将图片保存到系统相册

    // 保存图片到系统相册里    
    UIImageWriteToSavedPhotosAlbum(image, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
    // image:保存的图片
    // completionTarget:保存完成的监听者
    // completionSelector:保存完成的时候调用监听者的方法

}

// 保存图片完成的时候调用,必须调用系统的这个方法,不可以自定义别的方法
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo
{
    if (error) {
        [MBProgressHUD showError:@"保存失败"];       
    }else{
        [MBProgressHUD showSuccess:@"保存成功"];
    }
}

从系统相册选择图片

@interface ViewController () <UINavigationControllerDelegate, UIImagePickerControllerDelegate>
...
@end

// 选择照片
- (IBAction)photo:(UIBarButtonItem *)sender {
    // 创建照片选择控制器
    UIImagePickerController *imagePickerVc = [[UIImagePickerController alloc] init];
    // 设置数据源
    imagePickerVc.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum; // 系统相册
    // 设置代理
    imagePickerVc.delegate = self;
    
    //modal - 进入相册界面
    [self presentViewController:imagePickerVc animated:YES completion:nil];
    
}
#pragma mark - <UIImagePickerControllerDelegate>
// 用户选择照片的时候调用
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info{
    // 获取用户选择的照片
    UIImage *image = info[UIImagePickerControllerOriginalImage];
    // modal - 退出相册界面
    [self dismissViewControllerAnimated:YES completion:nil];
    
    _drawView.image = image;
    
}