Start work on menu interactivity.

With this commit, we get a ui.Interface and ui.Widget type. The
interface monitors hover and mouse click state and tells the widgets
about them; the widgets execute code specified by the application when
events occur.

Next step: have wh40k load the main menu and play sound, etc.

internal/ui/widget.go

@@ -0,0 +1,84 @@
+package ui
+
+import (
+	"image"
+
+	"github.com/hajimehoshi/ebiten"
+
+	"code.ur.gs/lupine/ordoor/internal/assetstore"
+	"code.ur.gs/lupine/ordoor/internal/menus"
+)
+
+// Widget represents an interactive area of the screen. Backgrounds and other
+// non-interactive areas are not widgets.
+type Widget struct {
+	// Position on the screen in original (i.e., unscaled) coordinates
+	Bounds image.Rectangle
+	// Tooltip string // TODO: show the tooltip when hovering?
+
+	OnHoverEnter func()
+	OnHoverLeave func()
+
+	// Mouse up can happen without a click taking place if, for instance, the
+	// mouse cursor leaves the bounds while still pressed.
+	OnMouseDown  func()
+	OnMouseClick func()
+	OnMouseUp    func()
+
+	// These are expected to have the same dimensions as the Bounds
+	hoverAnimation []*ebiten.Image
+	hoverState     bool
+
+	// FIXME: We assume right mouse button isn't needed here
+	// TODO: down, up, and click hooks.
+	mouseButtonDownImage *ebiten.Image
+	mouseButtonState     bool
+
+	path   []int
+	record *menus.Record
+	sprite *assetstore.Sprite
+}
+
+func (w *Widget) hovering(value bool) {
+	if w.OnHoverEnter != nil && !w.hoverState && value {
+		w.OnHoverEnter()
+	}
+
+	if w.OnHoverLeave != nil && w.hoverState && !value {
+		w.OnHoverLeave()
+	}
+
+	w.hoverState = value
+
+	return
+}
+
+func (w *Widget) mouseButton(value bool) {
+	if w.OnMouseDown != nil && !w.mouseButtonState && value {
+		w.OnMouseDown()
+	}
+
+	if w.mouseButtonState && !value {
+		if w.OnMouseClick != nil && w.hoverState {
+			w.OnMouseClick()
+		}
+
+		if w.OnMouseUp != nil {
+			w.OnMouseUp()
+		}
+	}
+
+	w.mouseButtonState = value
+}
+
+func (w *Widget) Image(aniStep int) (*ebiten.Image, error) {
+	if w.hoverState && w.mouseButtonState {
+		return w.mouseButtonDownImage, nil
+	}
+
+	if w.hoverState && len(w.hoverAnimation) > 0 {
+		return w.hoverAnimation[(aniStep)%len(w.hoverAnimation)], nil
+	}
+
+	return w.sprite.Image, nil
+}