HTML5 Game - Dragging an object

Introduction

To drag an object, update the object's position to match the coordinates of the mouse cursor.
Demo

<!doctype html>  
    <html>  
     <head>  
      <style>canvas{border:1px solid red;}</style>  
     </head>  
 <body>  
  <canvas id="canvas" width="400" height="400"></canvas>  
  <script>  
class Ball {  //from  w w  w  . j  a v  a 2 s  .c o  m
constructor(radius) {
     if (radius === undefined) { radius = 40; }  
       
     this.x = 0;  
     this.y = 0;  
     this.radius = radius;  
     this.vx = 0;  
     this.vy = 0;  
     this.rotation = 0;  
     this.scaleX = 1;  
     this.scaleY = 1;  
     this.color = "#ff0000";
     this.lineWidth = 1;  
   }  
     
   draw(context) {  
     context.save();  
     context.translate(this.x, this.y);  
     context.rotate(this.rotation);  
     context.scale(this.scaleX, this.scaleY);  
     context.lineWidth = this.lineWidth;  
     context.fillStyle = this.color;  
     context.beginPath();  
     context.arc(0, 0, this.radius, 0, (Math.PI * 2), true);  
     context.closePath();  
  context.fill();  
  if (this.lineWidth > 0) {  
    context.stroke();  
  }  
  context.restore();  
}
getBounds() {  
  return {  
    x: this.x - this.radius,  
    y: this.y - this.radius,  
    width: this.radius * 2,  
    height: this.radius * 2  
  };  
}

}
containsPoint = function (rect, x, y) {  
  return !(x < rect.x || x > rect.x + rect.width ||  
           y < rect.y || y > rect.y + rect.height);  
}  
function captureMouse(element) {
  let mouse = {x: 0, y: 0, event: null},
      body_scrollLeft = document.body.scrollLeft,
      element_scrollLeft = document.documentElement.scrollLeft,
      body_scrollTop = document.body.scrollTop,
      element_scrollTop = document.documentElement.scrollTop,
      offsetLeft = element.offsetLeft,
      offsetTop = element.offsetTop;
  
  element.addEventListener('mousemove', function (event) {
    let x, y;
    
    if (event.pageX || event.pageY) {
      x = event.pageX;
      y = event.pageY;
    } else {
      x = event.clientX + body_scrollLeft + element_scrollLeft;
      y = event.clientY + body_scrollTop + element_scrollTop;
    }
    x -= offsetLeft;
    y -= offsetTop;
    
    mouse.x = x;
    mouse.y = y;
    mouse.event = event;
  }, false);
  
  return mouse;
}  
  window.onload = function () {  
    let canvas = document.getElementById('canvas'),  
        context = canvas.getContext('2d'),  
        mouse = captureMouse(canvas),  
        ball = new Ball();  
  
    ball.x = canvas.width / 2;  
    ball.y = canvas.height / 2;  
  
    canvas.addEventListener('mousedown', function () {  
      if (containsPoint(ball.getBounds(), mouse.x, mouse.y)) {  
        canvas.addEventListener('mouseup', onMouseUp, false);  
        canvas.addEventListener('mousemove', onMouseMove, false);  
      }  
    }, false);  
  
    function onMouseUp () {  
      canvas.removeEventListener('mouseup', onMouseUp, false);  
      canvas.removeEventListener('mousemove', onMouseMove, false);  
    }  
  
    function onMouseMove (event) {  
      ball.x = mouse.x;  
      ball.y = mouse.y;  
    }  
  
    (function drawFrame () {  
      window.requestAnimationFrame(drawFrame, canvas);  
      context.clearRect(0, 0, canvas.width, canvas.height);  
  
      ball.draw(context);  
    }());  
  };  
  </script>  
 </body>  
</html>
HTML5 Game - Dragging an object

Introduction

Demo

Related Topic
