gui-cs.Terminal.Gui/Terminal.Gui/View/ViewKeyboard.cs

using System.ComponentModel;

namespace Terminal.Gui;

public partial class View
{
    private void AddCommands ()
    {
        // By default, the HotKey command sets the focus
        AddCommand (Command.HotKey, OnHotKey);

        // By default, the Accept command raises the Accept event
        AddCommand (Command.Accept, OnAccept);
    }

    #region HotKey Support

    /// <summary>
    /// Called when the HotKey command (<see cref="Command.HotKey"/>) is invoked. Causes this view to be focused.
    /// </summary>
    /// <returns>If <see langword="true"/> the command was canceled.</returns>
    private bool? OnHotKey ()
    {
        if (CanFocus)
        {
            SetFocus ();

            return true;
        }

        return false;
    }

    /// <summary>
    /// Cancelable event fired when the <see cref="Command.Accept"/> command is invoked. Set <see cref="CancelEventArgs.Cancel"/>
    /// to cancel the event.
    /// </summary>
    public event EventHandler<CancelEventArgs> Accept;

    /// <summary>
    /// Called when the <see cref="Command.Accept"/> command is invoked. Fires the <see cref="Accept"/>
    /// event.
    /// </summary>
    /// <returns>If <see langword="true"/> the event was canceled.</returns>
    public bool? OnAccept ()
    {
        var args = new CancelEventArgs ();
        Accept?.Invoke (this, args);
        return args.Cancel;
    }

    /// <summary>Invoked when the <see cref="HotKey"/> is changed.</summary>
    public event EventHandler<KeyChangedEventArgs> HotKeyChanged;

    private Key _hotKey = new ();
    private void TitleTextFormatter_HotKeyChanged (object sender, KeyChangedEventArgs e) { HotKeyChanged?.Invoke (this, e); }

    /// <summary>
    ///     Gets or sets the hot key defined for this view. Pressing the hot key on the keyboard while this view has focus will
    ///     invoke the <see cref="Command.HotKey"/> and <see cref="Command.Accept"/> commands. <see cref="Command.HotKey"/>
    ///     causes the view to be focused and <see cref="Command.Accept"/> does nothing. By default, the HotKey is
    ///     automatically set to the first character of <see cref="Text"/> that is prefixed with with
    ///     <see cref="HotKeySpecifier"/>.
    ///     <para>
    ///         A HotKey is a keypress that selects a visible UI item. For selecting items across <see cref="View"/>`s (e.g.a
    ///         <see cref="Button"/> in a <see cref="Dialog"/>) the keypress must include the <see cref="Key.WithAlt"/>
    ///         modifier. For selecting items within a View that are not Views themselves, the keypress 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
    ///         <see cref="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.
    ///     </para>
    /// </summary>
    /// <remarks>
    ///     <para>See <see href="../docs/keyboard.md"/> for an overview of Terminal.Gui keyboard APIs.</para>
    ///     <para>
    ///         This is a helper API for configuring a key binding for the hot key. By default, this property is set whenever
    ///         <see cref="Text"/> changes.
    ///     </para>
    ///     <para>
    ///         By default, when the Hot Key is set, key bindings are added for both the base key (e.g.
    ///         <see cref="Key.D3"/>) and the Alt-shifted key (e.g. <see cref="Key.D3"/>.
    ///         <see cref="Key.WithAlt"/>). This behavior can be overriden by overriding
    ///         <see cref="AddKeyBindingsForHotKey"/>.
    ///     </para>
    ///     <para>
    ///         By default, when the HotKey is set to <see cref="Key.A"/> through <see cref="Key.Z"/> key bindings will
    ///         be added for both the un-shifted and shifted versions. This means if the HotKey is <see cref="Key.A"/>, key
    ///         bindings for <c>Key.A</c> and <c>Key.A.WithShift</c> will be added. This behavior can be overriden by
    ///         overriding <see cref="AddKeyBindingsForHotKey"/>.
    ///     </para>
    ///     <para>If the hot key is changed, the <see cref="HotKeyChanged"/> event is fired.</para>
    ///     <para>Set to <see cref="Key.Empty"/> to disable the hot key.</para>
    /// </remarks>
    public virtual Key HotKey
    {
        get => _hotKey;
        set
        {
            if (value is null)
            {
                throw new ArgumentException (
                                             @"HotKey must not be null. Use Key.Empty to clear the HotKey.",
                                             nameof (value)
                                            );
            }

