gui-cs.Terminal.Gui/docfx/docs/navigation.md

Navigation Deep Dive

This document covers Terminal.Gui's navigation system, which determines:

• What are the visual cues that help the user know which element of an application is receiving keyboard and mouse input (which one has focus)?
• How does the user change which element of an application has focus?
• What are the visual cues that help the user know what keystrokes will change the focus?
• What are the visual cues that help the user know what keystrokes will cause action in elements of the application that don't currently have focus?
• What is the order in which UI elements are traversed when using keyboard navigation?
• What are the default actions for standard key/mouse input (e.g. Hotkey, Space, Enter, MouseClick)?

See Also

• Keyboard Deep Dive
• Mouse Deep Dive
• Lexicon & Taxonomy

Lexicon & Taxonomy

[!INCLUDE Navigation Lexicon]

Tenets for Terminal.Gui UI Navigation (Unless you know better ones...)

See the Keyboard Tenets as they apply as well.

Tenets higher in the list have precedence over tenets lower in the list.

• One Focus Per App - It should not be possible to have two views be the "most focused" view in an application. There is always exactly one view that is the target of keyboard input.

• There's Always a Way With The Keyboard - The framework strives to ensure users wanting to use the keyboard can't get into a situation where some element of the application is not accessible via the keyboard. For example, we have unit tests that ensure built-in Views will all have at least one navigation key that advances focus. Another example: As long as a View with a HotKey is visible and enabled, regardless of view-hierarchy, if the user presses that hotkey, the action defined by the hotkey will happen (and, by default the View that defines it will be focused).

• Flexible Overrides - The framework makes it easy for navigation changes to be made from code and enables changing of behavior to be done in flexible ways. For example a view can be prevented from getting focus by setting CanFocus to false or overriding OnHasFocusChanging and returning true to cancel.

• Decouple Concepts - In v1 CanFocus is tightly coupled with HasFocus, TabIndex, TabIndexes, and TabStop and vice-versa. There was a bunch of "magic" logic that automatically attempted to keep these concepts aligned. This resulted in a poorly specified, hard-to-test, and fragile API. In v2 we strive to keep the related navigation concepts decoupled. For example, CanFocus and TabStop are decoupled. A view with CanFocus == true can have TabStop == NoStop and still be focusable with the mouse.

Answering the Key Navigation Questions

Visual Cues for Focus

Current Focus Indicator:

• Views with focus are rendered using their ColorScheme.Focus attribute
• The focused view may display a cursor (for text input views)
• Views in the focus chain (SuperViews of the focused view) also use focused styling

Navigation Cues:

• HotKeys are indicated by underlined characters in Labels, Buttons, and MenuItems
• Tab order is generally left-to-right, top-to-bottom within containers
• Focus indicators (such as highlight rectangles) show which view will receive input

Changing Focus

Keyboard Methods:

• Tab / Shift+Tab - Navigate between TabStop views
• F6 / Shift+F6 - Navigate between TabGroup containers
  
• Arrow keys - Navigate within containers or between adjacent views
• HotKeys - Direct navigation to specific views (Alt+letter combinations)
• Enter / Space - Activate the focused view

Mouse Methods:

• Click on any focusable view to give it focus
• Focus behavior depends on whether the view was previously focused (RestoreFocus vs AdvanceFocus)

Navigation Order

Views are traversed based on their TabStop behavior and position in the view hierarchy:

1. TabStop Views - Navigated with Tab/Shift+Tab in layout order
2. TabGroup Views - Containers navigated with F6/Shift+F6
3. NoStop Views - Skipped during keyboard navigation but can receive mouse focus

Keyboard Navigation

The majority of the Terminal.Gui Navigation system is dedicated to enabling the keyboard to be used to navigate Views.

Terminal.Gui defines these keys for keyboard navigation:

• Application.NextTabStopKey (Key.Tab) - Navigates to the next subview that is a TabStop (see below). If there is no next, the first subview that is a TabStop will gain focus.
• Application.PrevTabStopKey (Key.Tab.WithShift) - Opposite of Application.NextTabStopKey.
• Key.CursorRight - Operates identically to Application.NextTabStopKey.
• Key.CursorDown - Operates identically to Application.NextTabStopKey.
• Key.CursorLeft - Operates identically to Application.PrevTabStopKey.
• Key.CursorUp - Operates identically to Application.PrevTabStopKey.
• Application.NextTabGroupKey (Key.F6) - Navigates to the next view in the view-hierarchy that is a TabGroup (see below). If there is no next, the first view that is a TabGroup will gain focus.
• Application.PrevTabGroupKey (Key.F6.WithShift) - Opposite of Application.NextTabGroupKey.

