringByAppendingPathComponent:[representation filename]];

[[NSFileManager defaultManager] createFileAtPath:filename contents:nil attributes:nil];
NSOutputStream *outPutStream = [NSOutputStream outputStreamToFileAtPath:filename append:YES];
[outPutStream open];

long long offset = 0;
long long bytesRead = 0;

NSError *error;
uint8_t * buffer = malloc(131072);
while (offset<[representation size] && [outPutStream hasSpaceAvailable]) {
bytesRead = [representation getBytes:buffer fromOffset:offset length:131072 error:&error];
[outPutStream write:buffer maxLength:bytesRead];
offset = offset+bytesRead;
}
[outPutStream close];
free(buffer);
}

//reading out the fullScreenImages and thumbnails
for (int j=0; j<[assetArray count]; j++)
{
@autoreleasepool
{

ALAsset *asset = [assetArray objectAtIndex:j];

NSString *orgFilename = [representation filename];
NSString *filenameFullScreen = [NSString stringWithFormat:@"%@_fullscreen.png",[orgFilename stringByDeletingPathExtension]]
NSString* pathFullScreen = [documentPath stringByAppendingPathComponent:filenameFullScreen];

CGImageRef imageRefFullScreen = [[asset defaultRepresentation] fullScreenImage];
UIImage *imageFullScreen = [UIImage imageWithCGImage:imageRefFullScreen];
NSData *imageDataFullScreen = UIImagePNGRepresentation(imageFullScreen);
[imageDataFullScreen writeToFile:pathFullScreen atomically:YES];

NSString *filenameThumb = [NSString stringWithFormat:@"%@_thumb.png",[orgFilename stringByDeletingPathExtension]]
NSString* pathThumb = [documentPath stringByAppendingPathComponent:filenameThumb];

CGImageRef imageRefThumb = [asset thumbnail];
UIImage *imageThumb = [UIImage imageWithCGImage:imageRefThumb];
NSData *imageDataThumb = UIImagePNGRepresentation(imageThumb);
[imageDataThumb writeToFile:pathThumb atomically:YES];

}
}