Avalonia使用.NET For Anroid API（以文件下载为例）

1. 项目结构

典型Avalonia解决方案结构：

MyApp/
├── MyApp/                    # 共享项目 (UI和业务逻辑)
│   ├── App.axaml
│   ├── ViewModels/
│   └── Views/
├── MyApp.Android/           # Android平台项目
│   ├── MainActivity.cs
│   ├── AndroidManifest.xml
│   └── Services/            # 这里添加Android特定实现
└── MyApp.Desktop/           # 桌面平台项目

服务注册与调用流程

[Android项目]                    [共享项目]
     |                               |
     v                               v
MainActivity.cs   ------->    App.cs（静态服务容器）
     |                               |
RegisterPlatformServices()           GetService<T>()
     |                               |
AndroidFileDownloadService      MainViewModel
                                     |
                                调用下载方法

2. 在共享项目中定义接口

// MyApp/Services/IFileDownloadService.cs
using System.Threading.Tasks;

namespace MyApp.Services
{
    public interface IFileDownloadService
    {
        Task<string> DownloadFileAsync(string url, string filename);
        
        // 可选：提供下载进度
        Task<string> DownloadFileWithProgressAsync(string url, string filename, 
                                               IProgress<double> progress);
    }
}

3. 在Android项目中实现接口

// MyApp.Android/Services/AndroidFileDownloadService.cs
using System;
using System.IO;
using System.Threading.Tasks;
using Android.App;
using Android.Content;
using Android.Database;
using Android.OS;
using MyApp.Services;

namespace MyApp.Android.Services
{
    public class AndroidFileDownloadService : IFileDownloadService
    {
        private readonly Context _context;
        
        public AndroidFileDownloadService()
        {
            _context = Application.Context;
        }
        
        public Task<string> DownloadFileAsync(string url, string filename)
        {
            var tcs = new TaskCompletionSource<string>();
            
            try
            {
                var downloadManager = DownloadManager.FromContext(_context);
                var request = new DownloadManager.Request(Android.Net.Uri.Parse(url));
                
                // 配置下载
                request.SetTitle(filename);
                request.SetDescription("正在下载文件...");
                request.SetNotificationVisibility(DownloadVisibility.VisibleNotifyCompleted);
                
                // 设置下载位置
                string destinationPath = Path.Combine(
                    Environment.GetExternalStoragePublicDirectory(
                        Environment.DirectoryDownloads).AbsolutePath, 
                    filename);
                
                request.SetDestinationUri(Android.Net.Uri.FromFile(new Java.IO.File(destinationPath)));
                
                // 注册下载完成的广播接收器
                var receiver = new DownloadCompletedReceiver(downloadManager, tcs, destinationPath);
                _context.RegisterReceiver(
                    receiver, 
                    new IntentFilter(DownloadManager.ActionDownloadComplete)
                );
                
                // 开始下载
                long downloadId = downloadManager.Enqueue(request);
                receiver.DownloadId = downloadId;
            }
            catch (Exception ex)
            {
                tcs.SetException(ex);
            }
            
            return tcs.Task;
        }
        
        public Task<string> DownloadFileWithProgressAsync(string url, string filename, 
                                                    IProgress<double> progress)
        {
            // 实现带进度的下载
            // 类似上面的方法，但添加了定期检查进度的逻辑
            // ...
            throw new NotImplementedException("进度下载尚未实现");
        }
        
        // 广播接收器处理下载完成事件
        private class DownloadCompletedReceiver : BroadcastReceiver
        {
            private readonly DownloadManager _downloadManager;
            private readonly TaskCompletionSource<string> _tcs;
            private readonly string _destinationPath;
            
            public long DownloadId { get; set; }
            
            public DownloadCompletedReceiver(DownloadManager downloadManager, 
                                     TaskCompletionSource<string> tcs,
                                     string destinationPath)
            {
                _downloadManager = downloadManager;
                _tcs = tcs;
                _destinationPath = destinationPath;
            }
            
            public override void OnReceive(Context context, Intent intent)
            {
                long receivedDownloadId = intent.GetLongExtra(DownloadManager.ExtraDownloadId, -1);
                
                if (receivedDownloadId == DownloadId)
                {
                    // 检查下载状态
                    DownloadManager.Query query = new DownloadManager.Query();
                    query.SetFilterById(DownloadId);
                    ICursor cursor = _downloadManager.InvokeQuery(query);
                    
                    if (cursor.MoveToFirst())
                    {
                        int statusColumnIndex = cursor.GetColumnIndex(DownloadManager.ColumnStatus);
                        int status = cursor.GetInt(statusColumnIndex);
                        
                        if (status == (int)DownloadStatus.Successful)
                        {
                            _tcs.SetResult(_destinationPath);
                        }
                        else
                        {
                            _tcs.SetException(new Exception($"下载失败，状态码: {status}"));
                        }
                    }
                    
                    cursor.Close();
                    context.UnregisterReceiver(this);
                }
            }
        }
    }
}

