package main

import (
    "github.com/hajimehoshi/ebiten"
)

const (
    screenWidth  = 640
    screenHeight = 480
    ballSpeed    = 4
)

var (
    ballX, ballY float64
)

func update(screen *ebiten.Image) error {
    if ebiten.IsKeyPressed(ebiten.KeyArrowRight) {
        ballX += ballSpeed
    }
    if ebiten.IsKeyPressed(ebiten.KeyArrowLeft) {
       手游开发者必读：Golang在游戏行业的发展前景  ballX -= ballSpeed
    }
    if ebiten.IsKeyPressed(ebiten.KeyArrowDown) {
        ballY += ballSpeed
    }
    if ebiten.IsKeyPressed(ebiten.KeyArrowUp) {
        ballY -= ballSpeed
    }

    if ballX < 0 {
        ballX = 0
    }
    if ballX > screenWidth {
        ballX = screenWidth
    }
    if ballY < 0 {
        ballY = 0
    }
    if ballY > screenHeight {
        ballY = screenHeight
    }

    return nil
}

func draw(screen *ebiten.Image) {
    op := &ebiten.DrawImageOptions{}
    op.GeoM.Translate(ballX, ballY)
    screen.DrawImage(ballImage, op)
}

func main() {
    if err := ebiten.Run(update, screenWidth, screenHeight, 1, "Simple Game"); err != nil {
        panic(err)
    }
}