HTML5重力感应小球冲撞动画实现教程

今天我们来分享一款很酷的HTML5重力感应动画教程，这款动画可以让你甩动页面中的小球，小球的大小都不同，并且鼠标点击空白区域时又可以生成一定数量的小球。当我们甩动小球时，各个小球之间就会发生互相碰撞的效果，并且在运动过程中模拟了重力感应的物理效果。你可以在DEMO演示中来尝试一下。

你也可以在这里查看在线演示

接下来我们来分析一下这款超酷的HTML5重力动画实现的思路及源码，主要由HTML代码和Javascript代码组成。

HTML代码：

<div id="canvas"></div>

还是很简单，HTML仅仅是列出了一个canvas容器，今后我们将在这里生成一些列canvas元素，这些小球就在canvas中运动。

另外由于该动画利用了box2d的js脚本库，所以在页面上你也需要引用它：

<script src="box2d.js"></script>

接下来是Javascript代码，在canvas上动态创建大小和样式不一的小球，并发生碰撞效果。

Javascript代码：

var canvas;

var delta = [ 0, 0 ];
var stage = [ window.screenX, window.screenY, window.innerWidth, window.innerHeight ];
getBrowserDimensions();

var themes = [ [ "#10222B", "#95AB63", "#BDD684", "#E2F0D6", "#F6FFE0" ],
        [ "#362C2A", "#732420", "#BF734C", "#FAD9A0", "#736859" ],
        [ "#0D1114", "#102C2E", "#695F4C", "#EBBC5E", "#FFFBB8" ],
        [ "#2E2F38", "#FFD63E", "#FFB54B", "#E88638", "#8A221C" ],
        [ "#121212", "#E6F2DA", "#C9F24B", "#4D7B85", "#23383D" ],
        [ "#343F40", "#736751", "#F2D7B6", "#BFAC95", "#8C3F3F" ],
        [ "#000000", "#2D2B2A", "#561812", "#B81111", "#FFFFFF" ],
        [ "#333B3A", "#B4BD51", "#543B38", "#61594D", "#B8925A" ] ];
var theme;

var worldAABB, world, iterations = 1, timeStep = 1 / 15;

var walls = [];
var wall_thickness = 200;
var wallsSetted = false;

var bodies, elements, text;

var createMode = false;
var destroyMode = false;

var isMouseDown = false;
var mouseJoint;
var mouse = { x: 0, y: 0 };
var gravity = { x: 0, y: 1 };

var PI2 = Math.PI * 2;

var timeOfLastTouch = 0;

init();
play();

function init() {

    canvas = document.getElementById( 'canvas' );

    document.onmousedown = onDocumentMouseDown;
    document.onmouseup = onDocumentMouseUp;
    document.onmousemove = onDocumentMouseMove;
    document.ondblclick = onDocumentDoubleClick;

    document.addEventListener( 'touchstart', onDocumentTouchStart, false );
    document.addEventListener( 'touchmove', onDocumentTouchMove, false );
    document.addEventListener( 'touchend', onDocumentTouchEnd, false );

    window.addEventListener( 'deviceorientation', onWindowDeviceOrientation, false );

    // init box2d

    worldAABB = new b2AABB();
    worldAABB.minVertex.Set( -200, -200 );
    worldAABB.maxVertex.Set( window.innerWidth + 200, window.innerHeight + 200 );

    world = new b2World( worldAABB, new b2Vec2( 0, 0 ), true );

    setWalls();
    reset();
}


function play() {

    setInterval( loop, 1000 / 40 );
}

function reset() {

    var i;

    if ( bodies ) {

        for ( i = 0; i < bodies.length; i++ ) {

            var body = bodies[ i ]
            canvas.removeChild( body.GetUserData().element );
            world.DestroyBody( body );
            body = null;
        }
    }

    // color theme
    theme = themes[ Math.random() * themes.length >> 0 ];
    document.body.style[ 'backgroundColor' ] = theme[ 0 ];

    bodies = [];
    elements = [];

    createInstructions();

    for( i = 0; i < 10; i++ ) {

        createBall();

    }

}

//

function onDocumentMouseDown() {

    isMouseDown = true;
    return false;
}

function onDocumentMouseUp() {

    isMouseDown = false;
    return false;
}

function onDocumentMouseMove( event ) {

    mouse.x = event.clientX;
    mouse.y = event.clientY;
}

function onDocumentDoubleClick() {

    reset();
}

function onDocumentTouchStart( event ) {

    if( event.touches.length == 1 ) {

        event.preventDefault();

