js实现画笔功能

实现基础画布与画笔功能

使用HTML5的Canvas API创建画布，并通过JavaScript监听鼠标事件实现画笔功能。以下代码展示如何初始化画布和基本绘图：

<canvas id="drawCanvas" width="800" height="600"></canvas>

const canvas = document.getElementById('drawCanvas');
const ctx = canvas.getContext('2d');
let isDrawing = false;

canvas.addEventListener('mousedown', startDrawing);
canvas.addEventListener('mousemove', draw);
canvas.addEventListener('mouseup', stopDrawing);
canvas.addEventListener('mouseout', stopDrawing);

function startDrawing(e) {
    isDrawing = true;
    ctx.beginPath();
    ctx.moveTo(e.offsetX, e.offsetY);
}

function draw(e) {
    if (!isDrawing) return;
    ctx.lineTo(e.offsetX, e.offsetY);
    ctx.stroke();
}

function stopDrawing() {
    isDrawing = false;
}

自定义画笔样式

扩展基础功能以支持自定义画笔属性，包括颜色、粗细和线条样式：

// 设置默认样式
ctx.strokeStyle = '#000000';
ctx.lineWidth = 5;
ctx.lineCap = 'round';
ctx.lineJoin = 'round';

// 动态修改样式示例
function setBrushColor(color) {
    ctx.strokeStyle = color;
}

function setBrushSize(size) {
    ctx.lineWidth = size;
}

实现触摸屏支持

为移动设备添加触摸事件处理，需注意坐标获取方式的差异：

canvas.addEventListener('touchstart', handleTouchStart);
canvas.addEventListener('touchmove', handleTouchMove);
canvas.addEventListener('touchend', handleTouchEnd);

function handleTouchStart(e) {
    e.preventDefault();
    const touch = e.touches[0];
    const mouseEvent = {
        offsetX: touch.clientX - canvas.offsetLeft,
        offsetY: touch.clientY - canvas.offsetTop
    };
    startDrawing(mouseEvent);
}

function handleTouchMove(e) {
    e.preventDefault();
    const touch = e.touches[0];
    const mouseEvent = {
        offsetX: touch.clientX - canvas.offsetLeft,
        offsetY: touch.clientY - canvas.offsetTop
    };
    draw(mouseEvent);
}

function handleTouchEnd() {
    stopDrawing();
}

添加撤销与重做功能

通过保存绘图状态实现历史记录功能：

const drawingHistory = [];
let historyIndex = -1;

function saveState() {
    historyIndex++;
    if (historyIndex < drawingHistory.length) {
        drawingHistory.length = historyIndex;
    }
    drawingHistory.push(canvas.toDataURL());
}

function undo() {
    if (historyIndex <= 0) return;
    historyIndex--;
    restoreState();
}

function redo() {
    if (historyIndex >= drawingHistory.length - 1) return;
    historyIndex++;
    restoreState();
}

function restoreState() {
    const img = new Image();
    img.onload = function() {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.drawImage(img, 0, 0);
    };
    img.src = drawingHistory[historyIndex];
}

// 在每次绘制结束时调用saveState()
function stopDrawing() {
    isDrawing = false;
    saveState();
}

实现压力敏感绘图（适用于支持压感的设备）

通过PointerEvent获取压力值，增强绘图体验：

canvas.addEventListener('pointerdown', startDrawing);
canvas.addEventListener('pointermove', draw);
canvas.addEventListener('pointerup', stopDrawing);

function draw(e) {
    if (!isDrawing) return;

    // 获取压力值（0-1），不支持时默认为1
    const pressure = e.pressure || 1;
    ctx.lineWidth = baseWidth * pressure;

    ctx.lineTo(e.offsetX, e.offsetY);
    ctx.stroke();
}

清除画布与导出图像

添加实用功能完善画笔应用：

function clearCanvas() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    saveState();
}

function exportAsPNG() {
    const link = document.createElement('a');
    link.download = 'drawing.png';
    link.href = canvas.toDataURL('image/png');
    link.click();
}

性能优化建议

对于复杂绘图场景，考虑以下优化措施：

// 使用requestAnimationFrame节流绘制操作
let lastX = 0;
let lastY = 0;

function draw(e) {
    if (!isDrawing) return;

    requestAnimationFrame(() => {
        ctx.beginPath();
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(e.offsetX, e.offsetY);
        ctx.stroke();

        lastX = e.offsetX;
        lastY = e.offsetY;
    });
}

以上实现方案可根据具体需求进行组合或扩展，例如添加多种画笔类型（毛笔、铅笔等）、实现图层功能或集成到现有Web应用中。