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。
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. Click on the 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;
}