F6 was chosen to match Windows conventions.

These keys are all registered as KeyBindingScope.Application key bindings by Application. Because application-scoped key bindings have the lowest priority, Views can override the behaviors of these keys (e.g. TextView overrides Key.Tab by default, enabling the user to enter \t into text). The AllViews_AtLeastOneNavKey_Leaves unit test ensures all built-in Views have at least one of the above keys that can advance focus.

Navigation Examples

// Basic focus management
var button = new Button() { Text = "Click Me", CanFocus = true, TabStop = TabBehavior.TabStop };
var textField = new TextField() { Text = "", CanFocus = true, TabStop = TabBehavior.TabStop };

// Container with group navigation
var frameView = new FrameView() 
{ 
    Title = "Options",
    CanFocus = true, 
    TabStop = TabBehavior.TabGroup 
};

// Programmatic focus control
button.SetFocus(); // Give focus to specific view
Application.Navigation.AdvanceFocus(NavigationDirection.Forward, TabBehavior.TabStop);

HotKeys

See also Keyboard where HotKey is covered more deeply...

HotKeys can be used to navigate across the entire application view-hierarchy. They work independently of Focus. This enables a user to navigate across a complex UI of nested subviews if needed (even in overlapped scenarios). An example use case is the AllViewsTester Scenario.

HotKeys are defined using the HotKey property and are activated using Alt+ the specified key:

var saveButton = new Button() { Text = "_Save", HotKey = Key.S };
var exitButton = new Button() { Text = "E_xit", HotKey = Key.X };

// Alt+S will activate save, Alt+X will activate exit, regardless of current focus

Additionally, multiple Views in an application (even within the same SuperView) can have the same HotKey.

Mouse Navigation

Mouse-based navigation is straightforward in comparison to keyboard: If a view is focusable and the user clicks on it, it gains focus. There are some nuances, though:

• If a View is focusable, and it has focusable sub-views, what happens when a user clicks on the Border of the View? Which sub-view (if any) will also get focus?

• If a View is focusable, and it has focusable sub-views, what happens when a user clicks on the ContentArea of the View? Which sub-view (if any) will also get focus?

The answer to both questions is:

If the View was previously focused, the system keeps a record of the SubView that was previously most-focused and restores focus to that SubView (RestoreFocus()).

If the View was not previously focused, AdvanceFocus() is called to find the next appropriate focus target.

For this to work properly, there must be logic that removes the focus-cache used by RestoreFocus() if something changes that makes the previously-focusable view not focusable (e.g. if Visible has changed).

Mouse Focus Examples

// Mouse click behavior
view.MouseEvent += (sender, e) => 
{
    if (e.Flags.HasFlag(MouseFlags.Button1Clicked) && view.CanFocus)
    {
        view.SetFocus();
        e.Handled = true;
    }
};

// Focus on mouse enter (optional behavior)
view.MouseEnter += (sender, e) => 
{
    if (view.CanFocus && focusOnHover)
    {
        view.SetFocus();
    }
};

Application Level Navigation

At the application level, navigation is encapsulated within the @Terminal.Gui.ApplicationNavigation helper class which is publicly exposed via the @Terminal.Gui.App.Application.Navigation property.

@Terminal.Gui.App.ApplicationNavigation.GetFocused gets the most-focused View in the application. Will return null if there is no view with focus (an extremely rare situation). This replaces View.MostFocused in v1.

The @Terminal.Gui.App.ApplicationNavigation.FocusedChanged and @Terminal.Gui.App.ApplicationNavigation.FocusedChanging events are raised when the most-focused View in the application is changing or has changed. FocusedChanged is useful for apps that want to do something with the most-focused view (e.g. see AdornmentsEditor). FocusChanging is useful for apps that want to override what view can be focused across an entire app.

The @Terminal.Gui.App.ApplicationNavigation.AdvanceFocus method causes the focus to advance (forward or backwards) to the next View in the application view-hierarchy, using behavior as a filter.

The implementation is simple:

return app.Current?.AdvanceFocus (direction, behavior);

This method is called from the Command handlers bound to the application-scoped keybindings created during app.Init(). It is public as a convenience.

Note: When accessing from within a View, use App?.Current instead of Application.Current (which is obsolete).

This method replaces about a dozen functions in v1 (scattered across Application and Toplevel).

