Application Architecture

Terminal.Gui v2 uses an instance-based application architecture with the IRunnable interface pattern that decouples views from the global application state, improving testability, enabling multiple application contexts, and providing type-safe result handling.

Key Features

• Instance-Based: Use Application.Create() to get an IApplication instance instead of static methods
• IRunnable Interface: Views implement IRunnable<TResult> to participate in session management without inheriting from Runnable
• Fluent API: Chain Init() and Run() for elegant, concise code
• IDisposable Pattern: Proper resource cleanup with Dispose() or using statements
• Automatic Disposal: Framework-created runnables are automatically disposed
• Type-Safe Results: Generic TResult parameter provides compile-time type safety
• CWP Compliance: All lifecycle events follow the Cancellable Work Pattern

View Hierarchy and Run Stack

graph TB
    subgraph ViewTree["View Hierarchy (SuperView/SubView)"]
        direction TB
        Top[app.TopRunnable<br/>Window]
        Menu[MenuBar]
        Status[StatusBar]
        Content[Content View]
        Button1[Button]
        Button2[Button]
        
        Top --> Menu
        Top --> Status
        Top --> Content
        Content --> Button1
        Content --> Button2
    end
    
    subgraph Stack["app.SessionStack"]
        direction TB
        S1[Window<br/>Currently Active]
        S2[Previous Runnable<br/>Waiting]
        S3[Base Runnable<br/>Waiting]
        
        S1 -.-> S2 -.-> S3
    end
    
    Top -.->|"same instance"| S1
    
    style Top fill:#ccffcc,stroke:#339933,stroke-width:3px
    style S1 fill:#ccffcc,stroke:#339933,stroke-width:3px

Usage Example Flow

sequenceDiagram
    participant App as IApplication
    participant Main as Main Window
    participant Dialog as Dialog
    
    Note over App: Initially empty SessionStack
    
    App->>Main: Run(mainWindow)
    activate Main
    Note over App: SessionStack: [Main]<br/>TopRunnable: Main
    
    Main->>Dialog: Run(dialog)
    activate Dialog
    Note over App: SessionStack: [Dialog, Main]<br/>TopRunnable: Dialog
    
    Dialog->>App: RequestStop()
    deactivate Dialog
    Note over App: SessionStack: [Main]<br/>TopRunnable: Main
    
    Main->>App: RequestStop()
    deactivate Main
    Note over App: SessionStack: []<br/>TopRunnable: null

Key Concepts

Instance-Based vs Static

Terminal.Gui v2 supports both static and instance-based patterns. The static Application class is marked obsolete but still functional for backward compatibility. The recommended pattern is to use Application.Create() to get an IApplication instance:

// OLD (v1 / early v2 - still works but obsolete):
Application.Init ();
Window top = new ();
top.Add (myView);
Application.Run (top);
top.Dispose ();
Application.Shutdown (); // Obsolete - use Dispose() instead

// RECOMMENDED (v2 - instance-based with using statement):
using (IApplication app = Application.Create ().Init ())
{
    Window top = new ();
    top.Add (myView);
    app.Run (top);
    top.Dispose ();
} // app.Dispose() called automatically

// WITH IRunnable (fluent API with automatic disposal):
using (IApplication app = Application.Create ().Init ())
{
    app.Run<ColorPickerDialog> ();
    Color? result = app.GetResult<Color> ();
}

// SIMPLEST (manual disposal):
IApplication app = Application.Create ().Init ();
app.Run<ColorPickerDialog> ();
Color? result = app.GetResult<Color> ();
app.Dispose ();

Note: The static Application class delegates to a singleton instance accessible via Application.Instance. Application.Create() creates a new application instance, enabling multiple application contexts and better testability.

