如何在Flutter插件代码中使用UIKit ViewController？遇UIDocumentPicker问题

遇到的错误 #

flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:16:15: No exact matches in call to initializer 
flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:16:15: Found candidate with type '([URL]) -> UIDocumentPickerViewController'
flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:16:15: Found candidate with type '([URL]) -> UIDocumentPickerViewController'
flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:16:72: Type 'Array<URL>.ArrayLiteralElement' (aka 'URL') has no member 'folder'
flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:21:41: Cannot find 'startingDirectory' in scope
flutter_download_manager_darwin/ios/Classes/SwiftFlutterDownloadManagerDarwinPlugin.swift:25:11: Cannot find 'present' in scope

错误原因及修复方案 #

1. 初始化方法错误 + .folder 未识别

• 原因：.folder 属于 UTType 类型，需导入对应框架才能使用，且初始化选择器时的参数类型不匹配。
• 修复：
  • 顶部添加导入语句：import UniformTypeIdentifiers
  • 修改初始化代码为：

    let documentPicker = UIDocumentPickerViewController(forOpeningContentTypes: [UTType.folder])
    

2. startingDirectory 未定义

• 原因：代码中直接使用了未声明的变量，需先定义初始目录的URL。
• 修复：根据需求指定初始目录，比如使用系统文档目录：

  guard let startingDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
      result(FlutterError(code: "DIRECTORY_ERROR", message: "无法获取初始目录", details: nil))
      return
  }
  

3. present 方法找不到

• 原因：Flutter插件类不是UIViewController，无法直接调用present，需通过根视图控制器弹出选择器。
• 修复：获取当前应用的根VC来present：

  if let rootVC = UIApplication.shared.windows.first?.rootViewController {
      rootVC.present(documentPicker, animated: true, completion: nil)
  } else {
      result(FlutterError(code: "VC_ERROR", message: "无法获取根视图控制器", details: nil))
      return
  }
  

4. 未实现UIDocumentPickerDelegate协议

• 原因：设置了documentPicker.delegate = self，但类未遵守该协议，会导致编译错误，同时需要实现回调将结果返回给Flutter。
• 修复：添加协议扩展并实现回调，同时在类中保存Flutter回调对象：

  // 类内添加属性保存回调
  private var result: FlutterResult?
  
  // 扩展实现协议
  extension SwiftFlutterDownloadManagerDarwinPlugin: UIDocumentPickerDelegate {
      public func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) {
          guard let selectedUrl = urls.first else { return }
          result?(selectedUrl.absoluteString)
          result = nil
      }
  
      public func documentPickerWasCancelled(_ controller: UIDocumentPickerViewController) {
          result?(nil)
          result = nil
      }
  }
  

完整修正后的代码 #

import Flutter
import UIKit
import UniformTypeIdentifiers

public class SwiftFlutterDownloadManagerDarwinPlugin: NSObject, FlutterPlugin {
    private var result: FlutterResult?
    
    public static func register(with registrar: FlutterPluginRegistrar) {
        let channel = FlutterMethodChannel(name: "flutter_download_manager_darwin", binaryMessenger: registrar.messenger())
        let instance = SwiftFlutterDownloadManagerDarwinPlugin()
        registrar.addMethodCallDelegate(instance, channel: channel)
    }

    public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
        switch call.method {
        case "pickDirectoryUrl":
            self.result = result
            
            // 创建目录选择器
            let documentPicker = UIDocumentPickerViewController(forOpeningContentTypes: [UTType.folder])
            documentPicker.delegate = self
            documentPicker.allowsMultipleSelection = false
            
            // 设置初始目录
            guard let startingDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
                result(FlutterError(code: "DIRECTORY_ERROR", message: "无法获取初始目录", details: nil))
                self.result = nil
                return
            }
            documentPicker.directoryURL = startingDirectory
            
            // 弹出选择器
            if let rootVC = UIApplication.shared.windows.first?.rootViewController {
                rootVC.present(documentPicker, animated: true, completion: nil)
            } else {
                result(FlutterError(code: "VC_ERROR", message: "无法获取根视图控制器", details: nil))
                self.result = nil
            }
        default:
            result(FlutterMethodNotImplemented)
        }
    }
}

extension SwiftFlutterDownloadManagerDarwinPlugin: UIDocumentPickerDelegate {
    public func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) {
        guard let selectedUrl = urls.first else {
            result?(nil)
            result = nil
            return
        }
        result?(selectedUrl.absoluteString)
        result = nil
    }

    public func documentPickerWasCancelled(_ controller: UIDocumentPickerViewController) {
        result?(nil)
        result = nil
    }
}

内容的提问来源于stack exchange，提问作者UnicornsOnLSD