        // Faking double click for touch devices

        var now = new Date().getTime();

        if ( now - timeOfLastTouch  < 250 ) {

            reset();
            return;
        }

        timeOfLastTouch = now;

        mouse.x = event.touches[ 0 ].pageX;
        mouse.y = event.touches[ 0 ].pageY;
        isMouseDown = true;
    }
}

function onDocumentTouchMove( event ) {

    if ( event.touches.length == 1 ) {

        event.preventDefault();

        mouse.x = event.touches[ 0 ].pageX;
        mouse.y = event.touches[ 0 ].pageY;

    }

}

function onDocumentTouchEnd( event ) {

    if ( event.touches.length == 0 ) {

        event.preventDefault();
        isMouseDown = false;

    }

}

function onWindowDeviceOrientation( event ) {

    if ( event.beta ) {

        gravity.x = Math.sin( event.gamma * Math.PI / 180 );
        gravity.y = Math.sin( ( Math.PI / 4 ) + event.beta * Math.PI / 180 );

    }

}

//

function createInstructions() {

    var size = 250;

    var element = document.createElement( 'div' );
    element.width = size;
    element.height = size;    
    element.style.position = 'absolute';
    element.style.left = -200 + 'px';
    element.style.top = -200 + 'px';
    element.style.cursor = "default";

    canvas.appendChild(element);
    elements.push( element );

    var circle = document.createElement( 'canvas' );
    circle.width = size;
    circle.height = size;

    var graphics = circle.getContext( '2d' );

    graphics.fillStyle = theme[ 3 ];
    graphics.beginPath();
    graphics.arc( size * .5, size * .5, size * .5, 0, PI2, true );
    graphics.closePath();
    graphics.fill();

    element.appendChild( circle );

    text = document.createElement( 'div' );
    text.onSelectStart = null;
    text.innerHTML = '<span style="color:' + theme[0] + ';font-size:40px;">Hello!</span><br /><br /><span style="font-size:15px;"><strong>This is how it works:</strong><br /><br />1. Drag a ball.<br />2.&nbsp;Click&nbsp;on&nbsp;the&nbsp;background.<br />3. Shake your browser.<br />4. Double click.<br />5. Play!</span>';
    text.style.color = theme[1];
    text.style.position = 'absolute';
    text.style.left = '0px';
    text.style.top = '0px';
    text.style.fontFamily = 'Georgia';
    text.style.textAlign = 'center';
    element.appendChild(text);

    text.style.left = ((250 - text.clientWidth) / 2) +'px';
    text.style.top = ((250 - text.clientHeight) / 2) +'px';    

    var b2body = new b2BodyDef();

    var circle = new b2CircleDef();
    circle.radius = size / 2;
    circle.density = 1;
    circle.friction = 0.3;
    circle.restitution = 0.3;
    b2body.AddShape(circle);
    b2body.userData = {element: element};

    b2body.position.Set( Math.random() * stage[2], Math.random() * -200 );
    b2body.linearVelocity.Set( Math.random() * 400 - 200, Math.random() * 400 - 200 );
    bodies.push( world.CreateBody(b2body) );    
}

function createBall( x, y ) {

    var x = x || Math.random() * stage[2];
    var y = y || Math.random() * -200;

    var size = (Math.random() * 100 >> 0) + 20;

    var element = document.createElement("canvas");
    element.width = size;
    element.height = size;
    element.style.position = 'absolute';
    element.style.left = -200 + 'px';
    element.style.top = -200 + 'px';
    element.style.WebkitTransform = 'translateZ(0)';
    element.style.MozTransform = 'translateZ(0)';
    element.style.OTransform = 'translateZ(0)';
    element.style.msTransform = 'translateZ(0)';
    element.style.transform = 'translateZ(0)';

    var graphics = element.getContext("2d");

    var num_circles = Math.random() * 10 >> 0;

    for (var i = size; i > 0; i-= (size/num_circles)) {

        graphics.fillStyle = theme[ (Math.random() * 4 >> 0) + 1];
        graphics.beginPath();
        graphics.arc(size * .5, size * .5, i * .5, 0, PI2, true); 
        graphics.closePath();
        graphics.fill();
    }

    canvas.appendChild(element);

    elements.push( element );

    var b2body = new b2BodyDef();

    var circle = new b2CircleDef();
    circle.radius = size >> 1;
    circle.density = 1;
    circle.friction = 0.3;
    circle.restitution = 0.3;
    b2body.AddShape(circle);
    b2body.userData = {element: element};

    b2body.position.Set( x, y );
    b2body.linearVelocity.Set( Math.random() * 400 - 200, Math.random() * 400 - 200 );
    bodies.push( world.CreateBody(b2body) );
}

//

function loop() {

    if (getBrowserDimensions()) {

        setWalls();