Application Navigation Examples

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

// Listen for global focus changes
app.Navigation.FocusedChanged += (sender, e) => 
{
    var focused = app.Navigation.GetFocused();
    StatusBar.Text = $"Focused: {focused?.GetType().Name ?? "None"}";
};

// Prevent certain views from getting focus
app.Navigation.FocusedChanging += (sender, e) => 
{
    if (e.NewView is SomeRestrictedView)
    {
        e.Cancel = true; // Prevent focus change
    }
};

// Programmatic navigation
Application.Navigation.AdvanceFocus(NavigationDirection.Forward, TabBehavior.TabStop);
Application.Navigation.AdvanceFocus(NavigationDirection.Backward, TabBehavior.TabGroup);

View Level Navigation

@Terminal.Gui.ViewBase.View.AdvanceFocus is the primary method for developers to cause a view to gain or lose focus.

Various events are raised when a View's focus is changing. For example, @Terminal.Gui.ViewBase.View.HasFocusChanging and @Terminal.Gui.ViewBase.View.HasFocusChanged.

View Focus Management

// Basic focus control
public class CustomView : View
{
    protected override void OnHasFocusChanging(CancelEventArgs<bool> e)
    {
        if (SomeCondition)
        {
            e.Cancel = true; // Prevent focus change
            return;
        }
        base.OnHasFocusChanging(e);
    }

    protected override void OnHasFocusChanged(EventArgs<bool> e)
    {
        if (e.CurrentValue)
        {
            // View gained focus
            UpdateAppearance();
        }
        base.OnHasFocusChanged(e);
    }
}

What makes a View focusable?

First, only Views that are visible and enabled can gain focus. Both Visible and Enabled must be true for a view to be focusable.

For visible and enabled Views, the CanFocus property is then used to determine whether the View is focusable. CanFocus must be true for a View to gain focus. However, even if CanFocus is true, other factors can prevent the view from gaining focus...

A visible, enabled, and CanFocus == true view can be focused if the user uses the mouse to clicks on it or if code explicitly calls View.SetFocus(). Of course, the view itself or some other code can cancel the focus (e.g. by overriding OnHasFocusChanging).

For keyboard navigation, the TabStop property is a filter for which views are focusable from the current most-focused. TabStop has no impact on mouse navigation. TabStop is of type TabBehavior.

TabBehavior Values

• null - This View is still being initialized; acts as a signal to set_CanFocus to set TabStop to TabBehavior.TabStop as convenience for the most common use-case. Equivalent to TabBehavior.NoStop when determining if a view is focusable by the keyboard or not.

• TabBehavior.NoStop - Prevents the user from using keyboard navigation to cause view (and by definition its subviews) to gain focus. Note: The view can still be focused using code or the mouse.

• TabBehavior.TabStop - Indicates a View is a focusable view with no focusable subviews. Application.Next/PrevTabStopKey will advance ONLY through the peer-Views (SuperView.SubViews).

• TabBehavior.TabGroup - Indicates a View is a focusable container for other focusable views and enables keyboard navigation across these containers. This applies to both tiled and overlapped views. For example, FrameView is a simple view designed to be a visible container of other views in tiled scenarios. It has TabStop set to TabBehavior.TabGroup (and Arrangement set to ViewArrangement.Fixed). Likewise, Window is a simple view designed to be a visible container of other views in overlapped scenarios. It has TabStop set to TabBehavior.TabGroup (and Arrangement set to ViewArrangement.Movable | ViewArrangement.Resizable | ViewArrangement.Overlapped). Application.Next/PrevGroupStopKey will advance across all TabGroup views in the application (unless blocked by a NoStop SuperView).

Focus Requirements Summary

For a view to be focusable:

1. Visible = true
2. Enabled = true
   
3. CanFocus = true

4. TabStop != TabBehavior.NoStop (for keyboard navigation only)

   // Example: Make a view focusable
   var view = new Label() 
   {
   Text = "Focusable Label",
   Visible = true,     // Must be visible
   Enabled = true,     // Must be enabled
   CanFocus = true,    // Must be able to focus
   TabStop = TabBehavior.TabStop  // Keyboard navigable
   };
   

How To Tell if a View has focus? And which view is the most-focused?

View.HasFocus indicates whether the View is focused or not. It is the definitive signal. If the view has no focusable SubViews then this property also indicates the view is the most-focused view in the application.

Setting this property to true has the same effect as calling View.SetFocus (), which also means the focus may not change as a result.

