JS+HTML5 Canvas实现简单的写字板功能示例

本文实例讲述了JS+HTML5 Canvas实现简单的写字板功能。分享给大家供大家参考，具体如下：

先来看运行效果：

具体代码如下：

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>www.jb51.net JS写字板</title>
    <script src="http://libs.baidu.com/jquery/2.0.0/jquery.min.js"></script>
    <style type="text/css">
        body,html {
            padding: 0;
            margin: 0;
        }
        a,div,span {
            font-family: "Arial","Microsoft YaHei","黑体","宋体",sans-serif;
        }
        .canvas-box {
            display: block;
            margin: 40px auto;
            background: #f5f5f5;
        }
        .color-box {
            width: 1080px;
            display: block;
            margin: 20px auto;
            text-align: center;
        }
        .color-item {
            display: inline-block;
            vertical-align: middle;
            width: 48px;
            height: 24px;
            margin: 10px;
            background: #989898;
            cursor: pointer;
        }
        .red {
            background: #e01d5b;
        }
        .blue {
            background: #1d6ae0;
        }
        .green {
            background: #1de087;
        }
        .yellow {
            background: #f3e41d;
        }
        .orange {
            background: #f9850b;
        }
        .black {
            background: #333;
        }
        .color-item.active {
            border: solid 3px #565656;
        }
        .btn {
            display: block;
            width: 120px;
            height: 40px;
            line-height: 40px;
            margin: 10px auto;
            text-align: center;
            font-size: 18px;
            border: solid 1px #999;
            border-radius: 5px;
            cursor: pointer;
        }
    </style>
</head>
<body>
    <canvas class="canvas-box" id="canvas"></canvas>
    <div class="color-box">
        <span class="color-item red"></span>
        <span class="color-item blue"></span>
        <span class="color-item green"></span>
        <span class="color-item yellow"></span>
        <span class="color-item orange"></span>
        <span class="color-item black active"></span>
    </div>
    <div class="btn" onclick="clearDraw()">清除</div>
<script type="text/javascript">
var    canvas = document.getElementById('canvas');
var    context = canvas.getContext("2d");
var    isDraw = false; //定义变量控制画笔是否可用
var movePos;         //定义变量存放初始画笔开始位置
var linWidth = 10;
var linColor = '#333';
window.onload = function(){
    draw();
}
function draw(){
    canvas.width = 500;
    canvas.height = 500;
    context.strokeStyle = "red";
    context.lineWidth = 10;
    context.beginPath();
    context.strokeRect(0,0,500,500);
    //设置画笔颜色，宽度
    context.strokeStyle = "red";
    context.lineWidth = 1;
    //使线段连续，圆滑
    context.lineCap = "round";
    context.lineJoin = "round";
    drawDashedLine(context,0,0,500,500);
    drawDashedLine(context,0,500,500,0);
    drawLine(context,0,250,500,250);
    drawLine(context,250,0,250,500);
}
//画虚线（参照网上大神）
function drawDashedLine(context, x1, y1, x2, y2, dashLength) {
  dashLength = dashLength === undefined ? 5 : dashLength;
  var deltaX = x2 - x1;
  var deltaY = y2 - y1;
  var numDashes = Math.floor(
   Math.sqrt(deltaX * deltaX + deltaY * deltaY) / dashLength);
   for (var i=0; i < numDashes; ++i) {
   context[ i % 2 === 0 ? 'moveTo' : 'lineTo' ]    (x1 + (deltaX / numDashes) * i, y1 + (deltaY / numDashes) * i);
   }
  context.stroke();
};
//画直线
function drawLine(context,x1,y1,x2,y2){
    context.moveTo(x1,y1);
    context.lineTo(x2,y2);
    context.stroke();
};
//获取鼠标相对与canvas位置
function getPos(x,y){
    var box = canvas.getBoundingClientRect();
    return {x: x-box.left,y: y-box.top};
};
//画笔
function drawing(e){
    if(isDraw){
        var position = getPos(e.clientX,e.clientY);
        context.strokeStyle = linColor;
        context.lineWidth = linWidth;
        context.save();
        context.beginPath();
        context.moveTo(movePos.x,movePos.y);
        context.lineTo(position.x,position.y);
        context.stroke();
        movePos = position;
        context.restore();
    }
}
//鼠标点下
canvas.onmousedown = function(e){
    isDraw = true;
    movePos = getPos(e.clientX,e.clientY);
    drawing(e);
}
//鼠标移动
canvas.onmousemove = function(e){
    drawing(e);
}
//鼠标松开
canvas.onmouseup = function(e){
    isDraw = false;
}
//鼠标离开
canvas.onmouseout =function(e){
    isDraw = false;
}
//选择画笔颜色
$('.color-item').on('click',function(){
    $(this).addClass('active').siblings().removeClass('active');
    linColor = $(this).css('background-color');
});
//清除
function clearDraw(){
    context.clearRect(0,0,500,500);
    draw();
}
</script>
</body>
</html>

感兴趣的朋友还可以使用在线HTML/CSS/JavaScript代码运行工具：http://tools.jb51.net/code/HtmlJsRun测试上述代码，亲身体验一下运行效果。

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希望本文所述对大家JavaScript程序设计有所帮助。