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              <canvas id="canvas" width="1500px" height="1500px"></canvas>
            
          
!
            
              // Canvas DOM 元素 
const canvas = document.getElementById('canvas')
const ctx = canvas.getContext('2d')

//起始點座標
let x1= 0
let y1= 0

// 終點座標
let x2= 0
let y2= 0

// 宣告一個 hasTouchEvent 變數,來檢查是否有 touch 的事件存在
const hasTouchEvent = 'ontouchstart' in window ? true : false

// 透過上方的 hasTouchEvent 來決定要監聽的是 mouse 還是 touch 的事件
const downEvent = hasTouchEvent ? 'ontouchstart' : 'mousedown'
const moveEvent = hasTouchEvent ? 'ontouchmove' : 'mousemove'
const upEvent = hasTouchEvent ? 'touchend' : 'mouseup'

// 宣告 isMouseActive 為滑鼠點擊的狀態,因為我們需要滑鼠在 mousedown 的狀態時,才會監聽 mousemove 的狀態
let isMouseActive = false

canvas.addEventListener(downEvent, function(e){
  isMouseActive = true
})

canvas.addEventListener(downEvent, function(e){
  isMouseActive = true  
  x1 = e.offsetX
  y1 = e.offsetY
  
  ctx.lineWidth = 5
  ctx.lineCap = 'round'
  ctx.lineJoin = 'round'
})

canvas.addEventListener(moveEvent, function(e){
      if(!isMouseActive){
        return
      }
      // 取得終點座標
      x2 = e.offsetX
      y2 = e.offsetY
      
      // 開始繪圖
      ctx.beginPath()
      ctx.moveTo(x1, y1)
      ctx.lineTo(x2, y2)
      ctx.stroke()
      
      // 更新起始點座標
      x1 = x2
      y1 = y2
})

canvas.addEventListener(upEvent, function(e){
  isMouseActive = false
})
            
          
!
999px
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