1. 在html页面中引入canvas标签,设置大小
<!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>
<!-- 引入js文件 -->
<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>
2. 随意写点样式,给画板加个边框
* {
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;
}
3. js代码
- 在页面加载完成后获取页面的canvas对象,并绑定监听事件,以实现拖动
window.onload = function () {
// 获取canvas对象
let canvas = document.getElementById("canvas");
canvasWidth = canvas.width;
canvasHeight = canvas.height;
let ctx = canvas.getContext("2d");
}
- 处理input标签选择的图片,创建图片添加至画板中
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]);
}
- 部分监听事件的函数,主要由mouseup,mousedown,mousemove事件完成拖拽
// 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 <= img_x + ctxWidth &&
y >= img_y &&
y <= img_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 < 0) {
y = 0;
}
if (x < 0) {
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;
}
};
4. js完整代码 补充了上面函数所使用到的全局变量
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 <= img_x + ctxWidth &&
y >= img_y &&
y <= img_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 < 0) {
y = 0;
}
if (x < 0) {
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]);
}