# How to Create a Cross-Platform Media Player using Avalonia MVVM and VisioForge SDK

This guide will walk you through the process of building a cross-platform media player application using Avalonia UI with the Model-View-ViewModel (MVVM) pattern and the VisioForge Media Player SDK. The application will be capable of playing video files on Windows, macOS, Linux, Android, and iOS.

We will be referencing the SimplePlayerMVVM example project, which demonstrates the core concepts and implementation details.

[SCREENSHOT: Final application running on multiple platforms]

Before you begin, ensure you have the following installed:

• .NET SDK (latest version, e.g., .NET 8 or newer)
• An IDE such as Visual Studio, JetBrains Rider, or VS Code with C# and Avalonia extensions.
• For Android development:
  • Android SDK
  • Java Development Kit (JDK)
• For iOS development (requires a macOS machine):
  • Xcode
  • Necessary provisioning profiles and certificates.
• VisioForge .NET SDK (MediaPlayer SDK X). You can obtain this from the VisioForge website. The necessary packages will be added via NuGet.

This section outlines how to set up the solution structure and include the necessary VisioForge SDK packages.

The SimplePlayerMVVM solution consists of several projects:

• SimplePlayerMVVM: A .NET Standard library containing the core application logic, including ViewModels, Views (AXAML), and shared interfaces. This is the main project where most of our application logic resides.
• SimplePlayerMVVM.Android: The Android-specific head project.
• SimplePlayerMVVM.Desktop: The desktop-specific head project (Windows, macOS, Linux).
• SimplePlayerMVVM.iOS: The iOS-specific head project.

[SCREENSHOT: Solution structure in the IDE]

The main project, SimplePlayerMVVM.csproj, targets multiple platforms. Key package references include:

• Avalonia: The core Avalonia UI framework.
• Avalonia.Themes.Fluent: Provides a Fluent Design theme.
• Avalonia.ReactiveUI: For MVVM support using ReactiveUI.
• VisioForge.DotNet.MediaBlocks: Core VisioForge media processing components.
• VisioForge.DotNet.Core.UI.Avalonia: VisioForge UI components for Avalonia, including the VideoView.

<Project Sdk="Microsoft.NET.Sdk">
 <PropertyGroup>
  <Nullable>enable</Nullable>
  <LangVersion>latest</LangVersion>
  <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
 </PropertyGroup>
 <ItemGroup>
  <AvaloniaResource Include="Assets\**" />
 </ItemGroup>
 <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('Windows'))">
  <TargetFrameworks>net8.0-android;net8.0-ios;net8.0-windows</TargetFrameworks>
 </PropertyGroup>
 <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('OSX'))">
  <TargetFrameworks>net8.0-android;net8.0-ios;net8.0-macos14.0</TargetFrameworks>
 </PropertyGroup>
 <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('Linux'))">
  <TargetFrameworks>net8.0-android;net8.0</TargetFrameworks>
 </PropertyGroup>
 <ItemGroup>
  <AvaloniaResource Include="Assets\**" />
 </ItemGroup>
 <ItemGroup>
  <PackageReference Include="Avalonia" Version="11.3.0" />
  <PackageReference Include="Avalonia.Themes.Fluent" Version="11.3.0" />
  <PackageReference Include="Avalonia.Fonts.Inter" Version="11.3.0" />
  <!--Condition below is needed to remove Avalonia.Diagnostics package from build output in Release configuration.-->
  <PackageReference Condition="'$(Configuration)' == 'Debug'" Include="Avalonia.Diagnostics" Version="11.3.0" />
  <PackageReference Include="Avalonia.ReactiveUI" Version="$(AvaloniaVersion)" />
 </ItemGroup>
 <ItemGroup Condition="'$(TargetFramework)' == 'net8.0-android'">
  <PackageReference Include="Avalonia.Android" Version="$(AvaloniaVersion)" />
 </ItemGroup>
 <ItemGroup>
  <PackageReference Include="VisioForge.DotNet.MediaBlocks" Version="2025.5.1" />
  <PackageReference Include="VisioForge.DotNet.Core.UI.Avalonia" Version="2025.5.1" />
 </ItemGroup>
</Project>

This setup allows the core logic to be shared across all target platforms.

Each platform head project (SimplePlayerMVVM.Android.csproj, SimplePlayerMVVM.Desktop.csproj, SimplePlayerMVVM.iOS.csproj) includes platform-specific dependencies and configurations.

Desktop (SimplePlayerMVVM.Desktop.csproj):

• References Avalonia.Desktop.
• Includes platform-specific VisioForge native libraries (e.g., VisioForge.CrossPlatform.Core.Windows.x64, VisioForge.CrossPlatform.Core.macOS).

