Reimplement cursor as a query operation

internal/ui/window.go

@@ -3,7 +3,6 @@ package ui
 import (
 	"flag"
 	"fmt"
-	"image"
 	"log"
 	"os"
 	"runtime/debug"
@@ -19,6 +18,11 @@ type Game interface {
 	Draw(*ebiten.Image) error
 }
 
+type CustomCursor interface {
+	// The cursor draw operation
+	Cursor() (*ebiten.Image, *ebiten.DrawImageOptions, error)
+}
+
 var (
 	screenScale = flag.Float64("screen-scale", 1.0, "Scale the window by this factor")
 	cpuprofile  = flag.String("cpuprofile", "", "write cpu profile to `file`")
@@ -32,8 +36,6 @@ type Window struct {
 	KeyUpHandlers     map[ebiten.Key]func()
 	MouseWheelHandler func(float64, float64)
 
-	cursor *ebiten.Image
-
 	// Allow the "game" to be switched out at any time
 	game Game
 
@@ -74,15 +76,26 @@ func (w *Window) Layout(_, _ int) (int, int) {
 	return w.xRes, w.yRes
 }
 
-func (w *Window) SetCursor(cursor *ebiten.Image) {
-	w.cursor = cursor
+func (w *Window) drawCursor(screen *ebiten.Image) error {
+	cIface, ok := w.game.(CustomCursor)
+	if !ok {
+		return nil
+	}
+
+	cursor, op, err := cIface.Cursor()
+	if err != nil {
+		return err
+	}
 
 	// Hide the system cursor if we have a custom one
 	if cursor == nil {
 		ebiten.SetCursorMode(ebiten.CursorModeVisible)
-	} else {
-		ebiten.SetCursorMode(ebiten.CursorModeHidden)
+		return nil
 	}
+
+	ebiten.SetCursorMode(ebiten.CursorModeHidden)
+
+	return screen.DrawImage(cursor, op)
 }
 
 func (w *Window) Update(screen *ebiten.Image) (outErr error) {
@@ -125,23 +138,14 @@ func (w *Window) Update(screen *ebiten.Image) (outErr error) {
 		return err
 	}
 
-	if w.cursor != nil {
-		// TODO: account for scaling, including the hotspot.
-		pos := image.Pt(ebiten.CursorPosition())
-		op := ebiten.DrawImageOptions{}
-		op.GeoM.Translate(float64(pos.X), float64(pos.Y))
-		if err := screen.DrawImage(w.cursor, &op); err != nil {
-			return err
-		}
-	}
-
 	if w.debug {
 		// Draw FPS, etc, to the screen
 		msg := fmt.Sprintf("tps=%0.2f fps=%0.2f", ebiten.CurrentTPS(), ebiten.CurrentFPS())
 		ebitenutil.DebugPrint(screen, msg)
 	}
 
-	return nil
+	// Draw the cursor last
+	return w.drawCursor(screen)
 }
 
 // TODO: a stop or other cancellation mechanism