实现画布拖拽js

实现画布拖拽的基本思路

通过监听鼠标事件（mousedown、mousemove、mouseup）实现画布拖拽功能。核心逻辑是记录鼠标按下时的初始位置，计算移动时的偏移量，并更新画布位置。

HTML 结构准备

创建一个包含画布的容器，画布元素需设置绝对定位以便移动：

<div id="canvas-container" style="position: relative; overflow: hidden; width: 800px; height: 600px;">
  <canvas id="canvas" width="1600" height="1200" style="position: absolute;"></canvas>
</div>

JavaScript 实现

const canvas = document.getElementById('canvas');
const container = document.getElementById('canvas-container');
let isDragging = false;
let lastX = 0;
let lastY = 0;
let offsetX = 0;
let offsetY = 0;

// 鼠标按下事件
canvas.addEventListener('mousedown', (e) => {
  isDragging = true;
  lastX = e.clientX;
  lastY = e.clientY;
  e.preventDefault(); // 阻止默认行为（如文本选中）
});

// 鼠标移动事件
document.addEventListener('mousemove', (e) => {
  if (!isDragging) return;
  const deltaX = e.clientX - lastX;
  const deltaY = e.clientY - lastY;
  offsetX += deltaX;
  offsetY += deltaY;
  lastX = e.clientX;
  lastY = e.clientY;
  updateCanvasPosition();
});

// 鼠标释放事件
document.addEventListener('mouseup', () => {
  isDragging = false;
});

// 更新画布位置
function updateCanvasPosition() {
  canvas.style.transform = `translate(${offsetX}px, ${offsetY}px)`;
}

边界限制处理

为避免画布被拖出可视区域，可在 updateCanvasPosition 中添加边界检查：

function updateCanvasPosition() {
  const maxX = 0;
  const maxY = 0;
  const minX = container.clientWidth - canvas.width;
  const minY = container.clientHeight - canvas.height;

  offsetX = Math.min(maxX, Math.max(minX, offsetX));
  offsetY = Math.min(maxY, Math.max(minY, offsetY));

  canvas.style.transform = `translate(${offsetX}px, ${offsetY}px)`;
}

平滑惯性拖拽（可选）

通过记录移动速度并在鼠标释放后继续缓动，实现更自然的拖拽效果：

let velocityX = 0;
let velocityY = 0;
let lastTime = 0;

document.addEventListener('mousemove', (e) => {
  if (!isDragging) return;
  const now = Date.now();
  const deltaTime = now - lastTime;
  if (deltaTime > 0) {
    velocityX = (e.clientX - lastX) / deltaTime * 10;
    velocityY = (e.clientY - lastY) / deltaTime * 10;
  }
  lastTime = now;
  // 其余逻辑同上...
});

function animate() {
  if (!isDragging && (Math.abs(velocityX) > 0.1 || Math.abs(velocityY) > 0.1)) {
    offsetX += velocityX;
    offsetY += velocityY;
    velocityX *= 0.95; // 摩擦系数
    velocityY *= 0.95;
    updateCanvasPosition();
  }
  requestAnimationFrame(animate);
}
animate();

触摸屏支持

为兼容移动设备，需添加触摸事件：

canvas.addEventListener('touchstart', (e) => {
  isDragging = true;
  lastX = e.touches[0].clientX;
  lastY = e.touches[0].clientY;
  e.preventDefault();
});

document.addEventListener('touchmove', (e) => {
  if (!isDragging) return;
  const deltaX = e.touches[0].clientX - lastX;
  const deltaY = e.touches[0].clientY - lastY;
  offsetX += deltaX;
  offsetY += deltaY;
  lastX = e.touches[0].clientX;
  lastY = e.touches[0].clientY;
  updateCanvasPosition();
});

document.addEventListener('touchend', () => {
  isDragging = false;
});