examples/scroll: update comments

examples/scroll/main.go

@@ -0,0 +1,193 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"log"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+type Game struct {
+	contentArea image.Rectangle
+
+	// dragging indicates whether a pointing device is dragging the content.
+	dragging bool
+
+	// prevY is the previous Y value of a pointing device in the last frame.
+	prevY int
+
+	// startY is the initial Y value of a pointing device where dragging started.
+	startY int
+
+	// offsetY is the content offset in the Y axis.
+	offsetY int
+
+	// offsetStartY is the content offset in the Y axis when dragging started.
+	offsetStartY int
+
+	// velocityY is the scrolling velocity in the Y axis.
+	velocityY int
+
+	itemCount  int
+	itemHeight int
+
+	touchIDs             []ebiten.TouchID
+	justReleasedTouchIDs []ebiten.TouchID
+}
+
+func (g *Game) updateInput() {
+	g.touchIDs = ebiten.AppendTouchIDs(g.touchIDs[:0])
+	g.justReleasedTouchIDs = inpututil.AppendJustReleasedTouchIDs(g.justReleasedTouchIDs[:0])
+}
+
+// pointingDevicePosition returns the position of a pointing device (a touch or a mouse).
+func (g *Game) pointingDevicePosition() (x, y int) {
+	if len(g.touchIDs) > 0 {
+		return ebiten.TouchPosition(g.touchIDs[0])
+	}
+	return ebiten.CursorPosition()
+}
+
+// isPointingDevicePressed reports whether a pointing device is pressed.
+func (g *Game) isPointingDevicePressed() bool {
+	if len(g.touchIDs) > 0 {
+		return true
+	}
+	return ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft)
+}
+
+// isPointingDeviceJustReleased reports whether a pointing device is just released.
+func (g *Game) isPointingDeviceJustReleased() bool {
+	if len(g.justReleasedTouchIDs) > 0 {
+		return false
+	}
+	return inpututil.IsMouseButtonJustReleased(ebiten.MouseButtonLeft)
+}
+
+func (g *Game) Update() error {
+	g.updateInput()
+
+	x, y := g.pointingDevicePosition()
+	defer func() {
+		g.prevY = y
+	}()
+
+	hovering := image.Pt(x, y).In(g.contentArea)
+
+	// If a pointing device is just released, start scrolling.
+	if g.isPointingDeviceJustReleased() && g.dragging {
+		g.dragging = false
+		g.velocityY = y - g.prevY
+		return nil
+	}
+
+	// Process a mouse wheel.
+	if _, wheelY := ebiten.Wheel(); wheelY != 0 && !g.dragging && hovering {
+		g.velocityY = int(wheelY)
+	}
+
+	// If a pointing device is NOT pressed, scroll by the inertia.
+	if !g.isPointingDevicePressed() {
+		g.dragging = false
+
+		g.setOffsetY(g.offsetY + g.velocityY)
+		if g.velocityY != 0 {
+			g.velocityY = int(float64(g.velocityY) * 15.0 / 16.0)
+		}
+		return nil
+	}
+
+	// As a pointing device is pressed, stop the inertia.
+	g.velocityY = 0
+
+	if !g.dragging && hovering {
+		g.dragging = true
+		g.offsetStartY = g.offsetY
+		g.startY = y
+	}
+
+	// If a pinting device is pressed, adjust the offset by the movement.
+	if g.dragging {
+		g.setOffsetY(g.offsetStartY + y - g.startY)
+	}
+
+	return nil
+}
+
+func (g *Game) setOffsetY(offsetY int) {
+	g.offsetY = offsetY
+	if g.offsetY > 0 {
+		g.offsetY = 0
+	}
+	h := g.contentHeight()
+	if g.offsetY < -h+g.contentArea.Dy() {
+		g.offsetY = -h + g.contentArea.Dy()
+	}
+}
+
+func (g *Game) contentHeight() int {
+	return g.itemCount * g.itemHeight
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	// Render the content. Use SubImage as a mask.
+	screenContentArea := screen.SubImage(g.contentArea).(*ebiten.Image)
+
+	for i := 0; i < g.itemCount; i++ {
+		itemRegion := image.Rect(0, i*g.itemHeight, g.contentArea.Dx(), (i+1)*g.itemHeight)
+		itemRegion = itemRegion.Add(image.Pt(g.contentArea.Min.X, g.contentArea.Min.Y))
+		itemRegion = itemRegion.Add(image.Pt(0, g.offsetY))
+
+		// Skip rendering if the item is out of the content area.
+		if itemRegion.Intersect(g.contentArea).Empty() {
+			continue
+		}
+
+		vector.DrawFilledRect(screenContentArea, float32(itemRegion.Min.X), float32(itemRegion.Min.Y), float32(itemRegion.Dx()), float32(itemRegion.Dy()), color.RGBA{byte(i), byte(i), byte(i), 0xff}, false)
+		text := fmt.Sprintf("Item %d", i)
+		if i == 0 {
+			text += " (drag or touch to scroll)"
+		}
+		ebitenutil.DebugPrintAt(screenContentArea, text, itemRegion.Min.X, itemRegion.Min.Y)
+	}
+
+	// Render the content border line.
+	vector.StrokeRect(screen, float32(g.contentArea.Min.X), float32(g.contentArea.Min.Y), float32(g.contentArea.Dx()), float32(g.contentArea.Dy()), 1, color.White, false)
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	const offset = 16
+	g.contentArea = image.Rect(offset, offset, outsideWidth-offset, outsideHeight-offset)
+	return outsideWidth, outsideHeight
+}
+
+func main() {
+	ebiten.SetWindowTitle("Scroll (Ebitengine Demo)")
+	ebiten.SetWindowResizingMode(ebiten.WindowResizingModeEnabled)
+	ebiten.SetWindowSizeLimits(320, 240, -1, -1)
+	if err := ebiten.RunGame(&Game{
+		itemCount:  256,
+		itemHeight: 24,
+	}); err != nil {
+		log.Fatal(err)
+	}
+}