            if (AddKeyBindingsForHotKey (_hotKey, value))
            {
                // This will cause TextFormatter_HotKeyChanged to be called, firing HotKeyChanged
                // BUGBUG: _hotkey should be set BEFORE setting TextFormatter.HotKey
                _hotKey = value;
                TitleTextFormatter.HotKey = value;
            }
        }
    }

    /// <summary>
    ///     Adds key bindings for the specified HotKey. Useful for views that contain multiple items that each have their
    ///     own HotKey such as <see cref="RadioGroup"/>.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         By default, key bindings are added for both the base key (e.g. <see cref="Key.D3"/>) and the Alt-shifted key
    ///         (e.g. <c>Key.D3.WithAlt</c>) This behavior can be overriden by overriding <see cref="AddKeyBindingsForHotKey"/>.
    ///     </para>
    ///     <para>
    ///         By default, when <paramref name="hotKey"/> is <see cref="Key.A"/> through <see cref="Key.Z"/> key bindings
    ///         will be added for both the un-shifted and shifted versions. This means if the HotKey is <see cref="Key.A"/>,
    ///         key bindings for <c>Key.A</c> and <c>Key.A.WithShift</c> will be added. This behavior can be overriden by
    ///         overriding <see cref="AddKeyBindingsForHotKey"/>.
    ///     </para>
    /// </remarks>
    /// <param name="prevHotKey">The HotKey <paramref name="hotKey"/> is replacing. Key bindings for this key will be removed.</param>
    /// <param name="hotKey">The new HotKey. If <see cref="Key.Empty"/> <paramref name="prevHotKey"/> bindings will be removed.</param>
    /// <returns><see langword="true"/> if the HotKey bindings were added.</returns>
    /// <exception cref="ArgumentException"></exception>
    public virtual bool AddKeyBindingsForHotKey (Key prevHotKey, Key hotKey)
    {
        if (_hotKey == hotKey)
        {
            return false;
        }

        Key newKey = hotKey;

        Key baseKey = newKey.NoAlt.NoShift.NoCtrl;

        if (newKey != Key.Empty && (baseKey == Key.Space || Rune.IsControl (baseKey.AsRune)))
        {
            throw new ArgumentException (@$"HotKey must be a printable (and non-space) key ({hotKey}).");
        }

        if (newKey != baseKey)
        {
            if (newKey.IsCtrl)
            {
                throw new ArgumentException (@$"HotKey does not support CtrlMask ({hotKey}).");
            }

            // Strip off the shift mask if it's A...Z
            if (baseKey.IsKeyCodeAtoZ)
            {
                newKey = newKey.NoShift;
            }

            // Strip off the Alt mask
            newKey = newKey.NoAlt;
        }

        // Remove base version
        if (KeyBindings.TryGet (prevHotKey, out _))
        {
            KeyBindings.Remove (prevHotKey);
        }

        // Remove the Alt version
        if (KeyBindings.TryGet (prevHotKey.WithAlt, out _))
        {
            KeyBindings.Remove (prevHotKey.WithAlt);
        }

        if (_hotKey.IsKeyCodeAtoZ)
        {
            // Remove the shift version
            if (KeyBindings.TryGet (prevHotKey.WithShift, out _))
            {
                KeyBindings.Remove (prevHotKey.WithShift);
            }

            // Remove alt | shift version
            if (KeyBindings.TryGet (prevHotKey.WithShift.WithAlt, out _))
            {
                KeyBindings.Remove (prevHotKey.WithShift.WithAlt);
            }
        }

