js实现画图工具

实现基础画布

使用HTML5的Canvas元素作为画布基础，通过JavaScript获取画布上下文。示例代码初始化一个全屏画布：

<canvas id="drawingCanvas" width="800" height="600"></canvas>
<script>
  const canvas = document.getElementById('drawingCanvas');
  const ctx = canvas.getContext('2d');
  ctx.strokeStyle = '#000000';
  ctx.lineWidth = 3;
</script>

鼠标事件监听

绑定鼠标事件实现绘图功能。需要处理mousedown、mousemove和mouseup事件：

let isDrawing = false;
let lastX = 0;
let lastY = 0;

canvas.addEventListener('mousedown', (e) => {
  isDrawing = true;
  [lastX, lastY] = [e.offsetX, e.offsetY];
});

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];
});

canvas.addEventListener('mouseup', () => isDrawing = false);

绘制工具切换

通过按钮切换画笔、橡皮擦等工具。示例实现画笔和橡皮擦功能：

function setTool(toolType) {
  if (toolType === 'eraser') {
    ctx.globalCompositeOperation = 'destination-out';
  } else {
    ctx.globalCompositeOperation = 'source-over';
    ctx.strokeStyle = document.getElementById('colorPicker').value;
  }
}

颜色与粗细选择

添加颜色选择器和线宽滑块控制：

<input type="color" id="colorPicker" value="#000000">
<input type="range" id="brushSize" min="1" max="50" value="3">
<script>
  document.getElementById('colorPicker').addEventListener('change', (e) => {
    ctx.strokeStyle = e.target.value;
  });
  document.getElementById('brushSize').addEventListener('change', (e) => {
    ctx.lineWidth = e.target.value;
  });
</script>

图形绘制功能

扩展功能支持绘制矩形、圆形等预设图形：

function drawShape(shape, startX, startY, endX, endY) {
  ctx.beginPath();
  if (shape === 'rectangle') {
    ctx.rect(startX, startY, endX - startX, endY - startY);
  } else if (shape === 'circle') {
    const radius = Math.sqrt(Math.pow(endX - startX, 2) + Math.pow(endY - startY, 2));
    ctx.arc(startX, startY, radius, 0, Math.PI * 2);
  }
  ctx.stroke();
}

撤销与重做功能

使用数组保存绘图历史记录：

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

function saveState() {
  historyIndex++;
  history.length = historyIndex;
  history.push(canvas.toDataURL());
}

function undo() {
  if (historyIndex <= 0) return;
  historyIndex--;
  const img = new Image();
  img.onload = () => ctx.drawImage(img, 0, 0);
  img.src = history[historyIndex];
}

保存绘图结果

添加导出图像功能：

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

触摸屏支持

添加触摸事件处理使工具兼容移动设备：

canvas.addEventListener('touchstart', (e) => {
  e.preventDefault();
  const touch = e.touches[0];
  const mouseEvent = new MouseEvent('mousedown', {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
});

canvas.addEventListener('touchmove', (e) => {
  e.preventDefault();
  const touch = e.touches[0];
  const mouseEvent = new MouseEvent('mousemove', {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
});

以上代码组合可实现基础绘图工具的核心功能，包括自由绘制、图形工具、撤销重做和导出图像等特性。实际开发中可根据需求进一步扩展功能如图层管理、滤镜效果等。