参考书目：《HTML5 Canvas核心技术 图形、动画与游戏开发》
以图像方式(离屏canvas)实现的时钟程序
1-13.html
<!DOCTYPE html>
<head>
<title>Image Clock</title>
<style>
body {
background: #dddddd;
}
#canvas {
display: none;
}
#snapshotImageElement {
position: absolute;
left: 10px;
margin: 20px;
border: thin solid #aaaaaa;
}
</style>
</head>
<body>
<img id='snapshotImageElement' />
<canvas id='canvas' width='400' height='400'>
Canvas not supported
</canvas>
<script src='1-14.js'></script>
</body>
</html>
1-14.js
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
snapshotButton = document.getElementById('snapshotButton'),
snapshotImageElement = document.getElementById('snapshotImageElement'),
FONT_HEIGHT = 15,
MARGIN = 35,
HAND_TRUNCATION = canvas.width / 25,
HOUR_HAND_TRUNCATION = canvas.width / 10,
NUMERAL_SPACING = 20,
RADIUS = canvas.width / 2 - MARGIN,
HAND_RADIUS = RADIUS + NUMERAL_SPACING,
loop;
function drawCircle() {
context.beginPath();
context.arc(canvas.width / 2, canvas.height / 2, RADIUS, 0, Math.PI * 2, true);
context.stroke();
}
function drawNumerals() {
var numerals = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12],
angle = 0,
numeralWidth = 0;
numerals.forEach(function (numeral) {
angle = Math.PI / 6 * (numeral - 3);
numeralWidth = context.measureText(numeral).width;
context.fillText(numeral,
canvas.width / 2 + Math.cos(angle) * (HAND_RADIUS) - numeralWidth / 2,
canvas.height / 2 + Math.sin(angle) * (HAND_RADIUS) + FONT_HEIGHT / 3);
});
}
function drawCenter() {
context.beginPath();
context.arc(canvas.width / 2, canvas.height / 2, 5, 0, Math.PI * 2, true);
context.fill();
}
function drawHand(loc, isHour) {
var angle = (Math.PI * 2) * (loc / 60) - Math.PI / 2,
handRadius = isHour ? RADIUS - HAND_TRUNCATION - HOUR_HAND_TRUNCATION :
RADIUS - HAND_TRUNCATION;
context.moveTo(canvas.width / 2, canvas.height / 2);
context.lineTo(canvas.width / 2 + Math.cos(angle) * handRadius,
canvas.height / 2 + Math.sin(angle) * handRadius);
context.stroke();
}
function drawHands() {
var date = new Date,
hour = date.getHours();
hour = hour > 12 ? hour - 12 : hour;
drawHand(hour * 5 + (date.getMinutes() / 60) * 5, true, 0.5);
drawHand(date.getMinutes(), false, 0.5);
drawHand(date.getSeconds(), false, 0.2);
}
function updateClockImage() {
dataUrl = canvas.toDataURL();
snapshotImageElement.src = dataUrl;
}
function drawClock() {
context.clearRect(0, 0, canvas.width, canvas.height);
context.save();
context.fillStyle = 'rgba(255,255,255,0.8)';
context.fillRect(0, 0, canvas.width, canvas.height);
drawCircle();
drawCenter();
drawHands();
context.restore();
drawNumerals();
updateClockImage();
}
context.font = FONT_HEIGHT + 'px Arial';
loop = setInterval(drawClock, 1000);
效果如图所示：
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