Table of Contents

Keyboard Deep Dive

Tenets for Terminal.Gui Keyboard Handling (Unless you know better ones...)

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

  • Users Have Control - Terminal.Gui provides default key bindings consistent with these tenets, but those defaults are configurable by the user. For example, ConfigurationManager allows users to redefine key bindings for the system, a user, or an application.

  • More Editor than Command Line - Once a Terminal.Gui app starts, the user is no longer using the command line. Users expect keyboard idioms in TUI apps to be consistent with GUI apps (such as VS Code, Vim, and Emacs). For example, in almost all GUI apps, Ctrl+V is Paste. But the Linux shells often use Shift+Insert. Terminal.Gui binds Ctrl+V by default.

  • Be Consistent With the User's Platform - Users get to choose the platform they run Terminal.Gui apps on and those apps should respond to keyboard input in a way that is consistent with the platform. For example, on Windows to erase a word to the left, users press Ctrl+Backspace. But on Linux, Ctrl+W is used.

  • The Source of Truth is Wikipedia - We use this Wikipedia article as our guide for default key bindings.

  • If It's Hot, It Works - If a View with a HotKey is visible, and the HotKey is visible, the user should be able to press that HotKey and whatever behavior is defined for it should work. For example, in v1, when a Modal view was active, the HotKeys on MenuBar continued to show "hot". In v2 we strive to ensure this doesn't happen.

Keyboard APIs

Terminal.Gui provides the following APIs for handling keyboard input:

  • Key - Key provides a platform-independent abstraction for common keyboard operations. It is used for processing keyboard input and raising keyboard events. This class provides a high-level abstraction with helper methods and properties for common keyboard operations. Use this class instead of the low-level KeyCode enum when possible.
  • Key Bindings - Key Bindings provide a declarative method for handling keyboard input in View implementations. The View calls @Terminal.Gui.AddCommand to declare it supports a particular command and then uses KeyBindings to indicate which key presses will invoke the command.
  • Key Events - The Key Bindings API is rich enough to support the vast majority of use-cases. However, in some cases subscribing directly to key events is needed (e.g. when capturing arbitrary typing by a user). Use KeyDown and related events in these cases.

Each of these APIs are described more fully below.

Key Bindings

Key Bindings is the preferred way of handling keyboard input in View implementations. The View calls @Terminal.Gui.AddCommand to declare it supports a particular command and then uses KeyBindings to indicate which key presses will invoke the command. For example, if a View wants to respond to the user pressing the up arrow key to scroll up it would do this

public MyView : View
{
  AddCommand (Command.ScrollUp, () => ScrollVertical (-1));
  KeyBindings.Add (Key.CursorUp, Command.ScrollUp);
}

The Character Map Scenario includes a View called CharMap that is a good example of the Key Bindings API.

The Command enum lists generic operations that are implemented by views. For example Command.Accept in a Button results in the Accepting event firing while in TableView it is bound to CellActivated. Not all commands are implemented by all views (e.g. you cannot scroll in a Button). Use the GetSupportedCommands() method to determine which commands are implemented by a View.

The default key for activating a button is Space. You can change this using KeyBindings.ReplaceKey():

var btn = new Button () { Title = "Press me" };
btn.KeyBindings.ReplaceKey (btn.KeyBindings.GetKeyFromCommands (Command.Accept));

Key Bindings can be added at the Application or View level.

For Application-scoped Key Bindings there are two categories of Application-scoped Key Bindings:

  1. Application Command Key Bindings - Bindings for Commands supported by Application. For example, QuitKey, which is bound to Command.Quit and results in @Terminal.Gui.Application.RequestStop being called.
  2. Application Key Bindings - Bindings for Commands supported on arbitrary Views that are meant to be invoked regardless of which part of the application is visible/active.

Use KeyBindings to add or modify Application-scoped Key Bindings.

View-scoped Key Bindings also have two categories:

  1. HotKey Bindings - These bind to Commands that will be invoked regardless of whether the View has focus or not. The most common use-case for HotKey bindings is HotKey. For example, a Button with a Title of _OK, the user can press Alt-O and the button will be accepted regardless of whether it has focus or not. Add and modify HotKey bindings with HotKeyBindings.
  2. Focused Bindings - These bind to Commands that will be invoked only when the View has focus. Focused Key Bindings are the easiest way to enable a View to support responding to key events. Add and modify Focused bindings with KeyBindings.

Application-Scoped Key Bindings

HotKey

A HotKey is a key press that selects a visible UI item. For selecting items across Views (e.g. a Button in a Dialog) the key press must have the Alt modifier. For selecting items within a View that are not Views themselves, the key press can be key without the Alt modifier. For example, in a Dialog, a Button with the text of "_Text" can be selected with Alt+T. Or, in a Menu with "_File _Edit", Alt+F will select (show) the "_File" menu. If the "_File" menu has a sub-menu of "_New" Alt+N or N will ONLY select the "_New" sub-menu if the "_File" menu is already opened.