  <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('Windows'))">
    <TargetFramework>net8.0-windows</TargetFramework>
    <OutputType>WinExe</OutputType>
  </PropertyGroup>
  <ItemGroup Condition="$([MSBuild]::IsOsPlatform('Windows'))">
    <PackageReference Include="VisioForge.CrossPlatform.Core.Windows.x64" Version="2025.4.9" />
    <PackageReference Include="VisioForge.CrossPlatform.Libav.Windows.x64.UPX" Version="2025.4.9" />
  </ItemGroup>
  <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('OSX'))">
    <TargetFramework>net8.0-macos14.0</TargetFramework>
    <OutputType>Exe</OutputType>
  </PropertyGroup>
  <ItemGroup Condition="$([MSBuild]::IsOsPlatform('OSX'))">
    <PackageReference Include="VisioForge.CrossPlatform.Core.macOS" Version="2025.2.15" />
  </ItemGroup>
  <PropertyGroup Condition="$([MSBuild]::IsOsPlatform('Linux'))">
    <TargetFramework>net8.0</TargetFramework>
    <OutputType>Exe</OutputType>
  </PropertyGroup>

Android (SimplePlayerMVVM.Android.csproj):

• References Avalonia.Android.
• Includes Android-specific VisioForge libraries and dependencies like VisioForge.CrossPlatform.Core.Android.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0-android</TargetFramework>
    <SupportedOSPlatformVersion>21</SupportedOSPlatformVersion>
    <Nullable>enable</Nullable>
    <ApplicationId>com.CompanyName.Simple_Player_MVVM</ApplicationId>
    <ApplicationVersion>1</ApplicationVersion>
    <ApplicationDisplayVersion>1.0</ApplicationDisplayVersion>
    <AndroidPackageFormat>apk</AndroidPackageFormat>
    <AndroidEnableProfiledAot>false</AndroidEnableProfiledAot>
  </PropertyGroup>
  <!-- ... other items ... -->
  <ItemGroup>
    <ProjectReference Include="..\..\..\..\AndroidDependency\VisioForge.Core.Android.X8.csproj" />
    <ProjectReference Include="..\SimplePlayerMVVM\SimplePlayerMVVM.csproj" />
  </ItemGroup>
  <ItemGroup>
    <PackageReference Include="VisioForge.CrossPlatform.Core.Android" Version="15.10.33" />
  </ItemGroup>
</Project>

iOS (SimplePlayerMVVM.iOS.csproj):

• References Avalonia.iOS.
• Includes iOS-specific VisioForge libraries like VisioForge.CrossPlatform.Core.iOS.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0-ios</TargetFramework>
    <SupportedOSPlatformVersion>13.0</SupportedOSPlatformVersion>
    <Nullable>enable</Nullable>
    <RootNamespace>Simple_Player_MVVM.iOS</RootNamespace>
    <ApplicationId>com.visioforge.avaloniaplayer</ApplicationId>
  </PropertyGroup>
  <!-- ... other items ... -->
  <ItemGroup>
    <PackageReference Include="Avalonia.iOS" Version="$(AvaloniaVersion)" />
    <PackageReference Include="VisioForge.CrossPlatform.Core.iOS" Version="2025.0.16" />
  </ItemGroup>
  <ItemGroup>
    <ProjectReference Include="..\SimplePlayerMVVM\SimplePlayerMVVM.csproj" />
  </ItemGroup>
</Project>

These project files are crucial for managing dependencies and build configurations for each platform.

The application follows the MVVM pattern, separating UI (Views) from logic (ViewModels) and data (Models). ReactiveUI is used to facilitate this pattern.

This abstract class serves as the base for all ViewModels in the application. It inherits from ReactiveObject, which is part of ReactiveUI and provides the necessary infrastructure for property change notifications.

using ReactiveUI;

namespace Simple_Player_MVVM.ViewModels
{
    public abstract class ViewModelBase : ReactiveObject
    {
    }
}

Any ViewModel that needs to notify the UI of property changes should inherit from ViewModelBase.

[SCREENSHOT: ViewModelBase.cs code]

The ViewLocator class is responsible for locating and instantiating Views based on the type of their corresponding ViewModel. It implements Avalonia's IDataTemplate interface.

using Avalonia.Controls;
using Avalonia.Controls.Templates;
using Simple_Player_MVVM.ViewModels;
using System;

namespace Simple_Player_MVVM
{
    public class ViewLocator : IDataTemplate
    {
        public Control? Build(object? data)
        {
            if (data is null)
                return null;

            var name = data.GetType().FullName!.Replace("ViewModel", "View", StringComparison.Ordinal);
            var type = Type.GetType(name);

            if (type != null)
            {
                return (Control)Activator.CreateInstance(type)!;
            }

            return new TextBlock { Text = "Not Found: " + name };
        }

        public bool Match(object? data)
        {
            return data is ViewModelBase;
        }
    }
}

When Avalonia needs to display a ViewModel, the ViewLocator's Match method checks if the data object is a ViewModelBase. If it is, the Build method attempts to find a corresponding View by replacing "ViewModel" with "View" in the ViewModel's class name and instantiates it.

This convention-based approach simplifies the association between Views and ViewModels.

[SCREENSHOT: ViewLocator.cs code]

The App.axaml file defines the application-level resources, including the ViewLocator as a data template and the theme (FluentTheme).

App.axaml:

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="using:Simple_Player_MVVM"
             x:Class="Simple_Player_MVVM.App"
             RequestedThemeVariant="Default">
    <Application.DataTemplates>
        <local:ViewLocator/>
    </Application.DataTemplates>

    <Application.Styles>
        <FluentTheme />
    </Application.Styles>
</Application>

App.axaml.cs: The App.axaml.cs file handles the application's initialization and lifecycle.

