canvas学习第一章

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canvas学习第一章

基本知识

  • ctx.fillStyle = ‘#000000’ 设置文本颜色
  • ctx.font = ‘20px _sans’ 设置字体大小与字号
  • ctx.textBaseline = ‘top’ 设置字体垂直对齐方式
  • ctx.fillText(‘Hello World’,195,80) 将测试文本输出到屏幕上
  • ctx.strokeStyle = ‘#000000’ 用于方块边框的颜色
  • ctx.strokeRect(5,5,490,290) 绘制矩形边框
  • ctx.fillRect(0,0,500,300) 填充矩形
  • canvas.toDataURL() 这个方法返回的数据是代表当前canvas对象产生的位图字符串，就像屏幕的一个快照，通过提供一个不同MIME类型作为参数，可以返回不同的数据格式。基本的格式是image/png，但也可以获取image/jpeg和其它格式。
  • 绘制图片 
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    var image = new Image();
    image.src = 'hello.gif';
    image.onload = function(){ //回调函数在image加载成功时触发
        ctx.drawImage(image,160,130); //图像对象，与坐标
    }

我们利用以上知识来制作一个猜字母小游戏

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<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>CH1Ex4: Guesss The Letter Game </title>
<script src="modernizr-1.6.min.js"></script>
<script type="text/javascript">
//当页面加载完成时触发eventWindowLoaded方法
window.addEventListener('load', eventWindowLoaded, false);
var Debugger = function () { };
Debugger.log = function (message) {
    try {
        console.log(message);
    } catch (exception) {
        return;
    }
}
function eventWindowLoaded() {
    canvasApp();
}
function canvasSupport () {
    return Modernizr.canvas;
}
function eventWindowLoaded() {
    canvasApp();
}
function canvasApp() {
    var guesses = 0; //猜的次数
    var message = "Guess The Letter From a (lower) to z (higher)";
    var letters = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
    var today = new Date();
    var letterToGuess = ""; //要猜的字母
    var higherOrLower = ""; //键盘按下的字母是大还是小
    var lettersGuessed; //键盘按下的字母
    var gameOver = false;
    
    if (!canvasSupport()) {
             return;
    }
    var theCanvas = document.getElementById("canvasOne");
    var context = theCanvas.getContext("2d");  //获得canvas的上下文
    initGame(); //初始化游戏
    function initGame() {
        var letterIndex = Math.floor(Math.random() * letters.length);
        letterToGuess = letters[letterIndex]; //随机生成一个字母
        guesses = 0;
        lettersGuessed = []; //存储已经猜的字母
        gameOver = false; //游戏成功与否标注位
        window.addEventListener("keyup",eventKeyPressed,true); //监听键盘事件
        var formElement = document.getElementById("createImageData"); //生成快照
        formElement.addEventListener('click', createImageDataPressed, false);       
        drawScreen();
    }
    
    function eventKeyPressed(e) {
        if (!gameOver) {
            var letterPressed = String.fromCharCode(e.keyCode); //获得键入的字母
            letterPressed = letterPressed.toLowerCase(); //转换成小写
            guesses++; //猜测次数加一
            lettersGuessed.push(letterPressed); //存储到数组中
            if (letterPressed == letterToGuess) { //如果猜对了
                gameOver = true; //标志位置true
            } else {
                letterIndex = letters.indexOf(letterToGuess); //被猜的字母在数组中的位置
                guessIndex = letters.indexOf(letterPressed); //键入的字母在数组中的位置
                Debugger.log(guessIndex);//控制台调试
                if (guessIndex < 0) { //如果输入的不是字母，返回-1
                    higherOrLower = "That is not a letter";
                } else if (guessIndex > letterIndex) { //输入偏大
                    higherOrLower = "Lower";
                } else {
                    higherOrLower = "Higher"; //输入偏小
                }                    
            }
            drawScreen();
          }
    }
    function drawScreen() {
        //绘制背景
        context.fillStyle = "#ffffaa";
        context.fillRect(0, 0, 500, 300);
        //绘制边框
        context.strokeStyle = "#000000"; 
        context.strokeRect(5,  5, 490, 290);
        context.textBaseline = "top";
        //绘制日期
        context.fillStyle    = "#000000";
        context.font         = "10px _san";
        context.fillText  (today, 150 ,10);
        //绘制消息
        context.fillStyle    = "#FF0000";
        context.font         = "14px _sans";
        context.fillText  (message, 125, 30);
        //绘制猜测次数
        context.fillStyle    = "#109910";
        context.font         = "16px _sans";
        context.fillText  ('Guesses: ' + guesses, 215, 50);
        //输入是大还是小
        context.fillStyle    = "#000000";
        context.font         = "16px _sans";
        context.fillText  ("Higher Or Lower: " + higherOrLower, 150,125);
        //Letters Guessed
        context.fillStyle    = "#FF0000";
        context.font         = "16px _sans";
        context.fillText  ("Letters Guessed: " + lettersGuessed.toString(), 10, 260);
        if (gameOver) {
            context.fillStyle    = "#FF0000";
            context.font         = "40px _sans";
            context.fillText  ( "You Got it!", 150, 180);
        }   
    }
    function createImageDataPressed(e) {
        window.open(theCanvas.toDataURL(),"canavsImage","left=0,top=0,width=" + theCanvas.width + ",height=" + theCanvas.height +",toolbar=0,resizable=0");   
    }       
}
</script> 
</head>
<body>
<div style="position: absolute; top: 50px; left: 50px;">
<canvas id="canvasOne" width="500" height="300">
 Your browser does not support HTML 5 Canvas. 
</canvas>
<form>
<input type="button" id="createImageData" value="Export Canvas Image">
</form>
</div>
</body>
</html>