example: add a mouse capture example (#2642)

Closes #2641

examples/capture/main.go

@@ -0,0 +1,132 @@
+// Copyright 2023 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.
+
+package main
+
+import (
+	"fmt"
+	"image/color"
+	"log"
+	"runtime"
+	"sync"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+)
+
+var pointerImage = ebiten.NewImage(8, 8)
+
+func init() {
+	pointerImage.Fill(color.RGBA{0xff, 0, 0, 0xff})
+}
+
+const (
+	screenWidth  = 640
+	screenHeight = 480
+)
+
+type Game struct {
+	initOnce sync.Once
+	mouseX   int
+	mouseY   int
+	x        int
+	y        int
+}
+
+func (g *Game) Update() error {
+	g.initOnce.Do(func() {
+		// initialize cursor position on first update
+		g.mouseX, g.mouseY = ebiten.CursorPosition()
+	})
+
+	if ebiten.CursorMode() == ebiten.CursorModeCaptured {
+		cursorX, cursorY := ebiten.CursorPosition()
+		deltaX, deltaY := cursorX-g.mouseX, cursorY-g.mouseY
+		g.mouseX, g.mouseY = cursorX, cursorY
+
+		if deltaX != 0 {
+			g.x += deltaX
+		}
+
+		if deltaY != 0 {
+			g.y += deltaY
+		}
+
+		// Constrain red dot within screen view.
+		if g.x < 0 {
+			g.x = 0
+		} else if g.x > screenWidth-pointerImage.Bounds().Dx() {
+			g.x = screenWidth - pointerImage.Bounds().Dx()
+		}
+
+		if g.y < 0 {
+			g.y = 0
+		} else if g.y > screenHeight-pointerImage.Bounds().Dy() {
+			g.y = screenHeight - pointerImage.Bounds().Dy()
+		}
+	}
+
+	if inpututil.IsKeyJustPressed(ebiten.KeySpace) {
+		if ebiten.CursorMode() == ebiten.CursorModeCaptured {
+			// Release mouse cursor capture.
+			ebiten.SetCursorMode(ebiten.CursorModeVisible)
+		} else {
+			// Recapture mouse cursor.
+			ebiten.SetCursorMode(ebiten.CursorModeCaptured)
+
+			// Reset mouse cursor position for its return.
+			g.mouseX, g.mouseY = ebiten.CursorPosition()
+		}
+	}
+
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	op := &ebiten.DrawImageOptions{}
+	op.GeoM.Translate(float64(g.x), float64(g.y))
+	screen.DrawImage(pointerImage, op)
+
+	var message string
+	if ebiten.CursorMode() == ebiten.CursorModeCaptured {
+		message = fmt.Sprintf("Move the red point with mouse captured\nPress Space to release mouse capture\n(%d, %d)", g.x, g.y)
+	} else {
+		message = fmt.Sprintf("The red point can only move when mouse captured\nPress Space to capture mouse\n(%d, %d)", g.x, g.y)
+	}
+	ebitenutil.DebugPrint(screen, message)
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	return screenWidth, screenHeight
+}
+
+func main() {
+	ebiten.SetWindowTitle("Mouse Capture (Ebitengine Demo)")
+	ebiten.SetWindowSize(screenWidth, screenHeight)
+
+	// Web browsers allow cursor mode capture only with user interaction.
+	// Start without cursor captured with message indicating how to capture.
+	if runtime.GOOS != "js" {
+		ebiten.SetCursorMode(ebiten.CursorModeCaptured)
+	}
+
+	g := &Game{
+		x: screenWidth / 2,
+		y: screenHeight / 2,
+	}
+	if err := ebiten.RunGame(g); err != nil {
+		log.Fatal(err)
+	}
+}