Key responsibilities in OnFrameworkInitializationCompleted:

1. Creates an instance of MainViewModel.
2. Sets up the main window or view based on the application lifetime (IClassicDesktopStyleApplicationLifetime for desktop, ISingleViewApplicationLifetime for mobile/web-like views).
3. Assigns the MainViewModel instance as the DataContext for the main window/view.
4. Retrieves the IVideoView instance from the MainView (hosted within MainWindow or directly as MainView).
5. Passes the IVideoView and the TopLevel control (necessary for file dialogs and other top-level interactions) to the MainViewModel.

using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using Simple_Player_MVVM.ViewModels;
using Simple_Player_MVVM.Views;
using VisioForge.Core.Types;

namespace Simple_Player_MVVM
{
    public partial class App : Application
    {
        public override void Initialize()
        {
            AvaloniaXamlLoader.Load(this);
        }

        public override void OnFrameworkInitializationCompleted()
        {
            IVideoView videoView = null;
            var model = new MainViewModel();
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            {
                desktop.MainWindow = new MainWindow
                {
                    DataContext = model
                };

                videoView = (desktop.MainWindow as MainWindow).GetVideoView();
                model.VideoViewIntf = videoView;
                model.TopLevel = desktop.MainWindow;
            }
            else if (ApplicationLifetime is ISingleViewApplicationLifetime singleViewPlatform)
            {
                singleViewPlatform.MainView = new MainView
                {
                    DataContext = model
                };

                videoView = (singleViewPlatform.MainView as MainView).GetVideoView();
                model.VideoViewIntf = videoView;
                model.TopLevel = TopLevel.GetTopLevel(singleViewPlatform.MainView);
            }

            base.OnFrameworkInitializationCompleted();
        }
    }
}

This setup ensures that the MainViewModel has access to the necessary UI components for video playback and interaction, regardless of the platform.

[SCREENSHOT: App.axaml.cs code focusing on OnFrameworkInitializationCompleted]

The MainViewModel is central to the media player's functionality. It manages the player's state, handles user interactions, and communicates with the VisioForge MediaPlayerCoreX engine.

[SCREENSHOT: MainViewModel.cs overall structure or class definition]

Key components of MainViewModel:

The ViewModel exposes several properties that are bound to UI elements in MainView.axaml. These properties use ReactiveUI's RaiseAndSetIfChanged to notify the UI of changes.

• VideoViewIntf (IVideoView): A reference to the VideoView control in the UI, passed from App.axaml.cs.
• TopLevel (TopLevel): A reference to the top-level control, used for displaying file dialogs.
• Position (string?): Current playback position (e.g., "00:01:23").
• Duration (string?): Total duration of the media file (e.g., "00:05:00").
• Filename (string? or Foundation.NSUrl? for iOS): The name or path of the currently loaded file.
• VolumeValue (double?): Current volume level (0-100).
• PlayPauseText (string?): Text for the Play/Pause button (e.g., "PLAY" or "PAUSE").
• SpeedText (string?): Text indicating the current playback speed (e.g., "SPEED: 1X").
• SeekingValue (double?): Current value of the seeking slider.
• SeekingMaximum (double?): Maximum value of the seeking slider (corresponds to media duration in milliseconds).

// Example property
private string? _Position = "00:00:00";
public string? Position
{
    get => _Position;
    set => this.RaiseAndSetIfChanged(ref _Position, value);
}

// ... other properties ...

ReactiveUI ReactiveCommand instances are used to handle actions triggered by UI elements (e.g., button clicks, slider value changes).

• OpenFileCommand: Opens a file dialog to select a media file.
• PlayPauseCommand: Plays or pauses the media.
• StopCommand: Stops playback.
• SpeedCommand: Cycles through playback speeds (1x, 2x, 0.5x).
• VolumeValueChangedCommand: Updates the player volume when the volume slider changes.
• SeekingValueChangedCommand: Seeks to a new position when the seeking slider changes.
• WindowClosingCommand: Handles cleanup when the application window is closing.

// Constructor - Command initialization
public MainViewModel()
{
    OpenFileCommand = ReactiveCommand.Create(OpenFileAsync);
    PlayPauseCommand = ReactiveCommand.CreateFromTask(PlayPauseAsync);
    StopCommand = ReactiveCommand.CreateFromTask(StopAsync);
    // ... other command initializations ...

    // Subscribe to property changes to trigger commands for sliders
    this.WhenAnyValue(x => x.VolumeValue).Subscribe(_ => VolumeValueChangedCommand.Execute().Subscribe());
    this.WhenAnyValue(x => x.SeekingValue).Subscribe(_ => SeekingValueChangedCommand.Execute().Subscribe());

    _tmPosition = new System.Timers.Timer(1000); // Timer for position updates
    _tmPosition.Elapsed += tmPosition_Elapsed;

    VisioForgeX.InitSDK(); // Initialize VisioForge SDK
}

Note: VisioForgeX.InitSDK() initializes the VisioForge SDK. This should be called once at application startup.

A private field _player of type MediaPlayerCoreX holds the instance of the VisioForge media player engine.

private MediaPlayerCoreX _player;

This asynchronous method initializes or re-initializes the MediaPlayerCoreX instance.

private async Task CreateEngineAsync()
{
    if (_player != null)
    {
        await _player.StopAsync();
        await _player.DisposeAsync();
    }

    _player = new MediaPlayerCoreX(VideoViewIntf); // Pass the Avalonia VideoView
    _player.OnError += _player_OnError; // Subscribe to error events
    _player.Audio_Play = true; // Ensure audio is enabled
    
    // Create source settings from the filename
    var sourceSettings = await UniversalSourceSettings.CreateAsync(Filename);
    await _player.OpenAsync(sourceSettings);
}

Key steps:

1. Disposes of any existing player instance.
2. Creates a new MediaPlayerCoreX, passing the IVideoView from the UI.
3. Subscribes to the OnError event for error handling.
4. Sets Audio_Play = true to enable audio playback by default.
5. Uses UniversalSourceSettings.CreateAsync(Filename) to create source settings appropriate for the selected file.
6. Opens the media source using _player.OpenAsync(sourceSettings).