If v.HasFocus == true then:

• All views up v's superview-hierarchy must be focusable.
• All views up v's superview-hierarchy will also have HasFocus == true.
• The deepest-subview of v that is focusable will also have HasFocus == true

In other words, v.HasFocus == true does not necessarily mean v is the most-focused view, receiving input. If it has focusable sub-views, one of those (or a further subview) will be the most-focused (Application.Navigation.GetFocused()).

The private bool _hasFocus field backs HasFocus and is the ultimate source of truth whether a View has focus or not.

Focus Chain Example

// In a hierarchy: Window -> Dialog -> Button
// If Button has focus, then:
window.HasFocus == true    // Part of focus chain
dialog.HasFocus == true    // Part of focus chain  
button.HasFocus == true    // Actually focused

// Application.Navigation.GetFocused() returns button
var mostFocused = Application.Navigation.GetFocused(); // Returns button

How does a user tell?

In short: ColorScheme.Focus - Views in the focus chain render with focused colors.

Views use their ColorScheme.Focus attribute when they are part of the focus chain. This provides visual feedback about which part of the application is active.

// Custom focus styling
protected override void OnDrawContent(Rectangle viewport)
{
    var attribute = HasFocus ? GetFocusColor() : GetNormalColor();
    Driver.SetAttribute(attribute);
    // ... draw content
}

How to make a View become focused?

The primary public method for developers to cause a view to get focus is View.SetFocus().

Unlike v1, in v2, this method can return false if the focus change doesn't happen (e.g. because the view wasn't focusable, or the focus change was cancelled).

// Programmatic focus control
if (myButton.SetFocus())
{
    Console.WriteLine("Button now has focus");
}
else
{
    Console.WriteLine("Could not focus button");
}

// Alternative: Set HasFocus property (same effect)
myButton.HasFocus = true;

How to make a View become NOT focused?

The typical method to make a view lose focus is to have another View gain focus.

// Focus another view to remove focus from current
otherView.SetFocus();

// Or advance focus programmatically
Application.Navigation.AdvanceFocus(NavigationDirection.Forward, TabBehavior.TabStop);

// Focus can also be lost when views become non-focusable
myView.CanFocus = false;  // Will lose focus if it had it
myView.Visible = false;   // Will lose focus if it had it
myView.Enabled = false;   // Will lose focus if it had it

Determining the Most Focused SubView

In v1 View had MostFocused property that traversed up the view-hierarchy returning the last view found with HasFocus == true. In v2, Application.Navigation.GetFocused() provides the same functionality with less overhead.

// v2 way to get the most focused view
var focused = Application.Navigation.GetFocused();

// This replaces the v1 pattern:
// var focused = Application.Current.MostFocused;

How Does View.Add/Remove Work?

In v1, calling super.Add (view) where view.CanFocus == true caused all views up the hierarchy (all SuperViews) to get CanFocus set to true as well.

Also, in v1, if view.CanFocus == true, Add would automatically set TabStop.

In v2, developers need to explicitly set CanFocus for any view in the view-hierarchy where focus is desired. This simplifies the implementation significantly and removes confusing behavior.

In v2, the automatic setting of TabStop in Add is retained because it is not overly complex to do so and is a nice convenience for developers to not have to set both TabStop and CanFocus. Note we do NOT automatically change CanFocus if TabStop is changed.

// v2 explicit focus setup
var container = new FrameView() 
{ 
    Title = "Container",
    CanFocus = true,                    // Must be explicitly set
    TabStop = TabBehavior.TabGroup 
};

var button = new Button() 
{ 
    Text = "Click Me",
    CanFocus = true,                    // Must be explicitly set
    TabStop = TabBehavior.TabStop       // Set automatically by Add(), but can override
};

container.Add(button);  // Does not automatically set CanFocus on container

Knowing When a View's Focus is Changing

@Terminal.Gui.ViewBase.View.HasFocusChanging and @Terminal.Gui.ViewBase.View.HasFocusChanged are raised when a View's focus is changing.