    }

    delta[0] += (0 - delta[0]) * .5;
    delta[1] += (0 - delta[1]) * .5;

    world.m_gravity.x = gravity.x * 350 + delta[0];
    world.m_gravity.y = gravity.y * 350 + delta[1];

    mouseDrag();
    world.Step(timeStep, iterations);

    for (i = 0; i < bodies.length; i++) {

        var body = bodies[i];
        var element = elements[i];

        element.style.left = (body.m_position0.x - (element.width >> 1)) + 'px';
        element.style.top = (body.m_position0.y - (element.height >> 1)) + 'px';

        if (element.tagName == 'DIV') {

            var style = 'rotate(' + (body.m_rotation0 * 57.2957795) + 'deg) translateZ(0)';
            text.style.WebkitTransform = style;
            text.style.MozTransform = style;
            text.style.OTransform = style;
            text.style.msTransform = style;
            text.style.transform = style;

        }

    }

}


// .. BOX2D UTILS

function createBox(world, x, y, width, height, fixed) {

    if (typeof(fixed) == 'undefined') {

        fixed = true;

    }

    var boxSd = new b2BoxDef();

    if (!fixed) {

        boxSd.density = 1.0;

    }

    boxSd.extents.Set(width, height);

    var boxBd = new b2BodyDef();
    boxBd.AddShape(boxSd);
    boxBd.position.Set(x,y);

    return world.CreateBody(boxBd);

}

function mouseDrag()
{
    // mouse press
    if (createMode) {

        createBall( mouse.x, mouse.y );

    } else if (isMouseDown && !mouseJoint) {

        var body = getBodyAtMouse();

        if (body) {

            var md = new b2MouseJointDef();
            md.body1 = world.m_groundBody;
            md.body2 = body;
            md.target.Set(mouse.x, mouse.y);
            md.maxForce = 30000 * body.m_mass;
            // md.timeStep = timeStep;
            mouseJoint = world.CreateJoint(md);
            body.WakeUp();

        } else {

            createMode = true;

        }

    }

    // mouse release
    if (!isMouseDown) {

        createMode = false;
        destroyMode = false;

        if (mouseJoint) {

            world.DestroyJoint(mouseJoint);
            mouseJoint = null;

        }

    }

    // mouse move
    if (mouseJoint) {

        var p2 = new b2Vec2(mouse.x, mouse.y);
        mouseJoint.SetTarget(p2);
    }
}

function getBodyAtMouse() {

    // Make a small box.
    var mousePVec = new b2Vec2();
    mousePVec.Set(mouse.x, mouse.y);

    var aabb = new b2AABB();
    aabb.minVertex.Set(mouse.x - 1, mouse.y - 1);
    aabb.maxVertex.Set(mouse.x + 1, mouse.y + 1);

    // Query the world for overlapping shapes.
    var k_maxCount = 10;
    var shapes = new Array();
    var count = world.Query(aabb, shapes, k_maxCount);
    var body = null;

    for (var i = 0; i < count; ++i) {

        if (shapes[i].m_body.IsStatic() == false) {

            if ( shapes[i].TestPoint(mousePVec) ) {

                body = shapes[i].m_body;
                break;

            }

        }

    }

    return body;

}

function setWalls() {

    if (wallsSetted) {

        world.DestroyBody(walls[0]);
        world.DestroyBody(walls[1]);
        world.DestroyBody(walls[2]);
        world.DestroyBody(walls[3]);

        walls[0] = null; 
        walls[1] = null;
        walls[2] = null;
        walls[3] = null;
    }

    walls[0] = createBox(world, stage[2] / 2, - wall_thickness, stage[2], wall_thickness);
    walls[1] = createBox(world, stage[2] / 2, stage[3] + wall_thickness, stage[2], wall_thickness);
    walls[2] = createBox(world, - wall_thickness, stage[3] / 2, wall_thickness, stage[3]);
    walls[3] = createBox(world, stage[2] + wall_thickness, stage[3] / 2, wall_thickness, stage[3]);    

    wallsSetted = true;

}

// BROWSER DIMENSIONS

function getBrowserDimensions() {

    var changed = false;

    if (stage[0] != window.screenX) {

        delta[0] = (window.screenX - stage[0]) * 50;
        stage[0] = window.screenX;
        changed = true;

    }

    if (stage[1] != window.screenY) {

        delta[1] = (window.screenY - stage[1]) * 50;
        stage[1] = window.screenY;
        changed = true;

    }

    if (stage[2] != window.innerWidth) {

        stage[2] = window.innerWidth;
        changed = true;

    }

    if (stage[3] != window.innerHeight) {

        stage[3] = window.innerHeight;
        changed = true;

    }

    return changed;

}

上面mouseDrag方法就实现了鼠标拖拽甩动小球的功能，这也是该动画最重要的方法。全部代码可以下载源码来研究。源码下载>>