知识库

html
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html,
    body {
      width: 100vw;
      height: 100vh;
      display: flex;
      flex-direction: column;
      justify-content: center;
      align-items: center;
    }
  </style>
</head>

<body>
  <canvas id="canvas"></canvas>

  <script>
    const canvas = document.querySelector('#canvas')
    const context = canvas.getContext('2d')

    canvas.style.background = '#ddd'

    const roundedRect = (x, y, width, height, borderRadius) => {
      context.moveTo(width > 0 ? x + borderRadius : x - borderRadius, y)
      // 不使用 beginPath，正好省略绘制直线的操作。
      // top
      context.arcTo(x + width, y, x + width, y + height, borderRadius)
      // right
      context.arcTo(x + width, y + height, x, y + height, borderRadius)
      // bottom
      context.arcTo(x, y + height, x, y, borderRadius)
      // left 
      // 上一个子路径连接到新子路径的的起点，
      // 新子路径的起点不是 (x, y)，而是绘制出来的圆弧的起点。
      context.arcTo(x, y, width > 0 ? x + borderRadius : x - borderRadius, y, borderRadius)
    }

    const drawRoundedRect = (x, y, width, height, borderRadius, strokeColor = 'black', fillColor = 'transparent') => {
      context.beginPath()
      roundedRect(x, y, width, height, borderRadius)
      context.strokeStyle = strokeColor
      context.fillStyle = fillColor
      context.stroke()
      context.fill()
    }

    drawRoundedRect(10, 10, 50, 50, 10, 'red')
    drawRoundedRect(120, 10, -50, 50, 15, 'green')
    drawRoundedRect(130, 60, 50, -50, 20, 'blue')
    drawRoundedRect(240, 60, -50, -50, 25, 'red')
  </script>
</body>

</html>