canvas-台球玩法

一、思路

绘制台球案板
画球方法
加上球移动的动作（控制透明度）
鼠标点击，球开始发射，匀速（实际有摩擦力，要速度减少），碰到案板要返回，速度或者说是每次位移的距离
鼠标移动，来放置球的位置
鼠标移出，停止动作

二、代码

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>controlBall</title>
</head>
<body>
    <canvas id="canvas" width="600" height="300"></canvas>

    <script>
      var canvas = document.getElementById('canvas');
      var ctx = canvas.getContext('2d');
      var raf;
      var running = false;


      // 绘制边框
      function drawRect() {
        ctx.beginPath();
        ctx.rect(0,0,canvas.width,canvas.height);
        ctx.strokeStyle = '#000';
        ctx.stroke();
      }

      var ball = {
        x: 100,
        y: 100,
        vx: 5,
        vy: 1,
        radius: 25,
        color: 'blue',
        draw: function() {
          ctx.beginPath();
          ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2, true);
          ctx.closePath();
          ctx.fillStyle = this.color;
          ctx.fill();

        }
      };

      function clear() {
        ctx.fillStyle = 'rgba(255,255,255,0.3)';
        ctx.fillRect(0,0,canvas.width,canvas.height);
      }

      function draw() {
        clear();
        drawRect();
        ball.x += ball.vx;
        ball.y += ball.vy;

        if (ball.y + ball.vy > canvas.height || ball.y + ball.vy < 0) {
          ball.vy = -ball.vy;
        }
        if (ball.x + ball.vx > canvas.width || ball.x + ball.vx < 0) {
          ball.vx = -ball.vx;
        }

        ball.draw();

        raf = window.requestAnimationFrame(draw);
      }

      canvas.addEventListener('mousemove', function(e){
        if (!running) {
          clear();
          drawRect();
          ball.x = e.clientX;
          ball.y = e.clientY;
          ball.draw();
        }
      });

      canvas.addEventListener('click',function(e){
        if (!running) {
          raf = window.requestAnimationFrame(draw);
          running = true;
        }
      });

      canvas.addEventListener('mouseout', function(e){
        window.cancelAnimationFrame(raf);
        running = false;
      });

      ball.draw();
      drawRect();
    </script>
</body>
</html>

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