10.1 Canvas的基础操作
Canvas API 给开发者提供了很多很酷的工具，用来在web应用里搞二维图形和动画。在这节课，我们会看看Canvas API的基础，包括创建元素，绘制图形和制作动画。
创建 <canvas> 元素
HTML里的 <canvas> 元素是用来创建绘图区域的，你可以在这里搞各种图形操作：
HTML
<canvas id="myCanvas" width="800" height="600"></canvas>
获取绘图上下文
想在 <canvas> 元素上动手，就得先拿到绘图上下文。二维图形用的是2d的上下文：
JavaScript
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
10.2 在Canvas上绘图
1. 矩形
Canvas API 提供了绘制填充和描边矩形的方法。
填充矩形 — fillRect：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.fillStyle = 'blue';
ctx.fillRect(10, 10, 150, 100);
</script>
</body>
</html>
描边矩形 — strokeRect：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.strokeStyle = 'red';
ctx.strokeRect(200, 10, 150, 100);
</script>
</body>
</html>
清除区域
方法 clearRect 用来清除指定的区域：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.strokeStyle = 'red';
ctx.strokeRect(200, 10, 150, 100);
ctx.clearRect(0, 0, 800, 600);
</script>
</body>
</html>
2. 线条
用方法 beginPath 来开始新路径，还有 moveTo 和 lineTo 方法来定义线条的坐标：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(50, 150);
ctx.lineTo(250, 150);
ctx.lineTo(150, 300);
ctx.closePath();
ctx.stroke();
</script>
</body>
</html>
3. 圆和弧
方法 arc 用来画圆和弧：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.arc(400, 75, 50, 0, Math.PI * 2);
ctx.fillStyle = 'green';
ctx.fill();
</script>
</body>
</html>
4. 文本
方法 fillText 和 strokeText 用于绘制文本：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
ctx.font = '30px Arial';
ctx.fillStyle = 'black';
ctx.fillText('Hello Canvas', 10, 350);
</script>
</body>
</html>
5. 渐变
Canvas API 支持创建线性和放射渐变。
线性渐变：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const gradient = ctx.createLinearGradient(0, 0, 200, 0);
gradient.addColorStop(0, 'red');
gradient.addColorStop(1, 'blue');
ctx.fillStyle = gradient;
ctx.fillRect(10, 10, 200, 100);
</script>
</body>
</html>
放射渐变：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const radialGradient = ctx.createRadialGradient(400, 200, 20, 400, 200, 100);
radialGradient.addColorStop(0, 'yellow');
radialGradient.addColorStop(1, 'green');
ctx.fillStyle = radialGradient;
ctx.fillRect(300, 100, 200, 200);
</script>
</body>
</html>
6. 图片
方法 drawImage 用于在 Canvas 上绘制图片：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const img = new Image();
img.onload = function() {
ctx.drawImage(img, 256, 256);
};
img.src = 'https://cdn.javarush.com/images/article/ebb06cf3-0e04-45bd-a33e-31fe096fd323/256.jpeg';
</script>
</body>
</html>
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const img = new Image();
img.onload = function() {
ctx.drawImage(img, 256, 256);
};
img.src = 'path/to/image-1.jpg';
</script>
</body>
</html>
10.3 在Canvas上做动画
想做动画，可以用 requestAnimationFrame 方法，它会调用你指定的函数来绘制新的一帧。
1. 动画基础
简单动画例子：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
let x = 0;
let y = 0;
function animate() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = 'blue';
ctx.fillRect(x, y, 50, 50);
x += 2;
y += 2;
requestAnimationFrame(animate);
}
animate();
</script>
</body>
</html>
2. 复杂的动画
循环动画示例：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
let angle = 0;
function animateCircle() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
const centerX = canvas.width / 2;
const centerY = canvas.height / 2;
const radius = 50;
ctx.beginPath();
ctx.arc(centerX, centerY, radius, 0, Math.PI * 2);
ctx.fillStyle = 'blue';
ctx.fill();
ctx.beginPath();
ctx.arc(centerX + Math.cos(angle) * 100, centerY + Math.sin(angle) * 100, radius, 0, Math.PI * 2);
ctx.fillStyle = 'red';
ctx.fill();
angle += 0.05;
requestAnimationFrame(animateCircle);
}
animateCircle();
</script>
</body>
</html>
3. 使用定时器
还可以用 setInterval 和 setTimeout 方法来搞动画。
用 setInterval 的动画例子：
HTML
<html>
<head>
<title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="800" height="600"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
let x = 0;
let y = 0;
function draw() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = 'blue';
ctx.fillRect(x, y, 50, 50);
x += 2;
y += 2;
}
setInterval(draw, 30);
</script>
</body>
</html>
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