Terminal.Gui/docfx/docs/application.md

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 Toplevel
• Fluent API: Chain Init(), Run(), and Shutdown() for elegant, concise code
• 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.Current<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 Toplevel<br/>Waiting]
        S3[Base Toplevel<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/>Current: Main
    
    Main->>Dialog: Run(dialog)
    activate Dialog
    Note over App: SessionStack: [Dialog, Main]<br/>Current: Dialog
    
    Dialog->>App: RequestStop()
    deactivate Dialog
    Note over App: SessionStack: [Main]<br/>Current: Main
    
    Main->>App: RequestStop()
    deactivate Main
    Note over App: SessionStack: []<br/>Current: 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();
var top = new Toplevel();
top.Add(myView);
Application.Run(top);
top.Dispose();
Application.Shutdown();

// NEW (v2 recommended - instance-based):
var app = Application.Create();
app.Init();
var top = new Toplevel();
top.Add(myView);
app.Run(top);
top.Dispose();
app.Shutdown();

// NEWEST (v2 with IRunnable and Fluent API):
Color? result = Application.Create()
                           .Init()
                           .Run<ColorPickerDialog>()
                           .Shutdown() as Color?;

Note: The static Application class delegates to ApplicationImpl.Instance (a singleton). Application.Create() creates a new ApplicationImpl 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?.Current?.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.Current?.SetNeedsDraw();
    }
}

IRunnable Architecture

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

Key Benefits

• Interface-Based: No forced inheritance from Toplevel
• 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:

// All-in-one: Create, initialize, run, shutdown, and extract result
Color? result = Application.Create()
                           .Init()
                           .Run<ColorPickerDialog>()
                           .Shutdown() as Color?;

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

Key Methods:

• Init() - Returns IApplication for chaining
• Run<TRunnable>() - Creates and runs runnable, returns IApplication
• Shutdown() - Disposes framework-owned runnables, returns object? result

Disposal Semantics

"Whoever creates it, owns it":

Method	Creator	Owner	Disposal
Run<TRunnable>()	Framework	Framework	Automatic in Shutdown()
Run(IRunnable)	Caller	Caller	Manual by caller

// Framework ownership - automatic disposal
var result = app.Run<MyDialog>().Shutdown();

// Caller ownership - manual disposal
var dialog = new MyDialog();
app.Run(dialog);
var 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 TextField { X = 1, Y = 1, Width = Dim.Fill(1) };
        
        var okButton = new Button { 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 RunnableSessionStack
• 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
IsModalChanging	✓	Before becoming/leaving top	Prevent activation
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())
        {
            int 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);
}

RunnableSessionStack

The RunnableSessionStack manages all running IRunnable sessions:

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

Stack Behavior:

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

IApplication Interface

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

public interface IApplication
{
    // Legacy Toplevel support
    Toplevel? Current { get; }
    ConcurrentStack<Toplevel> SessionStack { get; }
    
    // IRunnable support
    IRunnable? TopRunnable { get; }
    ConcurrentStack<RunnableSessionToken>? RunnableSessionStack { get; }
    IRunnable? FrameworkOwnedRunnable { get; set; }
    
    // Driver and lifecycle
    IDriver? Driver { get; }
    IMainLoopCoordinator? MainLoop { get; }
    
    // Fluent API methods
    IApplication Init(string? driverName = null);
    object? Shutdown();
    
    // Runnable methods
    RunnableSessionToken Begin(IRunnable runnable);
    void 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(RunnableSessionToken sessionToken);
    
    // Legacy Toplevel methods
    SessionToken? Begin(Toplevel toplevel);
    void Run(Toplevel view, Func<Exception, bool>? errorHandler = null);
    void End(SessionToken sessionToken);
    
    // ... 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 Toplevel on the top of the session stack (the active runnable session):

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

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

Why "TopRunnable"?

• Clearly indicates it's the top of the runnable session stack
• Aligns with the IRunnable architecture proposal
• Distinguishes from other concepts like "Current" which could be ambiguous

Application.SessionStack (formerly "TopLevels")

The SessionStack property is the stack of running sessions:

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

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

Why "SessionStack" instead of "TopLevels"?

• 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 ApplicationImpl.Instance (a singleton) 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 Toplevel? Current => ApplicationImpl.Instance.Current;
    
    [Obsolete("The legacy static Application object is going away.")]
    public static ConcurrentStack<Toplevel> SessionStack => ApplicationImpl.Instance.SessionStack;
    
    // ... other obsolete static members
}

Important: The static Application class uses a singleton (ApplicationImpl.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?.Current?.SetNeedsDraw();
}

Strategy 2: Pass IApplication

// OLD:
void ProcessSessions()
{
    foreach (var toplevel in Application.SessionStack)
    {
        // Process
    }
}

// NEW:
void ProcessSessions(IApplication app)
{
    foreach (var toplevel 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.Current?.Title = "Processing...";
    }
}

Session Management

Begin and End

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

var app = Application.Create ();
app.Init();

var toplevel = new Toplevel();

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

// Current now points to this toplevel
Debug.Assert(app.Current == toplevel);

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

// Current restored to previous toplevel (if any)

Nested Sessions

Multiple sessions can run nested:

var app = Application.Create ();
app.Init();

// Session 1
var main = new Toplevel { Title = "Main" };
var token1 = app.Begin(main);
// app.Current == main, SessionStack.Count == 1

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

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

// End main
app.End(token1);
// app.Current == 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
    var mockApp = new Mock<IApplication>();
    mockApp.Setup(a => a.Current).Returns(new Toplevel());
    
    // Create view with mock app
    var view = new MyView { App = mockApp.Object };
    
    // Test without Application.Init()!
    view.SetNeedsDraw();
    Assert.True(view.NeedsDraw);
    
    // No Application.Shutdown() needed!
}

Testing with Real ApplicationImpl

[Fact]
public void MyView_WorksWithRealApplication()
{
    var app = Application.Create ();
    try
    {
        app.Init(new FakeDriver());
        
        var view = new MyView();
        var top = new Toplevel();
        top.Add(view);
        
        app.Begin(top);
        
        // View.App automatically set
        Assert.NotNull(view.App);
        Assert.Same(app, view.App);
        
        // Test view behavior
        view.DoSomething();
        
    }
    finally
    {
        app.Shutdown();
    }
}

Best Practices

DO: Use View.App

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

DON'T: Use Static Application

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

✅ PREFERRED:
public void Refresh()
{
    App?.Current?.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
var app1 = Application.Create ();
app1.Init(new WindowsDriver());
var top1 = new Toplevel { Title = "App 1" };
// ... configure top1

// Application 2 (different driver!)
var app2 = Application.Create ();
app2.Init(new CursesDriver());
var top2 = new Toplevel { 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
        var app = GetApp();
        if (app != null)
        {
            var msg = new MessageBox(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