js代码

<script>
var wave = (function () {
	var ctx;
	var waveImage;
	var canvasWidth;
	var canvasHeight;
	var needAnimate = false;

	function init (callback) {
		var wave = document.getElementById('wave');
		var canvas = document.createElement('canvas');
		if (!canvas.getContext) return;
		ctx = canvas.getContext('2d');
		canvasWidth = wave.offsetWidth;
		canvasHeight = wave.offsetHeight;
		canvas.setAttribute('width', canvasWidth);
		canvas.setAttribute('height', canvasHeight);
		wave.appendChild(canvas);
		waveImage = new Image();
		waveImage.onload = function () {
			waveImage.onload = null;
			callback();
		}
		waveImage.src = 'images/wave.png';
	}

	function animate () {
		var waveX = 0;
		var waveY = 0;
		var waveX_min = -203;
		var waveY_max = canvasHeight * 0.7;
		var requestAnimationFrame = 
			window.requestAnimationFrame || 
			window.mozRequestAnimationFrame || 
			window.webkitRequestAnimationFrame || 
			window.msRequestAnimationFrame ||
			function (callback) { window.setTimeout(callback, 1000 / 60); };
		function loop () {
			ctx.clearRect(0, 0, canvasWidth, canvasHeight);
			if (!needAnimate) return;
			if (waveY < waveY_max) waveY += 1.5;
			if (waveX < waveX_min) waveX = 0; else waveX -= 3;
			
			ctx.globalCompositeOperation = 'source-over';
			ctx.beginPath();
			ctx.arc(canvasWidth/2, canvasHeight/2, canvasHeight/2, 0, Math.PI*2, true);
			ctx.closePath();
			ctx.fill();

			ctx.globalCompositeOperation = 'source-in';
			ctx.drawImage(waveImage, waveX, canvasHeight - waveY);
			
			requestAnimationFrame(loop);
		}
		loop();
	}

	function start () {
		if (!ctx) return init(start);
		needAnimate = true;
		setTimeout(function () {
			if (needAnimate) animate();
		}, 500);
	}
	function stop () {
		needAnimate = false;
	}
	return {start: start, stop: stop};
}());
wave.start();
</script>