[SCREENSHOT: CreateEngineAsync method code]

This method is responsible for allowing the user to select a media file.

private async Task OpenFileAsync()
{
    await StopAllAsync(); // Stop any current playback
    PlayPauseText = "PLAY";

#if __IOS__ && !__MACCATALYST__
    // iOS specific: Use IDocumentPickerService
    var filePicker = Locator.Current.GetService<IDocumentPickerService>();
    var res = await filePicker.PickVideoAsync();
    if (res != null)
    {
        Filename = (Foundation.NSUrl)res;
        var access = IOSHelper.CheckFileAccess(Filename); // Helper to check file access
        if (!access)
        {
            IOSHelper.ShowToast("File access error");
            return;
        }
    }
#else
    // Other platforms: Use Avalonia's StorageProvider
    try
    {
        var files = await TopLevel.StorageProvider.OpenFilePickerAsync(new FilePickerOpenOptions
        {
            Title = "Open video file",
            AllowMultiple = false
        });

        if (files.Count >= 1)
        {
            var file = files[0];
            Filename = file.Path.AbsoluteUri;

#if __ANDROID__
            // Android specific: Convert content URI to file path if necessary
            if (!Filename.StartsWith('/'))
            {
                Filename = global::VisioForge.Core.UI.Android.FileDialogHelper.GetFilePathFromUri(AndroidHelper.GetContext(), file.Path);
            }
#endif
        }
    }
    catch (Exception ex)
    {
        // Handle cancellation or errors
        Debug.WriteLine($"File open error: {ex.Message}");
    }
#endif
}

Platform-specific considerations:

• iOS: Uses an IDocumentPickerService (resolved via Locator.Current.GetService) to present the native document picker. IOSHelper.CheckFileAccess is used to ensure the app has permission to access the selected file. The filename is stored as an NSUrl.
• Android: If the path obtained from the file picker is a content URI, FileDialogHelper.GetFilePathFromUri (from VisioForge.Core.UI.Android) is used to convert it to an actual file path. This requires an IAndroidHelper to get the Android context.
• Desktop/Other: Uses TopLevel.StorageProvider.OpenFilePickerAsync for the standard Avalonia file dialog.

[SCREENSHOT: OpenFileAsync method with platform-specific blocks highlighted]

• PlayPauseAsync:

  • If the player is not initialized or stopped (PlaybackState.Free), it calls CreateEngineAsync and then _player.PlayAsync().

  • If playing (PlaybackState.Play), it calls _player.PauseAsync().

  • If paused (PlaybackState.Pause), it calls _player.ResumeAsync().

  • Updates PlayPauseText accordingly and starts/stops the _tmPosition timer.

    private async Task PlayPauseAsync()
    {
        // ... (null/empty filename check) ...
    
        if (_player == null || _player.State == PlaybackState.Free)
        {
            await CreateEngineAsync();
            await _player.PlayAsync();
            _tmPosition.Start();
            PlayPauseText = "PAUSE";
        }
        else if (_player.State == PlaybackState.Play)
        {
            await _player.PauseAsync();
            PlayPauseText = "PLAY";
        }
        else if (_player.State == PlaybackState.Pause)
        {
            await _player.ResumeAsync();
            PlayPauseText = "PAUSE";
        }
    }

• StopAsync:

  • Calls StopAllAsync to stop the player and reset UI elements.

  • Resets SpeedText and PlayPauseText.

    private async Task StopAsync()
    {
        await StopAllAsync();
        SpeedText = "SPEED: 1X";
        PlayPauseText = "PLAY";
    }

• StopAllAsync (Helper):

  • Stops the _tmPosition timer.

  • Calls _player.StopAsync().

  • Resets SeekingMaximum to null (so it gets re-calculated on next play).

    private async Task StopAllAsync()
    {
        if (_player == null) return;
        _tmPosition.Stop();
        if (_player != null) await _player.StopAsync();
        await Task.Delay(300); // Small delay to ensure stop completes
        SeekingMaximum = null;
    }

Cycles through playback rates: 1.0, 2.0, and 0.5.

private async Task SpeedAsync()
{
    if (SpeedText == "SPEED: 1X")
    {
        SpeedText = "SPEED: 2X";
        await _player.Rate_SetAsync(2.0);
    }
    else if (SpeedText == "SPEED: 2X")
    {
        SpeedText = "SPEED: 0.5X";
        await _player.Rate_SetAsync(0.5);
    }
    else if (SpeedText == "SPEED: 0.5X") // Assumes this was the previous state
    {
        SpeedText = "SPEED: 1X";
        await _player.Rate_SetAsync(1.0);
    }
}

Uses _player.Rate_SetAsync(double rate) to change the playback speed.