        // Add the new 
        if (newKey != Key.Empty)
        {
            // Add the base and Alt key
            KeyBindings.Add (newKey, KeyBindingScope.HotKey, Command.HotKey);
            KeyBindings.Add (newKey.WithAlt, KeyBindingScope.HotKey, Command.HotKey);

            // If the Key is A..Z, add ShiftMask and AltMask | ShiftMask
            if (newKey.IsKeyCodeAtoZ)
            {
                KeyBindings.Add (newKey.WithShift, KeyBindingScope.HotKey, Command.HotKey);
                KeyBindings.Add (newKey.WithShift.WithAlt, KeyBindingScope.HotKey, Command.HotKey);
            }
        }

        return true;
    }

    /// <summary>
    ///     Gets or sets the specifier character for the hot key (e.g. '_'). Set to '\xffff' to disable automatic hot key
    ///     setting support for this View instance. The default is '\xffff'.
    /// </summary>
    public virtual Rune HotKeySpecifier
    {
        get
        {
            return TitleTextFormatter.HotKeySpecifier;
        }
        set
        {
            TitleTextFormatter.HotKeySpecifier = value;
            SetHotKeyFromTitle ();
        }
    }

    private void SetHotKeyFromTitle ()
    {
        if (TitleTextFormatter == null || HotKeySpecifier == new Rune ('\xFFFF'))
        {
            return; // throw new InvalidOperationException ("Can't set HotKey unless a TextFormatter has been created");
        }

        if (TextFormatter.FindHotKey (_title, HotKeySpecifier, out _, out Key hk))
        {
            if (_hotKey != hk)
            {
                HotKey = hk;
            }
        }
        else
        {
            HotKey = Key.Empty;
        }
    }

    #endregion HotKey Support

    #region Tab/Focus Handling

    // This is null, and allocated on demand.
    private List<View> _tabIndexes;

    /// <summary>Gets a list of the subviews that are <see cref="TabStop"/>s.</summary>
    /// <value>The tabIndexes.</value>
    public IList<View> TabIndexes => _tabIndexes?.AsReadOnly () ?? _empty;

    private int _tabIndex = -1;
    private int _oldTabIndex;

    /// <summary>
    ///     Indicates the index of the current <see cref="View"/> from the <see cref="TabIndexes"/> list. See also:
    ///     <seealso cref="TabStop"/>.
    /// </summary>
    public int TabIndex
    {
        get => _tabIndex;
        set
        {
            if (!CanFocus)
            {
                _tabIndex = -1;

                return;
            }

            if (SuperView?._tabIndexes is null || SuperView?._tabIndexes.Count == 1)
            {
                _tabIndex = 0;

                return;
            }

            if (_tabIndex == value)
            {
                return;
            }

            _tabIndex = value > SuperView._tabIndexes.Count - 1 ? SuperView._tabIndexes.Count - 1 :
                        value < 0 ? 0 : value;
            _tabIndex = GetTabIndex (_tabIndex);

            if (SuperView._tabIndexes.IndexOf (this) != _tabIndex)
            {
                SuperView._tabIndexes.Remove (this);
                SuperView._tabIndexes.Insert (_tabIndex, this);
                SetTabIndex ();
            }
        }
    }

    private int GetTabIndex (int idx)
    {
        var i = 0;

        foreach (View v in SuperView._tabIndexes)
        {
            if (v._tabIndex == -1 || v == this)
            {
                continue;
            }

            i++;
        }

        return Math.Min (i, idx);
    }

    private void SetTabIndex ()
    {
        var i = 0;

        foreach (View v in SuperView._tabIndexes)
        {
            if (v._tabIndex == -1)
            {
                continue;
            }

            v._tabIndex = i;
            i++;
        }
    }

    private bool _tabStop = true;

