使用PHImageManager调整图像大小以保持aspectfit并添加透明背景

我想把图像的大小调整为正方形。如果原始图像不是正方形的,我想添加透明背景,使图像的纵横比保持不变。我的代码如下,我知道我现在没有做任何添加透明背景。有办法吗 requestImageForAsset

PHImageRequestOptions *requestOptions =  [[PHImageRequestOptions alloc] init];
            requestOptions.resizeMode   = PHImageRequestOptionsResizeModeExact;
            requestOptions.deliveryMode = PHImageRequestOptionsDeliveryModeHighQualityFormat;
            requestOptions.synchronous = true;
            NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
            NSString *documentsDirectory = [paths objectAtIndex:0];
            PHImageManager *manager = [PHImageManager defaultManager];

            [manager requestImageForAsset:_asset
                     targetSize:!_isLogo? LARGE_IMAGE_SIZE : COMPANY_LOGO_LARGE_SIZE
                     contentMode:PHImageContentModeAspectFit
                     options:requestOptions
                     resultHandler:^void(UIImage *image, NSDictionary *info) {

                         NSData *imageData = UIImagePNGRepresentation(image);
                         NSString *filename = [NSString stringWithFormat:@"/img_%f.png", [[Utils getCurrentDate] timeIntervalSince1970]];
                         NSString *tempPath = [documentsDirectory stringByAppendingString:filename];
                         [imageData writeToFile:tempPath atomically:YES];
                         largeImage = image;
            }];