View.App Property

Every view now has an App property that references its application context:

public class View
{
    /// <summary>
    /// Gets the application context for this view.
    /// </summary>
    public IApplication? App { get; internal set; }
    
    /// <summary>
    /// Gets the application context, checking parent hierarchy if needed.
    /// Override to customize application resolution.
    /// </summary>
    public virtual IApplication? GetApp () => App ?? SuperView?.GetApp ();
}

Benefits:

• Views can be tested without Application.Init()
• Multiple applications can coexist
• Clear ownership: views know their context
• Reduced global state dependencies

Accessing Application from Views

Recommended pattern:

public class MyView : View
{
    public override void OnEnter (View view)
    {
        // Use View.App instead of static Application
        App?.TopRunnable?.SetNeedsDraw ();
        
        // Access SessionStack
        if (App?.SessionStack.Count > 0)
        {
            // Work with sessions
        }
    }
}

Alternative - dependency injection:

public class MyView : View
{
    private readonly IApplication _app;
    
    public MyView (IApplication app)
    {
        _app = app;
        // Now completely decoupled from static Application
    }
    
    public void DoWork ()
    {
        _app.TopRunnable?.SetNeedsDraw ();
    }
}

IRunnable Architecture

Terminal.Gui v2 introduces the IRunnable interface pattern that decouples runnable behavior from the Runnable class hierarchy. Views can implement IRunnable<TResult> to participate in session management without inheritance constraints.

Key Benefits

• Interface-Based: No forced inheritance from Runnable
• Type-Safe Results: Generic TResult parameter provides compile-time type safety
• Fluent API: Method chaining for elegant, concise code
• Automatic Disposal: Framework manages lifecycle of created runnables
• CWP Compliance: All lifecycle events follow the Cancellable Work Pattern

Fluent API Pattern

The fluent API enables elegant method chaining with automatic resource management:

// Recommended: using statement with GetResult
using (IApplication app = Application.Create ().Init ())
{
    app.Run<ColorPickerDialog> ();
    Color? result = app.GetResult<Color> ();
    
    if (result is { })
    {
        ApplyColor (result);
    }
}

// Alternative: Manual disposal
IApplication app = Application.Create ().Init ();
app.Run<ColorPickerDialog> ();
Color? result = app.GetResult<Color> ();
app.Dispose ();

if (result is { })
{
    ApplyColor (result);
}

Key Methods:

• Init() - Returns IApplication for chaining
• Run<TRunnable>() - Creates and runs runnable, returns IApplication
• GetResult() / GetResult<T>() - Extract typed result after run
• Dispose() - Release all resources (called automatically with using)

Disposal Semantics

"Whoever creates it, owns it":

Method	Creator	Owner	Disposal
Run<TRunnable>()	Framework	Framework	Automatic when Run<T>() returns
Run(IRunnable)	Caller	Caller	Manual by caller

// Framework ownership - automatic disposal
using (IApplication app = Application.Create ().Init ())
{
    app.Run<MyDialog> (); // Dialog disposed automatically when Run returns
    MyResultType? result = app.GetResult<MyResultType> ();
}

// Caller ownership - manual disposal
using (IApplication app = Application.Create ().Init ())
{
    MyDialog dialog = new ();
    app.Run (dialog);
    MyResultType? result = dialog.Result;
    dialog.Dispose ();  // Caller must dispose
}

Creating Runnable Views

Derive from Runnable<TResult> or implement IRunnable<TResult>:

public class FileDialog : Runnable<string?>
{
    private TextField _pathField;
    
    public FileDialog ()
    {
        Title = "Select File";
        
        _pathField = new () { X = 1, Y = 1, Width = Dim.Fill (1) };
        
        Button okButton = new () { Text = "OK", IsDefault = true };
        okButton.Accepting += (s, e) =>
        {
            Result = _pathField.Text;
            Application.RequestStop ();
        };
        
        Add (_pathField, okButton);
    }
    
    protected override bool OnIsRunningChanging (bool oldValue, bool newValue)
    {
        if (!newValue)  // Stopping - extract result before disposal
        {
            Result = _pathField?.Text;
        }
        return base.OnIsRunningChanging (oldValue, newValue);
    }
}

Lifecycle Properties

• IsRunning - True when runnable is on SessionStack
• IsModal - True when runnable is at top of stack (capturing all input)
• Result - Typed result value set before stopping

Lifecycle Events (CWP-Compliant)

All events follow Terminal.Gui's Cancellable Work Pattern:

Event	Cancellable	When	Use Case
IsRunningChanging	✓	Before add/remove from stack	Extract result, prevent close
IsRunningChanged	✗	After stack change	Post-start/stop cleanup
IsModalChanged	✗	After modal state change	Update UI after focus change

