使用Cocoa Touch如何访问并修改用户的照片库

要访问和修改用户的照片库，可以使用UIImagePickerController类和UIImagePickerDelegate协议。以下是一个简单的示例代码，演示如何访问用户的照片库并显示选定的图片：

// 在视图控制器中创建一个UIImagePickerController属性
@property (nonatomic, strong) UIImagePickerController *imagePickerController;

// 实例化UIImagePickerController并设置代理
self.imagePickerController = [[UIImagePickerController alloc] init];
self.imagePickerController.delegate = self;

// 设置UIImagePickerController的sourceType为UIImagePickerControllerSourceTypePhotoLibrary，以访问用户的照片库
self.imagePickerController.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;

// 显示UIImagePickerController
[self presentViewController:self.imagePickerController animated:YES completion:nil];

// 实现UIImagePickerDelegate协议方法，处理用户选择的图片
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info {
    UIImage *selectedImage = info[UIImagePickerControllerOriginalImage];
    
    // 在这里可以对选定的图片进行处理或保存到本地
    
    // 关闭UIImagePickerController
    [picker dismissViewControllerAnimated:YES completion:nil];
}

// 实现UIImagePickerDelegate协议方法，处理用户取消选择
- (void)imagePickerControllerDidCancel:(UIImagePickerController *)picker {
    // 关闭UIImagePickerController
    [picker dismissViewControllerAnimated:YES completion:nil];
}

这段代码演示了如何访问用户的照片库并显示选定的图片。您可以在didFinishPickingMediaWithInfo方法中对选定的图片进行处理，例如显示在UIImageView中，保存到本地等操作。在imagePickerControllerDidCancel方法中处理用户取消选择的情况。