examples: add an example to drag a desktop mascot

Closes #2783

examples/dragmascot/main.go

@@ -0,0 +1,130 @@
+// Copyright 2024 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Mascot is a desktop mascot on cross platforms.
+// This is inspired by mattn's gopher (https://github.com/mattn/gopher).
+package main
+
+import (
+	"bytes"
+	"image"
+	_ "image/png"
+	"log"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	rmascot "github.com/hajimehoshi/ebiten/v2/examples/resources/images/mascot"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+)
+
+const (
+	width  = 200
+	height = 200
+)
+
+var (
+	gopher1 *ebiten.Image
+	gopher2 *ebiten.Image
+	gopher3 *ebiten.Image
+)
+
+func init() {
+	// Decode an image from the image file's byte slice.
+	img1, _, err := image.Decode(bytes.NewReader(rmascot.Out01_png))
+	if err != nil {
+		log.Fatal(err)
+	}
+	gopher1 = ebiten.NewImageFromImage(img1)
+
+	img2, _, err := image.Decode(bytes.NewReader(rmascot.Out02_png))
+	if err != nil {
+		log.Fatal(err)
+	}
+	gopher2 = ebiten.NewImageFromImage(img2)
+
+	img3, _, err := image.Decode(bytes.NewReader(rmascot.Out03_png))
+	if err != nil {
+		log.Fatal(err)
+	}
+	gopher3 = ebiten.NewImageFromImage(img3)
+}
+
+type mascot struct {
+	count int
+
+	dragging         bool
+	dragStartWindowX int
+	dragStartWindowY int
+	dragStartCursorX int
+	dragStartCursorY int
+
+	cursorToWindowX float64
+	cursorToWindowY float64
+}
+
+func (m *mascot) Layout(outsideWidth, outsideHeight int) (int, int) {
+	// The cursor position is in a "logical" coordinate, which is determined by the game width and height.
+	// Calculate the factors to convert a cursor position to a window position.
+	m.cursorToWindowX = float64(outsideWidth) / float64(width)
+	m.cursorToWindowY = float64(outsideHeight) / float64(height)
+	return width, height
+}
+
+func (m *mascot) Update() error {
+	if !ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
+		m.dragging = false
+	}
+	if !m.dragging && inpututil.IsMouseButtonJustPressed(ebiten.MouseButtonLeft) {
+		m.dragging = true
+		m.dragStartWindowX, m.dragStartWindowY = ebiten.WindowPosition()
+		m.dragStartCursorX, m.dragStartCursorY = ebiten.CursorPosition()
+	}
+	if m.dragging {
+		cx, cy := ebiten.CursorPosition()
+		dx := int(float64(cx-m.dragStartCursorX) * m.cursorToWindowX)
+		dy := int(float64(cy-m.dragStartCursorY) * m.cursorToWindowY)
+		wx, wy := ebiten.WindowPosition()
+		ebiten.SetWindowPosition(wx+dx, wy+dy)
+		m.count++
+	}
+
+	return nil
+}
+
+func (m *mascot) Draw(screen *ebiten.Image) {
+	img := gopher1
+	if m.dragging {
+		switch (m.count / 3) % 4 {
+		case 0:
+			img = gopher1
+		case 1, 3:
+			img = gopher2
+		case 2:
+			img = gopher3
+		}
+	}
+	screen.DrawImage(img, nil)
+}
+
+func main() {
+	ebiten.SetWindowDecorated(false)
+	ebiten.SetWindowFloating(true)
+	ebiten.SetWindowSize(width, height)
+
+	op := &ebiten.RunGameOptions{}
+	op.ScreenTransparent = true
+	op.SkipTaskbar = true
+	if err := ebiten.RunGameWithOptions(&mascot{}, op); err != nil {
+		log.Fatal(err)
+	}
+}