Example - Result Extraction:

protected override bool OnIsRunningChanging (bool oldValue, bool newValue)
{
    if (!newValue)  // Stopping
    {
        // Extract result before views are disposed
        Result = _colorPicker.SelectedColor;
        
        // Optionally cancel stop (e.g., unsaved changes)
        if (HasUnsavedChanges ())
        {
            var response = MessageBox.Query ("Save?", "Save changes?", "Yes", "No", "Cancel");
            if (response == 2)
            {
                return true;  // Cancel stop
            }
            if (response == 0)
            {
                Save ();
            }
        }
    }
    
    return base.OnIsRunningChanging (oldValue, newValue);
}

SessionStack

The SessionStack manages all running IRunnable sessions:

public interface IApplication
{
    /// <summary>
    /// Stack of running IRunnable sessions.
    /// Each entry is a SessionToken wrapping an IRunnable.
    /// </summary>
    ConcurrentStack<SessionToken>? SessionStack { get; }
    
    /// <summary>
    /// The IRunnable at the top of SessionStack (currently modal).
    /// </summary>
    IRunnable? TopRunnable { get; }
}

Stack Behavior:

• Push: Begin(IRunnable) adds to top of stack
• Pop: End(SessionToken) removes from stack
• Peek: TopRunnable returns current modal runnable
• All: SessionStack enumerates all running sessions

IApplication Interface

The IApplication interface defines the application contract with support for both legacy Runnable and modern IRunnable patterns:

public interface IApplication
{
    // IRunnable support (primary)
    IRunnable? TopRunnable { get; }
    View? TopRunnableView { get; }
    ConcurrentStack<SessionToken>? SessionStack { get; }
    
    // Driver and lifecycle
    IDriver? Driver { get; }
    IMainLoopCoordinator? Coordinator { get; }
    
    // Fluent API methods  
    IApplication Init (string? driverName = null);
    void Dispose (); // IDisposable
    
    // Runnable methods
    SessionToken? Begin (IRunnable runnable);
    object? Run (IRunnable runnable, Func<Exception, bool>? errorHandler = null);
    IApplication Run<TRunnable> (Func<Exception, bool>? errorHandler = null) where TRunnable : IRunnable, new();
    void RequestStop (IRunnable? runnable);
    void End (SessionToken sessionToken);
    
    // Result extraction
    object? GetResult ();
    T? GetResult<T> () where T : class;
    
    // ... other members
}

Terminology Changes

Terminal.Gui v2 modernized its terminology for clarity:

Application.TopRunnable (formerly "Current", and before that "Top")

The TopRunnable property represents the IRunnable on the top of the session stack (the active runnable session):

// Access the top runnable session
IRunnable? topRunnable = app.TopRunnable;

// From within a view  
IRunnable? topRunnable = App?.TopRunnable;

// Cast to View if needed
View? topView = app.TopRunnableView;

Why "TopRunnable"?

• Clearly indicates it's the top of the runnable session stack
• Aligns with the IRunnable architecture
• Distinguishes from other concepts like "Current" which could be ambiguous
• Works with any view that implements IRunnable, not just Runnable

Application.SessionStack (formerly "Runnables")

The SessionStack property is the stack of running sessions:

// Access all running sessions
foreach (SessionToken runnable in app.SessionStack)
{
    // Process each session
}

// From within a view
var sessionCount = App?.SessionStack.Count ?? 0;

Why "SessionStack" instead of "Runnables"?

• Describes both content (sessions) and structure (stack)
• Aligns with SessionToken terminology
• Follows .NET naming patterns (descriptive + collection type)

Migration from Static Application

The static Application class delegates to a singleton instance and is marked obsolete. All static methods and properties are marked with [Obsolete] but remain functional for backward compatibility:

public static partial class Application
{
    [Obsolete ("The legacy static Application object is going away.")]
    public static View? TopRunnableView => Instance.TopRunnableView;
    
    [Obsolete ("The legacy static Application object is going away.")]
    public static IRunnable? TopRunnable => Instance.TopRunnable;
    
    [Obsolete ("The legacy static Application object is going away.")]
    public static ConcurrentStack<SessionToken>? SessionStack => Instance.SessionStack;
    
    // ... other obsolete static members
}

Important: The static Application class uses a singleton (Application.Instance), while Application.Create() creates new instances. For new code, prefer the instance-based pattern using Application.Create().

