SwiftUI中如何根据iOS版本声明不同的ViewModel存储属性？

要在同一个ViewModel中兼容iOS 16+的PhotosPickerItem和旧版本的UIImage，可以通过条件编译结合通用逻辑抽离的方式实现，无需拆分ViewModel。

核心实现思路 #

1. 用条件编译指令分别声明不同iOS版本的@Published属性，确保对应版本的代码仅在目标系统编译
2. 将属性变化后的处理逻辑抽进通用方法，避免重复代码
3. 对iOS 16+专属的方法（如loadTransferable）也用条件编译包裹，防止旧版本编译报错

ViewModel代码示例 #

import SwiftUI
#if compiler(>=5.7) && os(iOS)
import PhotosUI
#endif

class NoteViewModel: ObservableObject {
    @Published var noteTitle: String = ""
    @Published var noteText: String = ""
    
    // 图片加载状态枚举，根据你的业务需求定义
    @Published var imageAttachmentState: ImageAttachmentState = .empty
    
    // MARK: - iOS 16+ 专属图片选择属性
    #if compiler(>=5.7) && os(iOS) && iOS_VERSION_GREATER_THAN_OR_EQUAL_TO(160000)
    @Published var imageSelection: PhotosPickerItem? = nil {
        didSet {
            handleImageSelectionUpdate(for: imageSelection)
        }
    }
    #endif
    
    // MARK: - iOS <16 图片选择属性
    #if os(iOS) && !iOS_VERSION_GREATER_THAN_OR_EQUAL_TO(160000)
    @Published var imageSelection: UIImage? = nil {
        didSet {
            handleImageSelectionUpdate(for: imageSelection)
        }
    }
    #endif
    
    // MARK: - 通用图片选择处理逻辑
    private func handleImageSelectionUpdate<T>(for selection: T?) {
        guard let selection = selection else {
            imageAttachmentState = .empty
            return
        }
        
        // 分类型处理不同版本的选择结果
        if let pickerItem = selection as? PhotosPickerItem {
            let progress = loadTransferable(from: pickerItem)
            imageAttachmentState = .loading(progress)
        } else if let image = selection as? UIImage {
            imageAttachmentState = .loaded(image)
            // 这里添加旧版本图片选择后的业务逻辑
        }
    }
    
    // MARK: - iOS 16+ 专属图片加载方法
    #if compiler(>=5.7) && os(iOS) && iOS_VERSION_GREATER_THAN_OR_EQUAL_TO(160000)
    private func loadTransferable(from item: PhotosPickerItem) -> Progress {
        let progress = Progress()
        Task {
            do {
                if let image = try await item.loadTransferable(type: UIImage.self) {
                    DispatchQueue.main.async {
                        self.imageAttachmentState = .loaded(image)
                    }
                }
            } catch {
                DispatchQueue.main.async {
                    self.imageAttachmentState = .error(error)
                }
            }
        }
        return progress
    }
    #endif
    
    // MARK: - 图片状态枚举
    enum ImageAttachmentState {
        case empty
        case loading(Progress)
        case loaded(UIImage)
        case error(Error)
    }
}

关键配置说明 #

• 编译宏设置: 如果项目中未定义iOS_VERSION_GREATER_THAN_OR_EQUAL_TO，需要在Build Settings -> Preprocessor Macros中添加：

  iOS_VERSION_GREATER_THAN_OR_EQUAL_TO(160000)=$(greaterThanOrEqualTo(ios16_0))
  

• View层适配: 在View中同样用条件编译分别渲染PhotosPicker（iOS16+）和UIImagePickerController（旧版本），确保与ViewModel属性对应：

  struct NoteView: View {
      @StateObject var viewModel = NoteViewModel()
      
      var body: some View {
          VStack {
              // 其他UI元素...
              
              #if compiler(>=5.7) && os(iOS) && iOS_VERSION_GREATER_THAN_OR_EQUAL_TO(160000)
              PhotosPicker(selection: $viewModel.imageSelection, matching: .images) {
                  Text("选择图片")
              }
              #else
              Button("选择图片") {
                  let picker = UIImagePickerController()
                  picker.delegate = self
                  // 获取顶层ViewController并弹出选择器
                  UIApplication.shared.topViewController?.present(picker, animated: true)
              }
              #endif
              
              // 根据状态渲染图片/加载提示
              switch viewModel.imageAttachmentState {
              case .empty: Text("未选择图片")
              case .loading(let progress): ProgressView(progress)
              case .loaded(let image): Image(uiImage: image).resizable().scaledToFit()
              case .error(let error): Text("加载失败: \(error.localizedDescription)")
              }
          }
      }
  }
  
  // 旧版本图片选择器代理实现
  extension NoteView: UIImagePickerControllerDelegate, UINavigationControllerDelegate {
      func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any]) {
          picker.dismiss(animated: true)
          if let image = info[.originalImage] as? UIImage {
              viewModel.imageSelection = image
          }
      }
  }
  
  // 辅助扩展：获取顶层ViewController
  extension UIApplication {
      var topViewController: UIViewController? {
          connectedScenes
              .filter { $0.activationState == .foregroundActive }
              .compactMap { $0 as? UIWindowScene }
              .first?
              .windows
              .first(where: \.isKeyWindow)?
              .rootViewController?
              .topMost()
      }
  }
  
  extension UIViewController {
      func topMost() -> UIViewController {
          presentedViewController?.topMost() ?? self
      }
  }
  

方案优势 #

• 无需拆分ViewModel，所有业务逻辑集中维护
• 条件编译确保代码仅在对应系统版本生效，避免运行时兼容性错误
• 通用方法抽离重复逻辑，减少代码冗余

内容的提问来源于stack exchange，提问作者Giumex