This method is called by the _tmPosition timer (typically every second) to update the UI with the current playback position and duration.

private async void tmPosition_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{
    if (_player == null) return;

    var pos = await _player.Position_GetAsync();
    var progress = (int)pos.TotalMilliseconds;

    try
    {
        await Dispatcher.UIThread.InvokeAsync(async () =>
        {
            if (_player == null) return;

            _isTimerUpdate = true; // Flag to prevent seeking loop

            if (SeekingMaximum == null)
            {
                SeekingMaximum = (int)(await _player.DurationAsync()).TotalMilliseconds;
            }

            SeekingValue = Math.Min(progress, (int)(SeekingMaximum ?? progress));
            
            Position = $"{pos.ToString(@"hh\:mm\:ss", CultureInfo.InvariantCulture)}";
            Duration = $"{(await _player.DurationAsync()).ToString(@"hh\:mm\:ss", CultureInfo.InvariantCulture)}";

            _isTimerUpdate = false;
        });
    }
    catch (Exception exception)
    {
        System.Diagnostics.Debug.WriteLine(exception);
    }
}

Key actions:

1. Retrieves current position (_player.Position_GetAsync()) and duration (_player.DurationAsync()).
2. Updates SeekingMaximum if it hasn't been set yet (usually after a file is opened).
3. Updates SeekingValue with the current progress.
4. Formats and updates Position and Duration strings.
5. Uses Dispatcher.UIThread.InvokeAsync to ensure UI updates happen on the UI thread.
6. Sets _isTimerUpdate = true before updating SeekingValue and false after, to prevent the OnSeekingValueChanged handler from re-seeking when the timer updates the slider position.

[SCREENSHOT: tmPosition_Elapsed method]

Called when the SeekingValue property changes (i.e., the user moves the seeking slider).

private async Task OnSeekingValueChanged()
{
    if (!_isTimerUpdate && _player != null && SeekingValue.HasValue)
    {
        await _player.Position_SetAsync(TimeSpan.FromMilliseconds(SeekingValue.Value));
    }
}

If not currently being updated by the timer (!_isTimerUpdate), it calls _player.Position_SetAsync() to seek to the new position.

Called when the VolumeValue property changes (i.e., the user moves the volume slider).

private void OnVolumeValueChanged()
{
    if (_player != null && VolumeValue.HasValue)
    {
        // Volume for MediaPlayerCoreX is 0.0 to 1.0
        _player.Audio_OutputDevice_Volume = VolumeValue.Value / 100.0;
    }
}

Sets _player.Audio_OutputDevice_Volume. Note that the ViewModel uses a 0-100 scale for VolumeValue, while the player expects 0.0-1.0.

A simple error handler that logs errors to the debug console.

private void _player_OnError(object sender, VisioForge.Core.Types.Events.ErrorsEventArgs e)
{
    Debug.WriteLine(e.Message);
}

More sophisticated error handling (e.g., showing a message to the user) could be implemented here.

This method is invoked when the main window is closing. It ensures that VisioForge SDK resources are properly released.

private void OnWindowClosing()
{
    if (_player != null)
    {
        _player.OnError -= _player_OnError; // Unsubscribe from events
        _player.Stop(); // Ensure player is stopped (sync version here for quick cleanup)
        _player.Dispose();
        _player = null;
    }

    VisioForgeX.DestroySDK(); // Destroy VisioForge SDK instance
}

It stops the player, disposes of it, and importantly, calls VisioForgeX.DestroySDK() to release all SDK resources. This is crucial to prevent memory leaks or issues when the application exits.

This ViewModel orchestrates all the core logic of the media player, from loading files to controlling playback and interacting with the VisioForge SDK.

The user interface is defined using Avalonia XAML (.axaml files).

This UserControl defines the layout and controls for the media player.

[SCREENSHOT: MainView.axaml rendered UI design]

Key UI Elements:

• avalonia:VideoView: This is the VisioForge control responsible for rendering video. It's placed in the main area of the grid and set to stretch.

  <avalonia:VideoView x:Name="videoView1" Margin="0,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="#0C0C0C"  />

• Seeking Slider (Slider Name="slSeeking"):

  • Maximum="{Binding SeekingMaximum}": Binds to the SeekingMaximum property in MainViewModel.
  • Value="{Binding SeekingValue}": Binds two-way to the SeekingValue property in MainViewModel. Changes to this slider by the user will update SeekingValue, triggering OnSeekingValueChanged. Updates to SeekingValue from the ViewModel (e.g., by the timer) will update the slider's position.

• Time Display (TextBlocks for Position and Duration):

  • Bound to Position and Duration properties in MainViewModel.
  • TextBlock Text="{Binding Filename}" displays the current file name.

• Playback Control Buttons (Buttons):

  • Open File: Command="{Binding OpenFileCommand}"
  • Play/Pause: Command="{Binding PlayPauseCommand}", Content="{Binding PlayPauseText}" (dynamically changes button text).
  • Stop: Command="{Binding StopCommand}"

• Volume and Speed Controls:

  • Volume Slider: Value="{Binding VolumeValue}" (binds to VolumeValue for volume control).
  • Speed Button: Command="{Binding SpeedCommand}", Content="{Binding SpeedText}".

Layout: The view uses a Grid to arrange the VideoView and a StackPanel for the controls at the bottom. The controls themselves are organized using nested StackPanels and Grids for alignment.

