使用 javascript 调整大小 html canvas

Question

我正在尝试平滑地调整 html5 canvas 元素的大小。我拼凑了一些代码，我认为以下代码应该有效：

<body>
    <header id='myheader' height='200'>
        <canvas id='mycanvas' width='200' height='200'></canvas>

        <script>

        var mycanvas = document.getElementById('mycanvas');
        var context = mycanvas.getContext('2d');
        var centerX = mycanvas.width / 2;
        var centerY = mycanvas.height / 2;
        var radius = 70;

        context.beginPath();
        context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
        context.fillStyle = 'blue';
        context.fill();
        context.lineWidth = 5;
        context.strokeStyle = '#003300';
        context.stroke();

        var animate = function(prop, val, duration) {
            var start = new Date().getTime();
            var end = start + duration;
            var current = mycanvas[prop];
            var distance = val - current;
            var step = function() {
                var timestamp = new Date().getTime();
                var progress = Math.min((duration - (end - timestamp)) / duration, 1);
                mycanvas[prop] = current + (distance * progress);
                if (progress < 1) requestAnimationFrame(step);
            };
            return step();
        };

        animate('mycanvas.height', 10, 1000);

        </script>
    </header>       
</body>

...但显然不是！我正在寻找的结果是 canvas 缩小到只显示圆圈的中间部分（稍后将添加比圆圈更有趣的东西）。有什么明显的我失踪了，还是我只是以错误的方式这样做？最终我想一起调整 canvas 和 header 的大小，所以让 canvas 调整大小工作是第 1 阶段。感谢任何帮助...

（编辑：实际上，我最终想调整 canvas 和 header 的大小以响应滚动事件——我认为这意味着避免 css 解决方案——但我想让这个位先工作！）

Answer 1

以下是对您的脚本的一些更改，我相信可以满足您的要求：

    var mycanvas = document.getElementById('mycanvas');
    var context = mycanvas.getContext('2d');
    var radius = 70;

    function draw() {
        var centerX = mycanvas.width / 2;
        var centerY = mycanvas.height / 2;

        context.beginPath();
        context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
        context.fillStyle = 'blue';
        context.fill();
        context.lineWidth = 5;
        context.strokeStyle = '#003300';
        context.stroke();
    }

    var animate = function(target, prop, val, duration, action) {
        var start = new Date().getTime();
        var end = start + duration;
        var current = target[prop];
        var distance = val - current;
        var step = function() {
            var timestamp = new Date().getTime();
            var progress = Math.min((duration - (end - timestamp)) / duration, 1);
            target[prop] = current + (distance * progress);
            action();
            if (progress < 1) requestAnimationFrame(step);
        };
        return step();
    };

    animate(mycanvas, 'height', 10, 1000, draw);

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