使用HTML 5画布自动滚动

Question

我在与HTML 5画布动画相关的问题。
我刚刚编写了整个代码，它的作用是使框保持向下移动并超过网页的高度，这归因于滚动选项的出现，因此我必须向下滚动以查看框在哪里。
所以我的问题是：有什么方法可以使框向下移动时自动帮助用户仅通过该动画向下滚动？ 意思是当盒子向下移动时，它也会向下滚动，以帮助用户显示盒子向何处移动。 如果涉及到背景，那么它将只是一个示例。

小提琴在这里： http : //jsfiddle.net/gamealchemist/MLHQK/我尝试使用背景滚动，但是那也不起作用...！

代码是：

CSS

  body {
    margin: 0px;
    padding: 0px;
  }

HTML

<canvas id="myCanvas" width="578" height="1000"></canvas>

JavaScript的

window.requestAnimFrame = (function (callback) {
    return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame || function (callback) {
        window.setTimeout(callback, 1000 / 60);
    };
})();

function drawRectangle(myRectangle, context) {
    context.beginPath();
    context.rect(myRectangle.x, myRectangle.y, myRectangle.width, myRectangle.height);
    context.fillStyle = '#8ED6FF';
    context.fill();
    context.lineWidth = myRectangle.borderWidth;
    context.strokeStyle = 'black';
    context.stroke();
}

function animate(myRectangle, canvas, context, startTime) {
    // update
    var time = Date.now() - startTime;

    // pixels / second^2
    var gravity = 200;

    myRectangle.y = 0.5 * gravity * Math.pow(time / 1000, 2);

    if (myRectangle.y > canvas.height - myRectangle.height - myRectangle.borderWidth / 2) {
        myRectangle.y = canvas.height - myRectangle.height - myRectangle.borderWidth / 2;
    }
    lastTime = time;

    // clear
    context.clearRect(0, 0, canvas.width, canvas.height);

    // draw
    drawRectangle(myRectangle, context);

    // request new frame
    requestAnimFrame(function () {
        animate(myRectangle, canvas, context, startTime);
    });
}
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');

var myRectangle = {
    x: 239,
    y: 0,
    width: 100,
    height: 50,
    borderWidth: 5
};

drawRectangle(myRectangle, context);

// wait one second before dropping rectangle
setTimeout(function () {
    var startTime = (new Date()).getTime();
    animate(myRectangle, canvas, context, startTime);
}, 2000);

Answer 1

将此添加到动画函数中即可解决问题

delay = 100;
// scroll
if( myRectangle.y - delay > 0 )
    window.scrollTo( 0, myRectangle.y - delay );

这是我根据您的代码制作的一个示例，其中还添加了背景图片，因此您可以看到该框在正确移动。

<!DOCTYPE html>
<html>
<head>
<script>
    window.requestAnimFrame = (function (callback) {
        return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame || function (callback) {
            window.setTimeout(callback, 1000 / 60);
        };
    })();

    function drawRectangle(myRectangle, context) {
        context.beginPath();
        context.rect(myRectangle.x, myRectangle.y, myRectangle.width, myRectangle.height);
        context.fillStyle = '#8ED6FF';
        context.fill();
        context.lineWidth = myRectangle.borderWidth;
        context.strokeStyle = 'black';
        context.stroke();
    }

    function animate(myRectangle, canvas, context, startTime) {
        // update
        var time = Date.now() - startTime;

        // pixels / second^2
        var gravity = 200;

        myRectangle.y = 0.5 * gravity * Math.pow(time / 1000, 2);

        if (myRectangle.y > canvas.height - myRectangle.height - myRectangle.borderWidth / 2) {
            myRectangle.y = canvas.height - myRectangle.height - myRectangle.borderWidth / 2;
        }
        lastTime = time;

        // clear
        context.clearRect(0, 0, canvas.width, canvas.height);

        // draw
        drawRectangle(myRectangle, context);

        delay = 100;
        // scroll
        if( myRectangle.y - delay > 0 )
            window.scrollTo( 0, myRectangle.y - delay );

        // request new frame
        requestAnimFrame(function () {
            animate(myRectangle, canvas, context, startTime);
        });
    }

    function load(){
        var canvas = document.getElementById('myCanvas');
        var context = canvas.getContext('2d');

        var myRectangle = {
            x: 239,
            y: 0,
            width: 100,
            height: 50,
            borderWidth: 5
        };

        drawRectangle(myRectangle, context);

        // wait one second before dropping rectangle
        setTimeout(function () {
            var startTime = (new Date()).getTime();
            animate(myRectangle, canvas, context, startTime);
        }, 2000);
    }
</script>
</head>
<body background="http://upload.wikimedia.org/wikipedia/commons/1/16/Appearance_of_sky_for_weather_forecast,_Dhaka,_Bangladesh.JPG" style="margin: 0px;padding: 0px;" onload="load();">
    <canvas id="myCanvas" width="578" height="5000"></canvas>
</body>
</html>