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="clr-namespace:Simple_Player_MVVM.ViewModels"
             xmlns:avalonia="clr-namespace:VisioForge.Core.UI.Avalonia;assembly=VisioForge.Core.UI.Avalonia"
             x:Class="Simple_Player_MVVM.Views.MainView"
             x:DataType="vm:MainViewModel">
  <Design.DataContext>
    <vm:MainViewModel />
  </Design.DataContext>

  <Grid RowDefinitions="*,Auto" ColumnDefinitions="*">
    <!-- Video View Placeholder -->
    <Border Grid.Row="0" Background="Black" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
      <avalonia:VideoView x:Name="videoView1" Margin="0,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="#0C0C0C"  />
    </Border>

    <!-- Controls -->
    <StackPanel Grid.Row="1" Background="#1e1e1e" Orientation="Vertical">
      <!-- Slider for seeking -->
      <Slider Name="slSeeking" Margin="16,16,16,0" VerticalAlignment="Center" Maximum="{Binding SeekingMaximum}" Value="{Binding SeekingValue}"/>

      <!-- Time and filename display -->
      <Grid Margin="0">
        <Grid.ColumnDefinitions>
          <ColumnDefinition Width="auto"/>
          <ColumnDefinition Width="*"/>
          <ColumnDefinition Width="auto"/>
        </Grid.ColumnDefinitions>
        <TextBlock Grid.Column="0" Text="{Binding Position}" Foreground="White" VerticalAlignment="Center" Margin="5,0,5,0"/>
        <TextBlock Grid.Column="1" Text="{Binding Filename}" Foreground="White" HorizontalAlignment="Center" />
        <TextBlock Grid.Column="2" Text="{Binding Duration}" Foreground="White" VerticalAlignment="Center" Margin="5,0,5,0"/>
      </Grid>

      <!-- Playback Controls -->
      <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Margin="16,0,5,0">
        <Button Command="{Binding OpenFileCommand}" Content="OPEN FILE" Margin="5" VerticalAlignment="Center"/>
        <Button Name="btPlayPause" Command="{Binding PlayPauseCommand}" Content="{Binding PlayPauseText}" Margin="5"/>
        <Button Name="btStop" Command="{Binding StopCommand}" Content="STOP" Margin="5"/>
      </StackPanel>

      <!-- Volume and Speed Controls -->
      <StackPanel Orientation="Horizontal" HorizontalAlignment="Left" Margin="16,0,5,5">
        <TextBlock Text="Volume" Foreground="White" VerticalAlignment="Center"/>
        <Slider Value="{Binding VolumeValue}" Minimum="0" Maximum="100" Width="150" Margin="15,0,5,0" VerticalAlignment="Center"/>
        <Button Command="{Binding SpeedCommand}" Content="{Binding SpeedText}" Margin="5"/>
      </StackPanel>
    </StackPanel>
  </Grid>
</UserControl>

The x:DataType="vm:MainViewModel" directive enables compiled bindings, providing better performance and compile-time checking of binding paths. The Design.DataContext is used to provide data for the XAML previewer in IDEs.

[SCREENSHOT: MainView.axaml XAML code, perhaps highlighting binding expressions]

The code-behind for MainView is minimal. Its primary purpose is to provide a way for the application setup code (in App.axaml.cs) to access the VideoView control instance.

using Avalonia.Controls;
using VisioForge.Core.Types;

namespace Simple_Player_MVVM.Views
{
    public partial class MainView : UserControl
    {
        // Provides access to the VideoView control instance
        public IVideoView GetVideoView()
        {
            return videoView1; // videoView1 is the x:Name of the VideoView in XAML
        }

        public MainView()
        {
            InitializeComponent(); // Standard Avalonia control initialization
        }
    }
}

This GetVideoView() method is called during application startup to pass the VideoView reference to the MainViewModel.

For desktop platforms, MainWindow.axaml serves as the top-level window that hosts the MainView.

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:Simple_Player_MVVM.ViewModels"
        xmlns:views="clr-namespace:Simple_Player_MVVM.Views"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="Simple_Player_MVVM.Views.MainWindow"
        Icon="/Assets/avalonia-logo.ico"
        Title="Simple_Player_MVVM">
  <views:MainView />
</Window>

It simply embeds the MainView control. The DataContext (which will be an instance of MainViewModel) is typically set in App.axaml.cs when the MainWindow is created.

The code-behind for MainWindow primarily handles two things:

1. Provides a way to get the VideoView from the contained MainView.
2. Hooks into the Closing event of the window to trigger the WindowClosingCommand in the MainViewModel for resource cleanup.

using Avalonia.Controls;
using Simple_Player_MVVM.ViewModels;
using System;
using VisioForge.Core.Types;

namespace Simple_Player_MVVM.Views
{
    public partial class MainWindow : Window
    {
        // Helper to get VideoView from the MainView content
        public IVideoView GetVideoView()
        {
            return (Content as MainView).GetVideoView();
        }

        public MainWindow()
        {
            InitializeComponent();

            // Handle the window closing event to trigger cleanup in ViewModel
            Closing += async (sender, e) =>
            {
                if (DataContext is MainViewModel viewModel)
                {
                    // Execute the command and handle potential errors or completion
                    viewModel.WindowClosingCommand.Execute()
                        .Subscribe(_ => { /* Optional: action on completion */ }, 
                                   ex => Console.WriteLine($"Error during closing: {ex.Message}"));
                }
            };
        }
    }
}

When the window closes, it checks if the DataContext is a MainViewModel and then executes its WindowClosingCommand. This ensures that the MainViewModel can perform necessary cleanup, such as disposing of the MediaPlayerCoreX instance and calling VisioForgeX.DestroySDK().

[SCREENSHOT: MainWindow.axaml.cs code, highlighting the Closing event handler]

While Avalonia and .NET provide a high degree of cross-platform compatibility, certain aspects like file system access and permissions require platform-specific handling.

To abstract platform-specific functionality, interfaces are defined in the core SimplePlayerMVVM project:

• IAndroidHelper.cs:

  namespace SimplePlayerMVVM
  {
      public interface IAndroidHelper
      {
  #if __ANDROID__
          global::Android.Content.Context GetContext();
  #endif
      }
  }

  This interface is used to get the Android Context, which is needed for operations like converting content URIs to file paths.