Migration Strategies

Strategy 1: Use View.App

// OLD:
void MyMethod ()
{
    Application.TopRunnable?.SetNeedsDraw ();
}

// NEW:
void MyMethod (View view)
{
    view.App?.TopRunnableView?.SetNeedsDraw ();
}

Strategy 2: Pass IApplication

// OLD:
void ProcessSessions ()
{
    foreach (SessionToken runnable in Application.SessionStack)
    {
        // Process
    }
}

// NEW:
void ProcessSessions (IApplication app)
{
    foreach (SessionToken runnable in app.SessionStack)
    {
        // Process
    }
}

Strategy 3: Store IApplication Reference

public class MyService
{
    private readonly IApplication _app;
    
    public MyService (IApplication app)
    {
        _app = app;
    }
    
    public void DoWork ()
    {
        _app.TopRunnable?.Title = "Processing...";
    }
}

Resource Management and Disposal

Terminal.Gui v2 implements the IDisposable pattern for proper resource cleanup. Applications must be disposed after use to:

• Stop the input thread cleanly
• Release driver resources
• Prevent thread leaks in tests
• Free unmanaged resources

Using the using Statement (Recommended)

// Automatic disposal with using statement
using (IApplication app = Application.Create ().Init ())
{
    app.Run<MyDialog> ();
    // app.Dispose() automatically called when scope exits
}

Manual Disposal

// Manual disposal
IApplication app = Application.Create ();
try
{
    app.Init ();
    app.Run<MyDialog> ();
}
finally
{
    app.Dispose (); // Ensure cleanup even if exception occurs
}

Dispose() and Result Retrieval

• Dispose() - Standard IDisposable pattern for resource cleanup (required)
• GetResult() / GetResult<T>() - Retrieve results after run completes
• Shutdown() - Obsolete (use Dispose() instead)

// RECOMMENDED (using statement):
using (IApplication app = Application.Create ().Init ())
{
    app.Run<MyDialog> ();
    MyResult? result = app.GetResult<MyResult> ();
    // app.Dispose() called automatically here
}

// ALTERNATIVE (manual disposal):
IApplication app = Application.Create ().Init ();
app.Run<MyDialog> ();
MyResult? result = app.GetResult<MyResult> ();
app.Dispose (); // Must call explicitly

// OLD (obsolete - do not use):
object? result = app.Run<MyDialog> ().Shutdown ();

Input Thread Lifecycle

When calling Init(), Terminal.Gui starts a dedicated input thread that continuously polls for console input. This thread must be stopped properly:

IApplication app = Application.Create ();
app.Init ("fake"); // Input thread starts here

// Input thread runs in background at ~50 polls/second (20ms throttle)

app.Dispose (); // Cancels input thread and waits for it to exit

Important for Tests: Always dispose applications in tests to prevent thread leaks:

[Fact]
public void My_Test ()
{
    using IApplication app = Application.Create ();
    app.Init ("fake");
    
    // Test code here
    
    // app.Dispose() called automatically
}

Singleton Re-initialization

The legacy static Application singleton can be re-initialized after disposal (for backward compatibility with old tests):

// Test 1
Application.Init ();
Application.Shutdown (); // Obsolete but still works for legacy singleton

// Test 2 - singleton resets and can be re-initialized
Application.Init (); // ✅ Works!
Application.Shutdown (); // Obsolete but still works for legacy singleton

However, instance-based applications follow standard IDisposable semantics and cannot be reused after disposal:

IApplication app = Application.Create ();
app.Init ();
app.Dispose ();

app.Init (); // ❌ Throws ObjectDisposedException

Session Management

Begin and End

Applications manage sessions through Begin() and End():

using IApplication app = Application.Create ();
app.Init ();

Window window = new ();

// Begin a new session - pushes to SessionStack
SessionToken? token = app.Begin (window);

// TopRunnable now points to this window
Debug.Assert (app.TopRunnable == window);

// End the session - pops from SessionStack
if (token != null)
{
    app.End (token);
}

// TopRunnable restored to previous runnable (if any)

Nested Sessions

Multiple sessions can run nested:

using IApplication app = Application.Create ();
app.Init ();