    /// <summary>
    ///     Gets or sets whether the view is a stop-point for keyboard navigation of focus. Will be <see langword="true"/>
    ///     only if the <see cref="CanFocus"/> is also <see langword="true"/>. Set to <see langword="false"/> to prevent the
    ///     view from being a stop-point for keyboard navigation.
    /// </summary>
    /// <remarks>
    ///     The default keyboard navigation keys are <c>Key.Tab</c> and <c>Key>Tab.WithShift</c>. These can be changed by
    ///     modifying the key bindings (see <see cref="KeyBindings.Add(Key, Command[])"/>) of the SuperView.
    /// </remarks>
    public bool TabStop
    {
        get => _tabStop;
        set
        {
            if (_tabStop == value)
            {
                return;
            }

            _tabStop = CanFocus && value;
        }
    }

    #endregion Tab/Focus Handling

    #region Low-level Key handling

    #region Key Down Event

    /// <summary>
    ///     If the view is enabled, processes a new key down event and returns <see langword="true"/> if the event was
    ///     handled.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         If the view has a sub view that is focused, <see cref="NewKeyDownEvent"/> will be called on the focused view
    ///         first.
    ///     </para>
    ///     <para>
    ///         If the focused sub view does not handle the key press, this method calls <see cref="OnKeyDown"/> to allow the
    ///         view to pre-process the key press. If <see cref="OnKeyDown"/> returns <see langword="false"/>, this method then
    ///         calls <see cref="OnInvokingKeyBindings"/> to invoke any key bindings. Then, only if no key bindings are
    ///         handled, <see cref="OnProcessKeyDown"/> will be called allowing the view to process the key press.
    ///     </para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    /// <param name="keyEvent"></param>
    /// <returns><see langword="true"/> if the event was handled.</returns>
    public bool NewKeyDownEvent (Key keyEvent)
    {
        if (!Enabled)
        {
            return false;
        }

        // By default the KeyBindingScope is View

        if (Focused?.NewKeyDownEvent (keyEvent) == true)
        {
            return true;
        }

        // Before (fire the cancellable event)
        if (OnKeyDown (keyEvent))
        {
            return true;
        }

        // During (this is what can be cancelled)
        InvokingKeyBindings?.Invoke (this, keyEvent);

        if (keyEvent.Handled)
        {
            return true;
        }

        bool? handled = OnInvokingKeyBindings (keyEvent);

        if (handled is { } && (bool)handled)
        {
            return true;
        }

        // TODO: The below is not right. OnXXX handlers are supposed to fire the events.
        // TODO: But I've moved it outside of the v-function to test something.
        // After (fire the cancellable event)
        // fire event
        ProcessKeyDown?.Invoke (this, keyEvent);

        if (!keyEvent.Handled && OnProcessKeyDown (keyEvent))
        {
            return true;
        }

        return keyEvent.Handled;
    }

    /// <summary>
    ///     Low-level API called when the user presses a key, allowing a view to pre-process the key down event. This is
    ///     called from <see cref="NewKeyDownEvent"/> before <see cref="OnInvokingKeyBindings"/>.
    /// </summary>
    /// <param name="keyEvent">Contains the details about the key that produced the event.</param>
    /// <returns>
    ///     <see langword="false"/> if the key press was not handled. <see langword="true"/> if the keypress was handled
    ///     and no other view should see it.
    /// </returns>
    /// <remarks>
    ///     <para>
    ///         For processing <see cref="HotKey"/>s and commands, use <see cref="Command"/> and
    ///         <see cref="KeyBindings.Add(Key, Command[])"/>instead.
    ///     </para>
    ///     <para>Fires the <see cref="KeyDown"/> event.</para>
    /// </remarks>
    public virtual bool OnKeyDown (Key keyEvent)
    {
        // fire event
        KeyDown?.Invoke (this, keyEvent);

        return keyEvent.Handled;
    }

    /// <summary>
    ///     Invoked when the user presses a key, allowing subscribers to pre-process the key down event. This is fired
    ///     from <see cref="OnKeyDown"/> before <see cref="OnInvokingKeyBindings"/>. Set <see cref="Key.Handled"/> to true to
    ///     stop the key from being processed by other views.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         Not all terminals support key distinct up notifications, Applications should avoid depending on distinct
    ///         KeyUp events.
    ///     </para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    public event EventHandler<Key> KeyDown;

