Animate a Canvas circle to draw on load

问题

OK Hello.

I've decided to start using HTML canvas for a small project I have.

There's no real start yet. I'm just learning the basics of Canvas and I want to Animate a circle

<!DOCTYPE html>
<html>
 <head>
  <meta charset="UTF-8">
  <title>title</title>
    <style>
      body {
        margin: 0px;
        padding: 0px;
        background: #222;
      }

     </style>
  <link rel="stylesheet" href="style.css" type="text/css">
 </head>
<body>
<canvas id="myCanvas" width="578" height="250"></canvas>

    <script>
      var canvas = document.getElementById('myCanvas');
      var context = canvas.getContext('2d');
      var x = canvas.width / 2;
      var y = canvas.height / 2;
      var radius = 75;
      var startAngle = 1.5 * Math.PI;
      var endAngle = 3.2 * Math.PI;
      var counterClockwise = false;
      context.beginPath();
      context.arc(x, y, radius, startAngle, endAngle, counterClockwise);
      context.lineWidth = 15;
      // line color
      context.strokeStyle = 'black';
      context.stroke();
    </script>

  <script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js" type="text/javascript"></script>
  <script src="//ajax.googleapis.com/ajax/libs/mootools/1.4.5/mootools-yui-compressed.js" type="text/javascript"></script>
 </body>
</html>

Here is a fiddle http://jsfiddle.net/oskar/Aapn8/ of what I'm trying to achieve. I'm not to fussed with the "Bounce" effect.

But i basically want it to Draw on page load and stop at the desired Angle of the Arc Here's the Fiddle of what I've created so far.

http://jsfiddle.net/skerwin/uhVj6/

Thanks

回答1:

Live Demo

// requestAnimationFrame Shim
(function() {
  var requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame ||
                              window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
  window.requestAnimationFrame = requestAnimationFrame;
})();



var canvas = document.getElementById('myCanvas');
 var context = canvas.getContext('2d');
 var x = canvas.width / 2;
 var y = canvas.height / 2;
 var radius = 75;
 var endPercent = 85;
 var curPerc = 0;
 var counterClockwise = false;
 var circ = Math.PI * 2;
 var quart = Math.PI / 2;

 context.lineWidth = 10;
 context.strokeStyle = '#ad2323';
 context.shadowOffsetX = 0;
 context.shadowOffsetY = 0;
 context.shadowBlur = 10;
 context.shadowColor = '#656565';


 function animate(current) {
     context.clearRect(0, 0, canvas.width, canvas.height);
     context.beginPath();
     context.arc(x, y, radius, -(quart), ((circ) * current) - quart, false);
     context.stroke();
     curPerc++;
     if (curPerc < endPercent) {
         requestAnimationFrame(function () {
             animate(curPerc / 100)
         });
     }
 }

 animate();

Basically I used the same formula from the demo link you posted. I then added an animation function that when called will update the circle until it reaches the desired ending percent.

The animation is continually called by requestAnimationFrame this is the preferred way of doing any kind of animations with canvas. Every time animate is called the current percent is increased by one, which is then used to redraw the arc.

回答2:

Three steps:

1) Make an "init" function which will assign the variables (they aren't in any   
function).  
2) Then use requestAnimationFrame, or setInterval with your   
drawing function you will create.  
3) In this "drawing/updating" function you're going to   
write your code to do the maths and then just animate the updated circle.

There aren't any functions in your code. If you don't know how to make functions and use them + what is global variable it's better to read first a tutorials about that but anyway I'll try to make you an example :)

来源：https://stackoverflow.com/questions/15692353/animate-a-canvas-circle-to-draw-on-load

标签

canvas

jquery-animate

draw

radial