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。
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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 = 'Hello!
This is how it works:
1. Drag a ball.
2. Click on the background.
3. Shake your browser.
4. Double click.
5. Play!'; 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; }
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