// Session 1
Window main = new () { Title = "Main" };
SessionToken? token1 = app.Begin (main);
// app.TopRunnable == main, SessionStack.Count == 1

// Session 2 (nested)
Dialog dialog = new () { Title = "Dialog" };
SessionToken? token2 = app.Begin (dialog);
// app.TopRunnable == dialog, SessionStack.Count == 2

// End dialog
app.End (token2);
// app.TopRunnable == main, SessionStack.Count == 1

// End main
app.End (token1);
// app.TopRunnable == null, SessionStack.Count == 0

View.Driver Property

Similar to View.App, views now have a Driver property:

public class View
{
    /// <summary>
    /// Gets the driver for this view.
    /// </summary>
    public IDriver? Driver => GetDriver ();
    
    /// <summary>
    /// Gets the driver, checking application context if needed.
    /// Override to customize driver resolution.
    /// </summary>
    public virtual IDriver? GetDriver () => App?.Driver;
}

Usage:

public override void OnDrawContent (Rectangle viewport)
{
    // Use view's driver instead of Application.Driver
    Driver?.Move (0, 0);
    Driver?.AddStr ("Hello");
}

Testing with the New Architecture

The instance-based architecture dramatically improves testability:

Testing Views in Isolation

[Fact]
public void MyView_DisplaysCorrectly ()
{
    // Create mock application
    Mock<IApplication> mockApp = new ();
    mockApp.Setup (a => a.TopRunnable).Returns (new Runnable ());
    
    // Create view with mock app
    MyView view = new () { App = mockApp.Object };
    
    // Test without Application.Init()!
    view.SetNeedsDraw ();
    Assert.True (view.NeedsDraw);
    
    // No Application.Shutdown() needed!
}

Testing with Real Application

[Fact]
public void MyView_WorksWithRealApplication ()
{
    using IApplication app = Application.Create ();
    app.Init ("fake");
    
    MyView view = new ();
    Window top = new ();
    top.Add (view);
    
    app.Begin (top);
    
    // View.App automatically set
    Assert.NotNull (view.App);
    Assert.Same (app, view.App);
    
    // Test view behavior
    view.DoSomething ();
}

Best Practices

DO: Use View.App

✅ GOOD:
public void Refresh ()
{
    App?.TopRunnableView?.SetNeedsDraw ();
}

DON'T: Use Static Application

❌ AVOID:
public void Refresh ()
{
    Application.TopRunnableView?.SetNeedsDraw (); // Obsolete!
}

DO: Pass IApplication as Dependency

✅ GOOD:
public class Service
{
    public Service (IApplication app) { }
}

DON'T: Use Static Application in New Code

❌ AVOID (obsolete pattern):
public void Refresh ()
{
    Application.TopRunnableView?.SetNeedsDraw (); // Obsolete static access
}

✅ PREFERRED:
public void Refresh ()
{
    App?.TopRunnableView?.SetNeedsDraw (); // Use View.App property
}

DO: Override GetApp() for Custom Resolution

✅ GOOD:
public class SpecialView : View
{
    private IApplication? _customApp;
    
    public override IApplication? GetApp ()
    {
        return _customApp ?? base.GetApp ();
    }
}

Advanced Scenarios

Multiple Applications

The instance-based architecture enables multiple applications:

// Application 1
using IApplication app1 = Application.Create ();
app1.Init ("windows");
Window top1 = new () { Title = "App 1" };
// ... configure top1

// Application 2 (different driver!)
using IApplication app2 = Application.Create ();
app2.Init ("unix");
Window top2 = new () { Title = "App 2" };
// ... configure top2

// Views in top1 use app1
// Views in top2 use app2

Application-Agnostic Views

Create views that work with any application:

public class UniversalView : View
{
    public void ShowMessage (string message)
    {
        // Works regardless of which application context
        IApplication? app = GetApp ();
        if (app != null)
        {
            MessageBox msg = new (message);
            app.Begin (msg);
        }
    }
}

See Also

• Navigation - Navigation with the instance-based architecture
• Keyboard - Keyboard handling through View.App
• Mouse - Mouse handling through View.App
• Drivers - Driver access through View.Driver
• Multitasking - Session management with SessionStack