Handling input events

Input handling follows a topological model whereby the top-most (visually) Drawable handles input prior to anything else in the game. Hierarchically between a single parent-child relationship, the child handles input before the parent.

Parent              < Handler #4
    Child_1         < Handler #3
    Child_2         < Handler #2
        Child_2_1   < Handler #1

Positional vs non-positional input

"Positional" input refers to any input that depends on a screen-space position (e.g. hover). "Non-positional" input refers to any other input (e.g. a button press).

Individual event handlers

Positional

protected virtual bool OnMouseMove(MouseMoveEvent e);

protected virtual bool OnHover(HoverEvent e);
protected virtual void OnHoverLost(HoverLostEvent e);

protected virtual bool OnMouseDown(MouseDownEvent e);
protected virtual bool OnMouseUp(MouseUpEvent e);

protected virtual bool OnClick(ClickEvent e);
protected virtual bool OnDoubleClick(DoubleClickEvent e);

protected virtual bool OnDragStart(DragStartEvent e);
protected virtual bool OnDrag(DragEvent e);
protected virtual bool OnDragEnd(DragEndEvent e);

protected virtual bool OnScroll(ScrollEvent e);

Non-positional

protected virtual bool OnKeyDown(KeyDownEvent e);
protected virtual bool OnKeyUp(KeyUpEvent e);

protected virtual bool OnJoystickPress(JoystickPressEvent e);
protected virtual bool OnJoystickRelease(JoystickReleaseEvent e);

These event handlers should be used when singular events are to be handled.

The boolean return value indicates whether the event should continue being propagated to other Drawables in the scene graph.
Example: If a child did not want its parent to receive an OnHover() event, it should override OnHover() to return true.

OnHoverLost() is an exception which is unconditionally invoked on every un-hovered Drawable.

Example:

protected override bool OnClick(ClickEvent e)
{
    this.ScaleTo(1.25f, 50).Then().ScaleTo(1f, 50);
    return true;
}

Aggregate event handler

protected virtual bool Handle(UIEvent e);

This event handler should be used when groups of events with identical implementations are to be handled.

This counts as both a positional and non-positional event handler and works well when combined with the HandleNonPositionalInput and HandlePositionalInput properties described below.

Example:

protected override bool Handle(UIEvent e)
{
    switch (e)
    {
        case MouseEvent _:
            // Stop all mouse events from being handled by other Drawables
            return true;
        default:
            // Let other Drawables handle everything else
            return false;
    }
}

Controlling whether input is to be handled

By default, any Drawable that implements the handlers described above will receive all events appropriate for the types of input listed.

public virtual bool HandleNonPositionalInput;
public virtual bool HandlePositionalInput;

The above properties are provided to control whether a Drawable should receive the types of input events.
Example: If a Drawable did not want to handle any click, drag, and hover events at some point, it could achieve this by overriding HandlePositionalInput to return false.

Warning: Overriding the above properties will cause the Drawable to be considered for (but not necessarily receive) that type of input, and thus serves as an anti-optimisation if the intention is to make the Drawable not considered for that type of input.

If a parent should control whether itself or its children should be considered for a type of input at all, the following properties are provided to control the behaviour:

public virtual bool PropagateNonPositionalInputSubTree;
public virtual bool PropagatePositionalInputSubTree;

Example:

class MyContainer : FillFlowContainer
{
    public MyContainer()
    {
        for (int i = 0; i < 1024; i++)
            Add(new Button { Size = new Vector2(50) });
    }

    // Whoah! The buttons we added are just for show, they don't actually handle clicks!
    public override bool PropagatePositionalInputSubTree => false;
}

Receiving and handling focus

public virtual bool RequestsFocus;
public virtual bool AcceptsFocus;

protected virtual void OnFocus(FocusEvent e);
protected virtual void OnFocusLost(FocusLostEvent e);

Focus may be received via both positional and non-positional input. A Drawable with AcceptsFocus = false will never receive focus.

Positional

A Drawable receives focus when OnClick() returns true.

Non-positional

The top-most Drawable with RequestsFocus = true and HandleNonPostionalInput = true will receive focus if nothing else has focus.

The OnFocusLost() method is unconditionally invoked on the un-focused Drawable.

Example:

class MyDrawable : CompositeDrawable
{
    public override bool AcceptsFocus => true;
    protected override bool OnClick(ClickEvent e) => true;

    protected override void OnFocus(FocusEvent e) => this.ScaleTo(1.5f, 50);
    protected override void OnFocusLost(FocusLostEvent e) => this.ScaleTo(1f, 50);
}