By default, the Text of a View is used to determine the HotKey by looking for the first occurrence of the HotKeySpecifier (which is underscore (_) by default). The character following the underscore is the HotKey. If the HotKeySpecifier is not found in Text, the first character of Text is used as the HotKey. The Text of a View can be changed at runtime, and the HotKey will be updated accordingly. HotKey is virtual enabling this behavior to be customized.

Shortcut

A Shortcut is an opinionated (visually & API) View for displaying a command, help text, key key press that invokes a Command.

The Command can be invoked even if the View that defines them is not focused or visible (but the View must be enabled). Shortcuts can be any key press; Key.A, Key.A.WithCtrl, Key.A.WithCtrl.WithAlt, Key.Del, and Key.F1, are all valid.

Shortcuts are used to define application-wide actions or actions that are not visible (e.g. Copy).

MenuBar, ContextMenu, and StatusBar support Shortcuts.

Key Events

Keyboard events are retrieved from Console Drivers each iteration of the Application Main Loop. The console driver raises the KeyDown and KeyUp events which invoke @Terminal.Gui.Application.RaiseKeyDown(Terminal.Gui.Key) and @Terminal.Gui.Application.RaiseKeyUp(Terminal.Gui.Key) respectively.

NOTE: Not all drivers/platforms support sensing distinct KeyUp events. These drivers will simulate KeyUp events by raising @Terminal.Gui.ConsoleDriver.KeyUp after @Terminal.Gui.ConsoleDriver.KeyDown.

@Terminal.Gui.Application.RaiseKeyDown(Terminal.Gui.Key) raises KeyDown and then calls NewKeyDownEvent(Key) on all toplevel Views. If no View handles the key event, any Application-scoped key bindings will be invoked.

@Terminal.Gui.Application.RaiseKeyDown(Terminal.Gui.Key) raises KeyDown and then calls NewKeyUpEvent(Key) on all toplevel Views.

If a view is enabled, the NewKeyDownEvent(Key) method will do the following:

  1. If the view has a subview that has focus, 'NewKeyDown' on the focused view will be called. This is recursive. If the most-focused view handles the key press, processing stops.
  2. If there is no most-focused sub-view, or a most-focused sub-view does not handle the key press, OnKeyDown(Key) will be called. If the view handles the key press, processing stops.
  3. If OnKeyDown(Key) does not handle the event. KeyDown will be raised.
  4. If the view does not handle the key down event, any bindings for the key will be invoked (see @Terminal.Gui.View.KeyBindings). If the key is bound and any of it's command handlers return true, processing stops.
  5. If the key is not bound, or the bound command handlers do not return true, OnKeyDownNotHandled(Key) is called.

Application Key Handling

To define application key handling logic for an entire application in cases where the methods listed above are not suitable, use the KeyDown event.

Key Down/Up Events

Terminal.Gui supports key up/down events with OnKeyDown(Key) and OnKeyUp(Key), but not all Console Drivers do. To receive these key down and key up events, you must use a driver that supports them (e.g. WindowsDriver).

General input model

  • Key Down and Up events are generated by ConsoleDriver.

  • Application subscribes to ConsoleDriver.Down/Up events and forwards them to the most-focused TopLevel view using View.NewKeyDownEvent and View.NewKeyUpEvent.

  • The base (View) implementation of NewKeyDownEvent follows a pattern of "Before", "During", and "After" processing:

    • Before
      • If Enabled == false that view should never see keyboard (or mouse input).
      • NewKeyDownEvent is called on the most-focused SubView (if any) that has focus. If that call returns true, the method returns.
      • Calls OnKeyDown.
    • During
      • Assuming OnKeyDown call returns false (indicating the key wasn't handled) any commands bound to the key will be invoked.
    • After
      • Assuming no keybinding was found or all invoked commands were not handled:
        • OnKeyDownNotHandled is called to process the key.
        • KeyDownNotHandled is raised.

  • Subclasses of View can (rarely) override OnKeyDown (or subscribe to KeyDown) to see keys before they are processed

  • Subclasses of View can (often) override OnKeyDownNotHandled to do key processing for keys that were not previously handled. TextField and TextView are examples.

ConsoleDriver

  • No concept of Command or KeyBindings
  • Use the low-level KeyCode enum.
  • Exposes non-cancelable KeyDown/Up events. The OnKey/Down/Up methods are public and can be used to simulate keyboard input (in addition to SendKeys).

Application

  • Implements support for KeyBindingScope.Application.
  • Exposes KeyBindings.
  • Exposes cancelable KeyDown/Up events (via Handled = true). The OnKey/Down/Up/ methods are public and can be used to simulate keyboard input.

View

  • Implements support for KeyBindings and HotKeyBindings.
  • Exposes cancelable non-virtual methods for a new key event: NewKeyDownEvent and NewKeyUpEvent. These methods are called by Application can be called to simulate keyboard input.
  • Exposes cancelable virtual methods for a new key event: OnKeyDown and OnKeyUp. These methods are called by NewKeyDownEvent and NewKeyUpEvent and can be overridden to handle keyboard input.
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