html页面创建canvas画板，在画板添加图片，并实现图片拖拽

DOCTYPE html>
html lang="en">

head>
  meta charset="UTF-8">
  meta name="viewport" content="width=device-width, initial-scale=1.0">
  title>Canvas画板title>
  link rel="stylesheet" href="./index.css">
head>

body>
  
  script src="./index.js">script>

  div class="content">
    input type="file" id="img_input" onchange="fileInput(this.files)">
    canvas width="600" height="500" id="canvas">
    canvas>
  div>
body>

html>

* {
  margin: 0;
  padding: 0;
}

.content {
  position: relative;
  width: 900px;
  height: 100vh;
  margin: 0 auto;
}

.content input {
  position: absolute;
  top: 40px;
  left: 200px;
  margin-left: 50px;
}

.content canvas {
  border: 1px solid #30336b;
  margin-top: 100px;
  margin-left: 150px;
}

window.onload = function () {
  // 获取canvas对象
  let canvas = document.getElementById("canvas");
  canvasWidth = canvas.width;
  canvasHeight = canvas.height;
  let ctx = canvas.getContext("2d");
}

function fileInput(files) {
  let reader = new FileReader();
  reader.onload = function (e) {
    // 创建图片对象
    img = new Image();
    // 读取图片路劲
    img.src = e.target.result;
    let ctx = canvas.getContext("2d");
    img.onload = function () {
      // 设置缩放比例
      let scale = 1;
      // 长或宽超过150进行缩放
      let maxNum = 150;
      if (this.width > maxNum || this.height > maxNum) {
        if (this.width > this.height) {
          scale = maxNum / this.width;
        } else {
          scale = maxNum / this.height;
        }
      }

      ctxHeight = scale * this.height;
      ctxWidth = scale * this.width;
      // 清除画布
      ctx.clearRect(0, 0, canvasWidth, canvasHeight);
      // 显示图片
      ctx.drawImage(img, 0, 0, ctxWidth, ctxHeight);
      // 重新选择图片时对img_x, img_y清零
      img_x = 0;
      img_y = 0;
    };
  };
  reader.readAsDataURL(files[0]);
}

  // 事件处理函数

  //鼠标按下
  function mouseDown(e) {
    let x = e.offsetX;
    let y = e.offsetY;
    // 记录鼠标相对于图片的位置
    mouseX = e.offsetX - img_x;
    mouseY = e.offsetY - img_y;

    // 判断是否在图片范围内
    if (
      x >= img_x &&
      x 
      y >= img_y &&
      y  ctxHeight
    ) {
      isDown = true;
    }
  }

  // 鼠标松开
  function mouseUp() {
    isDown = false;
  }

  // 鼠标移动
  function mouseMove(e) {
    if (isDown) {
      let x = e.offsetX - mouseX;
      let y = e.offsetY - mouseY;
      ctx.clearRect(0, 0, canvasWidth, canvasHeight);
      // 判断有无超过边界
      if (y ) {
        y = 0;
      }
      if (x ) {
        x = 0;
      }
      if (x > canvasWidth - ctxWidth) {
        x = canvasWidth - ctxWidth;
      }
      if (y > canvasHeight - ctxHeight) {
        y = canvasHeight - ctxHeight;
      }
      ctx.drawImage(img, x, y, ctxWidth, ctxHeight);
      img_x = x;
      img_y = y;
    }
  }

  function mouseLeave() {
    isDown = false;
  }
};

let ctxHeight, ctxWidth, canvasWidth, canvasHeight, mouseX, mouseY;
let isDown = false;
let img_x = 0;
let img_y = 0;

window.onload = function () {
  // 获取canvas对象
  let canvas = document.getElementById("canvas");
  canvasWidth = canvas.width;
  canvasHeight = canvas.height;
  let ctx = canvas.getContext("2d");
  // 绑定事件
  canvas.addEventListener("mousedown", mouseDown);
  canvas.addEventListener("mouseup", mouseUp);
  canvas.addEventListener("mousemove", mouseMove);
  canvas.addEventListener("mouseleave", mouseLeave);

  // 事件处理函数

  //鼠标按下
  function mouseDown(e) {
    let x = e.offsetX;
    let y = e.offsetY;
    // 记录鼠标相对于图片的位置
    mouseX = e.offsetX - img_x;
    mouseY = e.offsetY - img_y;

    // 判断是否在图片范围内
    if (
      x >= img_x &&
      x 
      y >= img_y &&
      y  ctxHeight
    ) {
      isDown = true;
    }
  }

  // 鼠标松开
  function mouseUp() {
    isDown = false;
  }

  // 鼠标移动
  function mouseMove(e) {
    if (isDown) {
      let x = e.offsetX - mouseX;
      let y = e.offsetY - mouseY;
      ctx.clearRect(0, 0, canvasWidth, canvasHeight);
      // 判断有无超过边界
      if (y ) {
        y = 0;
      }
      if (x ) {
        x = 0;
      }
      if (x > canvasWidth - ctxWidth) {
        x = canvasWidth - ctxWidth;
      }
      if (y > canvasHeight - ctxHeight) {
        y = canvasHeight - ctxHeight;
      }
      ctx.drawImage(img, x, y, ctxWidth, ctxHeight);
      img_x = x;
      img_y = y;
    }
  }

  function mouseLeave() {
    isDown = false;
  }
};

function fileInput(files) {
  let reader = new FileReader();
  reader.onload = function (e) {
    // 创建图片对象
    img = new Image();
    // 读取图片路劲
    img.src = e.target.result;
    let ctx = canvas.getContext("2d");
    img.onload = function () {
      // 设置缩放比例
      let scale = 1;
      // 长或宽超过150进行缩放
      let maxNum = 150;
      if (this.width > maxNum || this.height > maxNum) {
        if (this.width > this.height) {
          scale = maxNum / this.width;
        } else {
          scale = maxNum / this.height;
        }
      }

      ctxHeight = scale * this.height;
      ctxWidth = scale * this.width;
      // 清除画布
      ctx.clearRect(0, 0, canvasWidth, canvasHeight);
      // 显示图片
      ctx.drawImage(img, 0, 0, ctxWidth, ctxHeight);
      // 重新选择图片时对img_x, img_y清零
      img_x = 0;
      img_y = 0;
    };
  };
  reader.readAsDataURL(files[0]);
}