// Monitor focus changes
view.HasFocusChanging += (sender, e) => 
{
    if (e.NewValue && !ValidateCanFocus())
    {
        e.Cancel = true; // Prevent gaining focus
    }
};

view.HasFocusChanged += (sender, e) => 
{
    if (e.CurrentValue)
    {
        OnViewGainedFocus();
    }
    else
    {
        OnViewLostFocus();
    }
};

Built-In Views Interactivity

The following table summarizes how built-in views respond to various input methods:

View	States	Static	Default	HotKeys	Select Cmd	Accept Cmd	HotKey Cmd	Click Focus	DblClick	RightClick	GrabMouse
View	1	Yes	No	1	OnSelect	OnAccept	Focus	Focus	-	-	No
Label	1	Yes	No	1	OnSelect	OnAccept	FocusNext	Focus	-	FocusNext	No
Button	1	No	Yes	1	OnSelect	Focus+OnAccept	Focus+OnAccept	HotKey	-	Select	No
CheckBox	3	No	No	1	OnSelect+Advance	OnAccept	OnAccept	Select	-	Select	No
OptionSelector	>1	No	No	2+	Advance	SetValue+OnAccept	Focus+SetValue	SetFocus+SetCursor	-	SetFocus+SetCursor	No
Slider	>1	No	No	1	SetFocusedOption	SetFocusedOption+OnAccept	Focus	SetFocus+SetOption	-	SetFocus+SetOption	Yes
ListView	>1	No	No	1	MarkUnMarkRow	OpenSelected+OnAccept	OnAccept	SetMark+OnSelectedChanged	OpenSelected+OnAccept	-	No
TextField	1	No	No	1	-	OnAccept	Focus	Focus	SelectAll	ContextMenu	No
TextView	1	No	No	1	-	OnAccept	Focus	Focus	-	ContextMenu	Yes

Table Legend

• States: Number of visual/functional states the view can have
• Static: Whether the view is primarily for display (non-interactive)
• Default: Whether the view can be a default button (activated by Enter)
• HotKeys: Number of hotkeys the view typically supports
• Select Cmd: What happens when Command.Select is invoked
• Accept Cmd: What happens when Command.Accept is invoked
• HotKey Cmd: What happens when the view's hotkey is pressed
• Click Focus: Behavior when clicked (if CanFocus=true)
• DblClick: Behavior on double-click
• RightClick: Behavior on right-click
• GrabMouse: Whether the view captures mouse for drag operations

Common Navigation Patterns

Dialog Navigation

var dialog = new Dialog()
{
    Title = "Settings",
    CanFocus = true,
    TabStop = TabBehavior.TabGroup
};

var okButton = new Button() { Text = "OK", IsDefault = true };
var cancelButton = new Button() { Text = "Cancel" };

// Tab navigates between buttons, Enter activates default
dialog.Add(okButton, cancelButton);

Container Navigation

var leftPanel = new FrameView() 
{ 
    Title = "Options",
    TabStop = TabBehavior.TabGroup,
    X = 0, 
    Width = Dim.Percent(50)
};

var rightPanel = new FrameView() 
{ 
    Title = "Preview",
    TabStop = TabBehavior.TabGroup,
    X = Pos.Right(leftPanel), 
    Width = Dim.Fill()
};

// F6 navigates between panels, Tab navigates within panels

List Navigation

var listView = new ListView()
{
    CanFocus = true,
    TabStop = TabBehavior.TabStop
};

// Arrow keys navigate items, Enter selects, Space toggles
listView.KeyBindings.Add(Key.CursorUp, Command.Up);
listView.KeyBindings.Add(Key.CursorDown, Command.Down);
listView.KeyBindings.Add(Key.Enter, Command.Accept);

Accessibility Considerations

Terminal.Gui's navigation system is designed with accessibility in mind:

Keyboard Accessibility

• All functionality must be accessible via keyboard
• Tab order should be logical and predictable
• HotKeys provide direct access to important functions
• Arrow keys provide fine-grained navigation within controls

Visual Accessibility

• Focus indicators must be clearly visible
• Color is not the only indicator of focus state
• Text and background contrast meets accessibility standards
• HotKeys are visually indicated (underlined characters)

Screen Reader Support

• Focus changes are announced through system events
• View titles and labels provide context
• Status information is available programmatically

Best Practices for Accessible Navigation

// Provide meaningful labels
var button = new Button() { Text = "_Save Document", HotKey = Key.S };

// Set logical tab order
container.TabStop = TabBehavior.TabGroup;
foreach (var view in container.Subviews)
{
    view.TabStop = TabBehavior.TabStop;
}

// Provide keyboard alternatives to mouse actions
view.KeyBindings.Add(Key.F10, Command.Context); // Right-click equivalent
view.KeyBindings.Add(Key.Space, Command.Select); // Click equivalent

For more information on accessibility standards, see:

• Web Content Accessibility Guidelines (WCAG)
• Microsoft Accessibility Guidelines
• .NET Accessibility Documentation