• IDocumentPickerService.cs:

  using System.Threading.Tasks;
  
  namespace SimplePlayerMVVM;
  
  public interface IDocumentPickerService
  {
      Task<object?> PickVideoAsync();
  }

  This interface abstracts the file picking mechanism, specifically for iOS where a native document picker is preferred.

[SCREENSHOT: Android project structure or MainActivity.cs]

• MainActivity.cs:

  • Inherits from AvaloniaMainActivity<App> and implements IAndroidHelper.

  • CustomizeAppBuilder: Standard Avalonia Android setup.

  • Permissions: In OnCreate, it calls RequestPermissionsAsync to request necessary permissions like Manifest.Permission.Internet, Manifest.Permission.ReadExternalStorage, and Manifest.Permission.ReadMediaVideo (for newer Android versions).

    ```csharp
    // In MainActivity.cs
    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        MainViewModel.AndroidHelper = this; // Provide IAndroidHelper implementation to ViewModel
        RequestPermissionsAsync();
    }
    
    private async void RequestPermissionsAsync()
    {
        RequestPermissions(
            new String[]{
                    Manifest.Permission.Internet,
                    Manifest.Permission.ReadExternalStorage,
                    Manifest.Permission.ReadMediaVideo}, 1004);
    }
    
    public Context GetContext() // IAndroidHelper implementation
    {
        return this;
    }
    ```

  • The MainViewModel.AndroidHelper = this; line makes the MainActivity instance (which implements IAndroidHelper) available to the MainViewModel for getting the Android context.

  • The Android Manifest (AndroidManifest.xml, not explicitly shown in the provided files but essential) must also declare these permissions.

• File Path Handling: As seen in MainViewModel.OpenFileAsync, the FileDialogHelper.GetFilePathFromUri method uses the context obtained via IAndroidHelper to resolve file paths from content URIs, which is common when using Android's file picker.

• Project File (SimplePlayerMVVM.Android.csproj): Configures the Android-specific build, target SDK versions, and includes necessary VisioForge Android libraries.

[SCREENSHOT: iOS project structure or AppDelegate.cs]

• AppDelegate.cs:

  • Inherits from AvaloniaAppDelegate<App>.

  • CustomizeAppBuilder: Registers the IOSDocumentPickerService with the Splat dependency resolver (Locator.CurrentMutable.RegisterConstant). This makes the IDocumentPickerService available for injection or service location in the MainViewModel.

    ```csharp
    // In AppDelegate.cs
    protected override AppBuilder CustomizeAppBuilder(AppBuilder builder)
    {
        Locator.CurrentMutable.RegisterConstant(new IOSDocumentPickerService(), typeof(IDocumentPickerService));
    
        return base.CustomizeAppBuilder(builder)
            .WithInterFont()
            .UseReactiveUI();
    }
    ```

• IOSDocumentPickerService.cs:

  • Implements IDocumentPickerService.

  • Uses UIDocumentPickerViewController to present the native iOS file picker for video files (UTType.Video, UTType.Movie).

  • Handles the DidPickDocumentAtUrls and WasCancelled events from the picker.

  • Returns the selected file URL (NSUrl) via a TaskCompletionSource.

  • Includes utility code (GetTopViewController) to find the topmost view controller to present the picker from.

    // Snippet from IOSDocumentPickerService.cs
    public Task<object?> PickVideoAsync()
    {
        _tcs = new TaskCompletionSource<object?>();
        string[] allowedUTIs = { UTType.Video, UTType.Movie };
        var picker = new UIDocumentPickerViewController(allowedUTIs, UIDocumentPickerMode.Import);
        // ... event subscriptions and presentation ...
        return _tcs.Task;
    }
    
    private void OnDocumentPicked(object sender, UIDocumentPickedAtUrlsEventArgs e)
    {
        // ... handles picked URL, resolves _tcs ...
        NSUrl fileUrl = e.Urls[0];
        _tcs?.TrySetResult(fileUrl);
    }

    [SCREENSHOT: IOSDocumentPickerService.cs showing picker setup or result handling]

• Info.plist:

  • This file is crucial for iOS apps. It must include keys like NSPhotoLibraryUsageDescription if accessing the photo library, or other relevant permissions depending on where files are stored/accessed. The provided Info.plist includes:

    ```xml
    <key>NSPhotoLibraryUsageDescription</key>
    <string>Photo library used to play files</string>
    ```

  • It also defines bundle identifiers, version numbers, supported orientations, etc.

• Project File (SimplePlayerMVVM.iOS.csproj): Configures the iOS-specific build, target OS version, and includes VisioForge iOS libraries.

[SCREENSHOT: Desktop project structure or Program.cs]

• Program.cs:

  • Contains the Main entry point for desktop applications.

  • Uses BuildAvaloniaApp().StartWithClassicDesktopLifetime(args); to initialize and run the Avalonia application for desktop.

  • BuildAvaloniaApp() configures Avalonia with platform detection, fonts, ReactiveUI, and logging.

    internal sealed class Program
    {
        [STAThread]
        public static void Main(string[] args) => BuildAvaloniaApp()
            .StartWithClassicDesktopLifetime(args);
    
        public static AppBuilder BuildAvaloniaApp()
            => AppBuilder.Configure<App>()
                .UsePlatformDetect()
                .WithInterFont()
                .UseReactiveUI()
                .LogToTrace();
    }

• File Access: On desktop, file access is generally more straightforward. The MainViewModel uses TopLevel.StorageProvider.OpenFilePickerAsync which works across Windows, macOS, and Linux without specific helper services like those for Android or iOS URI/permission complexities.