4. 在Android项目中注册服务

// MyApp.Android/MainActivity.cs
using Android.App;
using Android.Content.PM;
using Android.OS;
using Avalonia;
using Avalonia.Android;
using MyApp.Services;
using MyApp.Android.Services;

namespace MyApp.Android
{
    [Activity(Label = "MyApp", 
              Theme = "@style/MyTheme.NoActionBar",
              Icon = "@drawable/icon",
              LaunchMode = LaunchMode.SingleInstance, 
              ConfigurationChanges = ConfigChanges.Orientation | ConfigChanges.ScreenSize)]
    public class MainActivity : AvaloniaMainActivity<App>
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            
            // 注册Android特定服务
            App.RegisterPlatformServices(services => 
            {
                services.AddSingleton<IFileDownloadService, AndroidFileDownloadService>();
            });
        }
    }
}

5. 在共享项目中添加服务注册支持

// MyApp/App.axaml.cs
using System;
using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using Microsoft.Extensions.DependencyInjection;
using MyApp.Services;
using MyApp.ViewModels;
using MyApp.Views;

namespace MyApp
{
    public class App : Application
    {
        private static readonly ServiceCollection _services = new ServiceCollection();
        private static IServiceProvider _serviceProvider;
        
        public override void Initialize()
        {
            AvaloniaXamlLoader.Load(this);
        }

        public override void OnFrameworkInitializationCompleted()
        {
            // 构建服务提供程序
            _serviceProvider = _services.BuildServiceProvider();
            
            // 注册视图模型
            _services.AddTransient<MainViewModel>();
            
            // 配置应用程序
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            {
                desktop.MainWindow = new MainWindow
                {
                    DataContext = _serviceProvider.GetRequiredService<MainViewModel>()
                };
            }
            
            base.OnFrameworkInitializationCompleted();
        }
        
        // 用于平台特定服务注册
        public static void RegisterPlatformServices(Action<IServiceCollection> configureServices)
        {
            configureServices(_services);
        }
        
        // 获取服务的辅助方法
        public static T GetService<T>() where T : class
        {
            return _serviceProvider?.GetService<T>();
        }
    }
}

6. 在ViewModel中使用下载服务

// MyApp/ViewModels/MainViewModel.cs
using System;
using System.Threading.Tasks;
using MyApp.Services;
using ReactiveUI;

namespace MyApp.ViewModels
{
    public class MainViewModel : ViewModelBase
    {
        private readonly IFileDownloadService _downloadService;
        private string _status;
        
        public string Status
        {
            get => _status;
            set => this.RaiseAndSetIfChanged(ref _status, value);
        }
        
        public ReactiveCommand<string, string> DownloadCommand { get; }
        
        public MainViewModel()
        {
            // 获取平台特定服务
            _downloadService = App.GetService<IFileDownloadService>();
            
            DownloadCommand = ReactiveCommand.CreateFromTask<string, string>(DownloadFileAsync);
            Status = "准备下载";
        }
        
        private async Task<string> DownloadFileAsync(string url)
        {
            try
            {
                Status = "开始下载...";
                
                // 如果服务不可用（例如在设计时），则返回错误
                if (_downloadService == null)
                {
                    Status = "下载服务不可用";
                    return null;
                }
                
                // 执行下载
                string filename = $"download_{DateTime.Now:yyyyMMddHHmmss}.pdf";
                string filePath = await _downloadService.DownloadFileAsync(url, filename);
                
                Status = $"下载完成: {filePath}";
                return filePath;
            }
            catch (Exception ex)
            {
                Status = $"下载失败: {ex.Message}";
                return null;
            }
        }
    }
}

7. 在XAML中添加下载按钮

<!-- MyApp/Views/MainView.axaml -->
<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:MyApp.ViewModels"
             x:Class="MyApp.Views.MainView">
  <Design.DataContext>
    <vm:MainViewModel />
  </Design.DataContext>

  <StackPanel Spacing="20" Margin="20">
    <Button Content="下载PDF示例"
            Command="{Binding DownloadCommand}"
            CommandParameter="https://example.com/sample.pdf" />
    
    <TextBlock Text="{Binding Status}" />
  </StackPanel>
</UserControl>

8. 完整流程

1. 应用启动

• 首先Android项目的MainActivity初始化

• 调用App.RegisterPlatformServices()注册Android服务

2. 服务注册

• AndroidFileDownloadService注册为IFileDownloadService的实现

• 这个服务被添加到App类中的静态_services集合

3. 框架初始化

• App.OnFrameworkInitializationCompleted()被调用

• _serviceProvider从_services构建

• ViewModel被创建和注册

4. ViewModel获取服务

• MainViewModel构造函数中调用App.GetService()

• 返回之前注册的AndroidFileDownloadService实例

5. 调用下载方法

• 用户点击UI中的下载按钮

• ViewModel调用_downloadService.DownloadFileAsync()

• 实际执行的是AndroidFileDownloadService中的实现