1.概要

最近在迁移 GeneralUpdate.Tool的时候需要用到文件夹选择，在MAUI中可以使用FolderPicker进行选择。注意，和上篇文章的文件选择不一样。因为在.NET MAUI中目前还没有傻瓜式直接可用的FolderPicker供开发者使用所以需要自己动手做一些修改。

完整示例代码：https://gitee.com/Juster-zhu/GeneralUpdate/tree/master/src/c%23/GeneralUpdate.PacketTool

2.详细内容

实现步骤如下：

1. 定义接口

    public interface IFolderPickerService
    {
        Task<string> PickFolderTaskAsync();
    }

2.在每个受支持的平台上实现接口

using GeneralUpdate.Infrastructure.DataServices.Pick;
using WindowsFolderPicker = Windows.Storage.Pickers.FolderPicker;

namespace GeneralUpdate.PacketTool.Platforms.Windows
{
    public class FolderPicker : IFolderPickerService
    {
        public async Task<string> PickFolderTaskAsync()
        {
            var folderPicker = new WindowsFolderPicker();
            // Might be needed to make it work on Windows 10
            folderPicker.FileTypeFilter.Add("*");

            // Get the current window's HWND by passing in the Window object 
            var hwnd = ((MauiWinUIWindow)App.Current.Windows[0].Handler.PlatformView).WindowHandle;

            // Associate the HWND with the file picker
            WinRT.Interop.InitializeWithWindow.Initialize(folderPicker, hwnd);

            var result = await folderPicker.PickSingleFolderAsync();

            return result?.Path;
        }
    }
}

3.向.NET MAUI框架容器中注入FolderPicker注册实现

一定需要记住下面代码中的这个using引用。

using GeneralUpdate.Infrastructure.DataServices.Pick;

不可以删除因为加入了环境的判断会导致在编码时认为是无效应用，实际运行时会使用到该命名空间。

#if WINDOWS
        mauiAppBuilder.Services.AddTransient<IFolderPickerService, Platforms.Windows.FolderPicker>();
#elif MACCATALYST
        mauiAppBuilder.Services.AddTransient<IFolderPickerService, Platforms.MacCatalyst.FolderPicker>();
#endif

实际代码如下。

using GeneralUpdate.Infrastructure.DataServices.Pick;
using GeneralUpdate.PacketTool.ViewModels;

namespace GeneralUpdate.PacketTool
{
    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .RegisterViewModels()
                .RegisterView()
                .RegisterAppServices()
                .RegisterOther()
                .ConfigureFonts(fonts =>
                {
                    fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
                    fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
                });
            return builder.Build();
        }

        public static MauiAppBuilder RegisterOther(this MauiAppBuilder mauiAppBuilder)
        {
            mauiAppBuilder.Services.AddTransient<App>();
            return mauiAppBuilder;
        }

        public static MauiAppBuilder RegisterView(this MauiAppBuilder mauiAppBuilder)
        {
            mauiAppBuilder.Services.AddTransient<MainPage>();
            return mauiAppBuilder;
        }

        public static MauiAppBuilder RegisterViewModels(this MauiAppBuilder mauiAppBuilder)
        {
            mauiAppBuilder.Services.AddTransient<MainViewModel>();
            return mauiAppBuilder;
        }

        public static MauiAppBuilder RegisterAppServices(this MauiAppBuilder mauiAppBuilder)
        {
#if WINDOWS
        mauiAppBuilder.Services.AddTransient<IFolderPickerService, Platforms.Windows.FolderPicker>();
#elif MACCATALYST
        mauiAppBuilder.Services.AddTransient<IFolderPickerService, Platforms.MacCatalyst.FolderPicker>();
#endif 
            return mauiAppBuilder;
        }
    }
}

4.使用功能

• 如何使用

将我们刚刚在容器中注入好的FolderPickerService取出来，并初始化ViewModel中的引用。

    public class MainViewModel : ViewModeBase
    {
        //code...

        public MainViewModel(IFolderPickerService folderPickerService) 
        {
            _folderPickerService = folderPickerService;
        }

        //code...
     }

FolderPickerService调用。

        /// <summary> 
        /// Choose a path 
        /// </summary> 
        /// <param name="value"></param> 
        private async Task SelectFolderAction(string value)
        {
            var pickerResult = await _folderPickerService.PickFolderTaskAsync();
            if (pickerResult == null)
            {
                await Shell.Current.DisplayAlert("Pick options", "No results were selected !", "ok");
                return;
            }
            switch (value)
            {
                case "Source":
                    SourcePath = pickerResult;
                    break;
                case "Target":
                    TargetPath = pickerResult;
                    break;
                case "Patch":
                    PatchPath = pickerResult;
                    break;
            }
        }

• 运行效果如下