    /// <summary>
    ///     Low-level API called when the user presses a key, allowing views do things during key down events. This is
    ///     called from <see cref="NewKeyDownEvent"/> after <see cref="OnInvokingKeyBindings"/>.
    /// </summary>
    /// <param name="keyEvent">Contains the details about the key that produced the event.</param>
    /// <returns>
    ///     <see langword="false"/> if the key press was not handled. <see langword="true"/> if the keypress was handled
    ///     and no other view should see it.
    /// </returns>
    /// <remarks>
    ///     <para>
    ///         Override <see cref="OnProcessKeyDown"/> to override the behavior of how the base class processes key down
    ///         events.
    ///     </para>
    ///     <para>
    ///         For processing <see cref="HotKey"/>s and commands, use <see cref="Command"/> and
    ///         <see cref="KeyBindings.Add(Key, Command[])"/>instead.
    ///     </para>
    ///     <para>Fires the <see cref="ProcessKeyDown"/> event.</para>
    ///     <para>
    ///         Not all terminals support distinct key up notifications; applications should avoid depending on distinct
    ///         KeyUp events.
    ///     </para>
    /// </remarks>
    public virtual bool OnProcessKeyDown (Key keyEvent)
    {
        //ProcessKeyDown?.Invoke (this, keyEvent);
        return keyEvent.Handled;
    }

    /// <summary>
    ///     Invoked when the users presses a key, allowing subscribers to do things during key down events. Set
    ///     <see cref="Key.Handled"/> to true to stop the key from being processed by other views. Invoked after
    ///     <see cref="KeyDown"/> and before <see cref="InvokingKeyBindings"/>.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         SubViews can use the <see cref="ProcessKeyDown"/> of their super view override the default behavior of when
    ///         key bindings are invoked.
    ///     </para>
    ///     <para>
    ///         Not all terminals support distinct key up notifications; applications should avoid depending on distinct
    ///         KeyUp events.
    ///     </para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    public event EventHandler<Key> ProcessKeyDown;

    #endregion KeyDown Event

    #region KeyUp Event

    /// <summary>
    ///     If the view is enabled, processes a new key up event and returns <see langword="true"/> if the event was
    ///     handled. Called before <see cref="NewKeyDownEvent"/>.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         Not all terminals support key distinct down/up notifications, Applications should avoid depending on distinct
    ///         KeyUp events.
    ///     </para>
    ///     <para>
    ///         If the view has a sub view that is focused, <see cref="NewKeyUpEvent"/> will be called on the focused view
    ///         first.
    ///     </para>
    ///     <para>
    ///         If the focused sub view does not handle the key press, this method calls <see cref="OnKeyUp"/>, which is
    ///         cancellable.
    ///     </para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    /// <param name="keyEvent"></param>
    /// <returns><see langword="true"/> if the event was handled.</returns>
    public bool NewKeyUpEvent (Key keyEvent)
    {
        if (!Enabled)
        {
            return false;
        }

        if (Focused?.NewKeyUpEvent (keyEvent) == true)
        {
            return true;
        }

        // Before (fire the cancellable event)
        if (OnKeyUp (keyEvent))
        {
            return true;
        }

        // During (this is what can be cancelled)
        // TODO: Until there's a clear use-case, we will not define 'during' event (e.g. OnDuringKeyUp). 

        // After (fire the cancellable event InvokingKeyBindings)
        // TODO: Until there's a clear use-case, we will not define an 'after' event (e.g. OnAfterKeyUp). 

