勻速運動：指的是物體在一條直線上運動，並且物體在任何相等時間間隔內通過的位移都是相等的。其實就是勻速直線運動，它的特點是加速度為0，從定義可知，在任何相等的時間間隔內，速度大小和方向是相同的。
1 <head> 2 <meta charset='utf-8' /> 3 <style> 4 #canvas { 5 border: 1px dashed #aaa; 6 } 7 </style> 8 <script> 9 window.onload = function () { 10 var oCanvas = document.querySelector("#canvas"), 11 oGc = oCanvas.getContext('2d'), 12 width = oCanvas.width, height = oCanvas.height, 13 x = 0; 14 function drawBall( x, y, cxt ){ 15 cxt.fillStyle = '#09f'; 16 cxt.beginPath(); 17 cxt.arc( x, y, 20, 0, 2 * Math.PI ); 18 cxt.closePath(); 19 cxt.fill(); 20 } 21 ( function linear(){ 22 oGc.clearRect( 0, 0, width, height ); 23 drawBall( x, height / 2, oGc ); 24 x += 2; 25 console.log( x ); 26 requestAnimationFrame( linear ); 27 } )(); 28 } 29 </script> 30 </head> 31 <body> 32 <canvas id="canvas" width="1200" height="600"></canvas> 33 </body>
上述實例讓一個半徑20px的小球 從x=0, y=canvas高度的一半，以每幀2px的速度向右勻速運動.
我們可以把小球封裝成一個對象:
ball.js文件：
1 function Ball( x, y, r, color ){ 2 this.x = x || 0; 3 this.y = y || 0; 4 this.radius = r || 20; 5 this.color = color || '#09f'; 6 } 7 Ball.prototype = { 8 constructor : Ball, 9 stroke : function( cxt ){ 10 cxt.strokeStyle = this.color; 11 cxt.beginPath(); 12 cxt.arc( this.x, this.y, this.radius, 0, 2 * Math.PI ); 13 cxt.closePath(); 14 cxt.stroke(); 15 }, 16 fill : function( cxt ){ 17 cxt.fillStyle = this.color; 18 cxt.beginPath(); 19 cxt.arc( this.x, this.y, this.radius, 0, 2 * Math.PI ); 20 cxt.closePath(); 21 cxt.fill(); 22 } 23 }
該小球對象，可以定制位置半徑和顏色，支持兩種渲染方式（描邊和填充）
1 <head> 2 <meta charset='utf-8' /> 3 <style> 4 #canvas { 5 border: 1px dashed #aaa; 6 } 7 </style> 8 <script src="./ball.js"></script> 9 <script> 10 window.onload = function () { 11 var oCanvas = document.querySelector("#canvas"), 12 oGc = oCanvas.getContext('2d'), 13 width = oCanvas.width, height = oCanvas.height, 14 ball = new Ball( 0, height / 2 ); 15 (function linear() { 16 oGc.clearRect(0, 0, width, height); 17 ball.fill( oGc ); 18 ball.x += 2; 19 requestAnimationFrame(linear); 20 })(); 21 } 22 </script> 23 </head> 24 25 <body> 26 <canvas id="canvas" width="1200" height="600"></canvas> 27 </body>
斜線勻速運動：
1 <head> 2 <meta charset='utf-8' /> 3 <style> 4 #canvas { 5 border: 1px dashed #aaa; 6 } 7 </style> 8 <script src="./ball.js"></script> 9 <script> 10 window.onload = function () { 11 var oCanvas = document.querySelector("#canvas"), 12 oGc = oCanvas.getContext('2d'), 13 width = oCanvas.width, height = oCanvas.height, 14 ball = new Ball( 0, height ); 15 (function linear() { 16 oGc.clearRect(0, 0, width, height); 17 ball.fill( oGc ); 18 ball.x += 2; 19 ball.y -= 1; 20 requestAnimationFrame(linear); 21 })(); 22 } 23 </script> 24 </head> 25 26 <body> 27 <canvas id="canvas" width="1200" height="600"></canvas> 28 </body>
任意方向的勻速運動（速度分解）
1 <head> 2 <meta charset='utf-8' /> 3 <style> 4 #canvas { 5 border: 1px dashed #aaa; 6 } 7 </style> 8 <script src="./ball.js"></script> 9 <script> 10 window.onload = function () { 11 var oCanvas = document.querySelector("#canvas"), 12 oGc = oCanvas.getContext('2d'), 13 width = oCanvas.width, height = oCanvas.height, 14 ball = new Ball( 0, 0 ), 15 speed = 5, 16 vx = speed * Math.cos( 10 * Math.PI / 180 ), 17 vy = speed * Math.sin( 10 * Math.PI / 180 ); 18 19 (function linear() { 20 oGc.clearRect(0, 0, width, height); 21 ball.fill( oGc ); 22 ball.x += vx; 23 ball.y += vy; 24 requestAnimationFrame(linear); 25 })(); 26 } 27 </script> 28 </head> 29 <body> 30 <canvas id="canvas" width="1200" height="600"></canvas> 31 </body>