• Project File (SimplePlayerMVVM.Desktop.csproj):

  • Targets specific desktop frameworks (e.g., net8.0-windows, net8.0-macos14.0, net8.0 for Linux).

  • Includes Avalonia.Desktop.

  • Includes platform-specific VisioForge native libraries for Windows (x64) and macOS through PackageReference conditions.

    ```xml
    <ItemGroup Condition="$([MSBuild]::IsOsPlatform('Windows'))">
      <PackageReference Include="VisioForge.CrossPlatform.Core.Windows.x64" Version="..." />
      <PackageReference Include="VisioForge.CrossPlatform.Libav.Windows.x64.UPX" Version="..." />
    </ItemGroup>
    <ItemGroup Condition="$([MSBuild]::IsOsPlatform('OSX'))">
      <PackageReference Include="VisioForge.CrossPlatform.Core.macOS" Version="..." />
    </ItemGroup>
    ```

• Windows Manifest (app.manifest): Used on Windows for application settings, such as compatibility with specific Windows versions.

• macOS Info (Info.plist in Desktop project): Provides bundle information for macOS applications.

These platform-specific projects and configurations ensure that the shared core logic in SimplePlayerMVVM can interact correctly with the native features and requirements of each operating system.

This application leverages several key components from the VisioForge Media Player SDK X:

• VisioForge.Core.MediaPlayerX.MediaPlayerCoreX:
  • The central engine for media playback. It handles opening media sources, controlling playback (play, pause, stop, seek, rate), managing audio and video, and providing status information (position, duration).
  • Initialized in MainViewModel and linked to the VideoViewIntf.
• VisioForge.Core.UI.Avalonia.VideoView:
  • An Avalonia control that serves as the rendering surface for video. It is declared in MainView.axaml and its reference (IVideoView) is passed to MediaPlayerCoreX.
• VisioForge.Core.Types.X.Sources.UniversalSourceSettings:
  • Used to configure the media source. UniversalSourceSettings.CreateAsync(filename) automatically determines the best way to open a given file or URL.
• VisioForge.Core.VisioForgeX static class:
  • InitSDK(): Initializes the VisioForge SDK. This must be called once at application startup (done in MainViewModel constructor in this example, but can also be done in App.axaml.cs).
  • DestroySDK(): Releases all SDK resources. This must be called when the application is closing to prevent resource leaks (done in MainViewModel.OnWindowClosing).
• Platform-Specific Libraries:
  • As detailed in the project setup and platform-specific sections, various NuGet packages like VisioForge.CrossPlatform.Core.Windows.x64, VisioForge.CrossPlatform.Core.macOS, VisioForge.CrossPlatform.Core.Android, and VisioForge.CrossPlatform.Core.iOS provide the necessary native binaries and bindings for each platform.
• VisioForge.Core.UI.Android.FileDialogHelper (for Android):
  • Contains helper methods like GetFilePathFromUri to work with Android's file system and content URIs.

Understanding these components is crucial for working with the VisioForge SDK and extending the player's functionality.

[SCREENSHOT: Diagram showing interaction between MainViewModel, MediaPlayerCoreX, and VideoView]

1. Clone/Download the Source Code: Obtain the SimplePlayerMVVM example project.
2. Restore NuGet Packages: Open the solution in your IDE and ensure all NuGet packages are restored for all projects.
3. Select Startup Project and Target:
   • Desktop: Set SimplePlayerMVVM.Desktop as the startup project. You can then run it directly on Windows, macOS, or Linux (ensure you have the .NET runtime for your OS).
   • Android: Set SimplePlayerMVVM.Android as the startup project. Select an Android emulator or connected device. Build and deploy.
     • Ensure Android SDK and emulators/devices are correctly configured in your IDE.
     • You might need to accept permission prompts on the device/emulator upon first launch.
   • iOS: Set SimplePlayerMVVM.iOS as the startup project. Select an iOS simulator or connected device (requires a macOS build machine and appropriate Apple Developer provisioning).
     • Ensure Xcode and developer tools are correctly configured.
     • You might need to trust the developer certificate on the device.
4. Build and Run: Build the selected startup project and run it.

[SCREENSHOT: IDE showing startup project selection and run button] [SCREENSHOT: Application running on Android emulator] [SCREENSHOT: Application running on iOS simulator] [SCREENSHOT: Application running on Windows/macOS/Linux desktop]

This guide has demonstrated how to build a cross-platform media player using Avalonia UI with the MVVM pattern and the VisioForge Media Player SDK X. By leveraging a shared core project for ViewModels and Views, and handling platform-specifics in dedicated head projects, we can create a maintainable application that runs on a wide range of devices.

Key takeaways:

• The MVVM pattern helps separate concerns and improves testability.
• ReactiveUI simplifies implementing MVVM with Avalonia.
• VisioForge Media Player SDK X provides powerful media playback capabilities, with MediaPlayerCoreX as the core engine and VideoView for Avalonia UI integration.
• Platform-specific considerations, especially for file access and permissions, are handled through interfaces and platform-specific implementations.
• Proper initialization (VisioForgeX.InitSDK()) and cleanup (VisioForgeX.DestroySDK()) of the VisioForge SDK are essential.

You can extend this example by adding more features like playlist support, network streaming, video effects, or more advanced UI controls.