        return false;
    }

    /// <summary>Method invoked when a key is released. This method is called from <see cref="NewKeyUpEvent"/>.</summary>
    /// <param name="keyEvent">Contains the details about the key that produced the event.</param>
    /// <returns>
    ///     <see langword="false"/> if the key stroke was not handled. <see langword="true"/> if no other view should see
    ///     it.
    /// </returns>
    /// <remarks>
    ///     Not all terminals support key distinct down/up notifications, Applications should avoid depending on distinct KeyUp
    ///     events.
    ///     <para>
    ///         Overrides must call into the base and return <see langword="true"/> if the base returns
    ///         <see langword="true"/>.
    ///     </para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    public virtual bool OnKeyUp (Key keyEvent)
    {
        // fire event
        KeyUp?.Invoke (this, keyEvent);

        if (keyEvent.Handled)
        {
            return true;
        }

        return false;
    }

    /// <summary>
    ///     Invoked when a key is released. Set <see cref="Key.Handled"/> to true to stop the key up event from being processed
    ///     by other views.
    ///     <remarks>
    ///         Not all terminals support key distinct down/up notifications, Applications should avoid depending on
    ///         distinct KeyDown and KeyUp events and instead should use <see cref="KeyDown"/>.
    ///         <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    ///     </remarks>
    /// </summary>
    public event EventHandler<Key> KeyUp;

    #endregion KeyUp Event

    #endregion Low-level Key handling

    #region Key Bindings

    /// <summary>Gets the key bindings for this view.</summary>
    public KeyBindings KeyBindings { get; } = new ();

    private Dictionary<Command, Func<bool?>> CommandImplementations { get; } = new ();

    /// <summary>
    ///     Low-level API called when a user presses a key; invokes any key bindings set on the view. This is called
    ///     during <see cref="NewKeyDownEvent"/> after <see cref="OnKeyDown"/> has returned.
    /// </summary>
    /// <remarks>
    ///     <para>Fires the <see cref="InvokingKeyBindings"/> event.</para>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </remarks>
    /// <param name="keyEvent">Contains the details about the key that produced the event.</param>
    /// <returns>
    ///     <see langword="false"/> if the key press was not handled. <see langword="true"/> if the keypress was handled
    ///     and no other view should see it.
    /// </returns>
    public virtual bool? OnInvokingKeyBindings (Key keyEvent)
    {
        // fire event only if there's an app or hotkey binding for the key
        if (KeyBindings.TryGet (keyEvent, KeyBindingScope.Application | KeyBindingScope.HotKey, out KeyBinding _))
        {
            InvokingKeyBindings?.Invoke (this, keyEvent);
            if (keyEvent.Handled)
            {
                return true;
            }

        }

        // * If no key binding was found, `InvokeKeyBindings` returns `null`.
        //   Continue passing the event (return `false` from `OnInvokeKeyBindings`).
        // * If key bindings were found, but none handled the key (all `Command`s returned `false`),
        //   `InvokeKeyBindings` returns `false`. Continue passing the event (return `false` from `OnInvokeKeyBindings`)..
        // * If key bindings were found, and any handled the key (at least one `Command` returned `true`),
        //   `InvokeKeyBindings` returns `true`. Continue passing the event (return `false` from `OnInvokeKeyBindings`).
        bool? handled = InvokeKeyBindings (keyEvent);

        if (handled is { } && (bool)handled)
        {
            // Stop processing if any key binding handled the key.
            // DO NOT stop processing if there are no matching key bindings or none of the key bindings handled the key
            return true;
        }

        // Now, process any key bindings in the subviews that are tagged to KeyBindingScope.HotKey.
        foreach (View view in Subviews.Where (
                                              v => v.KeyBindings.TryGet (
                                                                         keyEvent,
                                                                         KeyBindingScope.HotKey,
                                                                         out KeyBinding _
                                                                        )
                                             ))
        {
            // TODO: I think this TryGet is not needed due to the one in the lambda above. Use `Get` instead?
            if (view.KeyBindings.TryGet (keyEvent, KeyBindingScope.HotKey, out KeyBinding binding))
            {
                //keyEvent.Scope = KeyBindingScope.HotKey;
                handled = view.OnInvokingKeyBindings (keyEvent);

                if (handled is { } && (bool)handled)
                {
                    return true;
                }
            }
        }

        return handled;
    }

    /// <summary>
    ///     Invoked when a key is pressed that may be mapped to a key binding. Set <see cref="Key.Handled"/> to true to
    ///     stop the key from being processed by other views.
    /// </summary>
    public event EventHandler<Key> InvokingKeyBindings;

