使用dataWithContentsOfURL读取本地资产时出错

Question

读取本地资产时出错。 assetURL具有所选图片的url，但是读取图片会生成错误并且NSData返回nil

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    UIImagePickerControllerSourceType sourceType = [picker sourceType];

    if ( sourceType != UIImagePickerControllerSourceTypeCamera )
    {
        //IT’S a local asset

        NSURL *assetURL = [info objectForKey:UIImagePickerControllerReferenceURL];

        NSError *error;

        NSData *data = [NSData dataWithContentsOfURL:assetURL options:NSDataReadingMappedAlways error:&error];

        if (error != nil)
        {
            NSLog(@“%@“,error.description);

            /*
            Error printed:

            2014-07-10 10:45:16.447 ApplicationName[91958:60b] 
            Error Domain=NSCocoaErrorDomain 
            Code=256 "The operation couldn’t be completed. (Cocoa error 256.)" 
            UserInfo=0x112d15d00 
            {NSURL=assets-library://asset/asset.PNG?id=9FE11E80-805B-4EB9-88DC-EE9C3A3051EC&ext=PNG}
            */
        }
        else
        {
            self.image = [UIImage imageWithData:data];
        }
    }
}