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js实现白板

2026-02-01 15:26:26JavaScript

实现白板的基本思路

使用HTML5的Canvas API可以轻松实现一个简单的白板功能。Canvas提供了绘制路径、形状、文本和图像的方法，结合鼠标事件监听可以实现自由绘制。

基础代码结构

<!DOCTYPE html>
<html>
<head>
    <title>简易白板</title>
    <style>
        #drawing-board {
            border: 1px solid #000;
            cursor: crosshair;
        }
    </style>
</head>
<body>
    <canvas id="drawing-board" width="800" height="600"></canvas>
    <script src="script.js"></script>
</body>
</html>

JavaScript实现

const canvas = document.getElementById('drawing-board');
const ctx = canvas.getContext('2d');
let isDrawing = false;
let lastX = 0;
let lastY = 0;

// 设置画笔样式
ctx.strokeStyle = '#000';
ctx.lineWidth = 5;
ctx.lineCap = 'round';
ctx.lineJoin = 'round';

// 鼠标按下事件
canvas.addEventListener('mousedown', (e) => {
    isDrawing = true;
    [lastX, lastY] = [e.offsetX, e.offsetY];
});

// 鼠标移动事件
canvas.addEventListener('mousemove', (e) => {
    if (!isDrawing) return;
    ctx.beginPath();
    ctx.moveTo(lastX, lastY);
    ctx.lineTo(e.offsetX, e.offsetY);
    ctx.stroke();
    [lastX, lastY] = [e.offsetX, e.offsetY];
});

// 鼠标释放事件
canvas.addEventListener('mouseup', () => isDrawing = false);
canvas.addEventListener('mouseout', () => isDrawing = false);

功能扩展

颜色选择器

const colorPicker = document.createElement('input');
colorPicker.type = 'color';
document.body.prepend(colorPicker);

colorPicker.addEventListener('change', (e) => {
    ctx.strokeStyle = e.target.value;
});

画笔粗细调整

const brushSize = document.createElement('input');
brushSize.type = 'range';
brushSize.min = '1';
brushSize.max = '50';
document.body.prepend(brushSize);

brushSize.addEventListener('change', (e) => {
    ctx.lineWidth = e.target.value;
});

清除画布

const clearBtn = document.createElement('button');
clearBtn.textContent = '清除';
document.body.prepend(clearBtn);

clearBtn.addEventListener('click', () => {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
});

触摸屏支持

// 触摸开始
canvas.addEventListener('touchstart', (e) => {
    e.preventDefault();
    isDrawing = true;
    const rect = canvas.getBoundingClientRect();
    [lastX, lastY] = [e.touches[0].clientX - rect.left, e.touches[0].clientY - rect.top];
});

// 触摸移动
canvas.addEventListener('touchmove', (e) => {
    e.preventDefault();
    if (!isDrawing) return;
    const rect = canvas.getBoundingClientRect();
    ctx.beginPath();
    ctx.moveTo(lastX, lastY);
    ctx.lineTo(e.touches[0].clientX - rect.left, e.touches[0].clientY - rect.top);
    ctx.stroke();
    [lastX, lastY] = [e.touches[0].clientX - rect.left, e.touches[0].clientY - rect.top];
});

// 触摸结束
canvas.addEventListener('touchend', () => isDrawing = false);

保存绘图

const saveBtn = document.createElement('button');
saveBtn.textContent = '保存';
document.body.prepend(saveBtn);

saveBtn.addEventListener('click', () => {
    const link = document.createElement('a');
    link.download = 'drawing.png';
    link.href = canvas.toDataURL('image/png');
    link.click();
});

优化绘图性能

对于复杂绘图，可以使用requestAnimationFrame优化性能：

function draw(x, y) {
    ctx.beginPath();
    ctx.moveTo(lastX, lastY);
    ctx.lineTo(x, y);
    ctx.stroke();
    [lastX, lastY] = [x, y];
}

function throttle(callback, delay) {
    let previousCall = new Date().getTime();
    return function() {
        const time = new Date().getTime();
        if ((time - previousCall) >= delay) {
            previousCall = time;
            callback.apply(null, arguments);
        }
    };
}

canvas.addEventListener('mousemove', throttle((e) => {
    if (!isDrawing) return;
    draw(e.offsetX, e.offsetY);
}, 10));

以上代码实现了一个功能完整的简易白板，支持鼠标和触摸绘制、颜色选择、画笔粗细调整、清除画布和保存图片等功能。可以根据需要进一步扩展，如添加形状工具、文字工具或撤销重做功能。

js实现白板