    /// <summary>
    ///     Invokes any binding that is registered on this <see cref="View"/> and matches the <paramref name="key"/>
    ///     <para>See <see href="../docs/keyboard.md">for an overview of Terminal.Gui keyboard APIs.</see></para>
    /// </summary>
    /// <param name="key">The key event passed.</param>
    /// <returns>
    ///     <see langword="null"/> if no command was bound the <paramref name="key"/>. <see langword="true"/> if
    ///     commands were invoked and at least one handled the command. <see langword="false"/> if commands were invoked and at
    ///     none handled the command.
    /// </returns>
    protected bool? InvokeKeyBindings (Key key)
    {
        bool? toReturn = null;

        if (!KeyBindings.TryGet (key, out KeyBinding binding))
        {
            return null;
        }

        foreach (Command command in binding.Commands)
        {
            if (!CommandImplementations.ContainsKey (command))
            {
                throw new NotSupportedException (
                                                 @$"A KeyBinding was set up for the command {command} ({key}) but that command is not supported by this View ({GetType ().Name})"
                                                );
            }

            // each command has its own return value
            bool? thisReturn = InvokeCommand (command);

            // if we haven't got anything yet, the current command result should be used
            toReturn ??= thisReturn;

            // if ever see a true then that's what we will return
            if (thisReturn ?? false)
            {
                toReturn = true;
            }
        }

        return toReturn;
    }

    /// <summary>
    ///     Invokes the specified commands.
    /// </summary>
    /// <param name="commands"></param>
    /// <returns>
    ///     <see langword="null"/> if no command was found.
    ///     <see langword="true"/> if the command was invoked and it handled the command.
    ///     <see langword="false"/> if the command was invoked and it did not handle the command.
    /// </returns>
    public bool? InvokeCommands (Command [] commands)
    {
        bool? toReturn = null;

        foreach (Command command in commands)
        {
            if (!CommandImplementations.ContainsKey (command))
            {
                throw new NotSupportedException (@$"{command} is not supported by ({GetType ().Name}).");
            }

            // each command has its own return value
            bool? thisReturn = InvokeCommand (command);

            // if we haven't got anything yet, the current command result should be used
            toReturn ??= thisReturn;

            // if ever see a true then that's what we will return
            if (thisReturn ?? false)
            {
                toReturn = true;
            }
        }

        return toReturn;
    }

    /// <summary>Invokes the specified command.</summary>
    /// <param name="command"></param>
    /// <returns>
    ///     <see langword="null"/> if no command was found. <see langword="true"/> if the command was invoked and it
    ///     handled the command. <see langword="false"/> if the command was invoked and it did not handle the command.
    /// </returns>
    public bool? InvokeCommand (Command command)
    {
        if (!CommandImplementations.ContainsKey (command))
        {
            return null;
        }

        return CommandImplementations [command] ();
    }

    /// <summary>
    ///     <para>
    ///         Sets the function that will be invoked for a <see cref="Command"/>. Views should call
    ///         <see cref="AddCommand"/> for each command they support.
    ///     </para>
    ///     <para>
    ///         If <see cref="AddCommand"/> has already been called for <paramref name="command"/> <paramref name="f"/> will
    ///         replace the old one.
    ///     </para>
    /// </summary>
    /// <param name="command">The command.</param>
    /// <param name="f">The function.</param>
    protected void AddCommand (Command command, Func<bool?> f)
    {
        // if there is already an implementation of this command
        // replace that implementation
        // else record how to perform the action (this should be the normal case)
        if (CommandImplementations is { })
        {
            CommandImplementations [command] = f;
        }
    }

    /// <summary>Returns all commands that are supported by this <see cref="View"/>.</summary>
    /// <returns></returns>
    public IEnumerable<Command> GetSupportedCommands () { return CommandImplementations.Keys; }

    // TODO: Add GetKeysBoundToCommand() - given a Command, return all Keys that would invoke it

    #endregion Key Bindings
}