//動畫循環(huán)
    if (!window.requestAnimationFrame) {
      window.requestAnimationFrame = (window.webkitRequestAnimationFrame ||
                                      window.mozRequestAnimationFrame ||
                                      window.msRequestAnimationFrame ||
                                      window.oRequestAnimationFrame ||
                                      function (callback) {
                                        return window.setTimeout(callback, 17 );
                                      });
    }

 if (!window.cancelAnimationFrame) {
      window.cancelAnimationFrame = (window.cancelRequestAnimationFrame ||
                                     window.webkitCancelAnimationFrame || window.webkitCancelRequestAnimationFrame ||
                                     window.mozCancelAnimationFrame || window.mozCancelRequestAnimationFrame ||
                                     window.msCancelAnimationFrame || window.msCancelRequestAnimationFrame ||
                                     window.oCancelAnimationFrame || window.oCancelRequestAnimationFrame ||
                                     window.clearTimeout);
    }

var cc = document.getElementById("canvas");   
var cxt = cc.getContext("2d");   
var ca = document.createElement("canvas");   
ca.width=500;   
ca.height=500;   
var cxt2 = ca.getContext("2d");   
var x = y = 0;  
//加載多個圖片
function loadImages(sources, callback) {  
    var count = 0,    
    images = {},   
    imgNum = 0;   
    for (k in sources) {   
        imgNum++;   
    }    
    for (k in sources) {   
        images[k] = new Image();   
        images[k].onload = function() {   
            if (++count >= imgNum) {   
                callback(images);   
            }   
        }   
        images[k].src = sources[k];   
    }   
}   
var sources = []; 

function drawGameImage() {    
    sources = ["gameimage/0.png", "gameimage/1.png", "gameimage/2.png", "gameimage/3.png", "gameimage/4.png", "gameimage/5.png", "gameimage/6.png", "gameimage/7.png", "gameimage/8.png", "gameimage/2.png", "gameimage/3.png", "gameimage/4.png", "gameimage/5.png", "gameimage/6.png", "gameimage/9.png", "gameimage/10.png"];
    //調(diào)用圖片預(yù)加載函數(shù)，實現(xiàn)回調(diào)函數(shù)     
    loadImages(sources, function(images) {   
        var x = y = 0;   
        for (var i = 0; i < sources.length; i++) {   
            cxt2.drawImage(images[i], x * 100, y * 100, 100, 100);   
            if (i < 4) {
                 x++; 
             } else if (i >= 4 && i < 8) {   
                  y++;   
             } else if (i >= 8 && i < 12) {   
                  x--;   
             } else {   
                   y--;
             } 
        }    
     });   
}    
drawGameImage();   
var times = Math.floor(Math.random() * 10) + sources.length * 2;   
var v = 0;    
function move() {   
     var moveX = 400;
     var speed = 100; 
     if (x < moveX && y == 0) {   
        x += speed;
     } else if (x == moveX && y < moveX) { 
        y += speed;   
     } else if (y == moveX && x > 0) {   
         x -= speed;   
     } else if (x == 0 && y > 0) {   
         y -= speed;   
     }   
     cxt.clearRect(0, 0, 500, 500);   
     cxt.drawImage(ca,0,0,500,500);   
     cxt.fillStyle = "rgba(0,0,0,.5)";   
     cxt.fillRect(x, y, 100, 100);   
     if (v > times) {   
         cancelAnimationFrame();   
     }   
     v++;   
      requestAnimationFrame(move);   
}   
move();

function loadImages(sources, callback) {  
    var count = 0,    
    images = {},   
    imgNum = 0;   
    for (k in sources) {   
        imgNum++;   
    }    
    for (k in sources) {   
        images[k] = new Image();   
        images[k].onload = function() {   
            if (++count >= imgNum) {   
                callback(images);   
            }   
        }   
        images[k].src = sources[k];   
    }   
}   

1. 在loadImages方法中，我們應(yīng)該先綁定image.onload事件，后加載圖片
   

1. for...in循環(huán) 與 for循環(huán)的區(qū)別
   

var ca = document.createElement("canvas");   
ca.width=500;   
ca.height=500;   
var cxt2 = ca.getContext("2d");   

 drawGameImage();