canvas实现刮奖功能-蒲公英云

canvas刮奖原理很简单，就是在刮奖区添加两个canvas，第一个canvas用于显示刮开后显示的内容，可以是一张图片或一个字符串，第二个canvas用于显示涂层，可以用一张图片或用纯色填充，第二个canvas覆盖在第一个canvas上面。

当在第二个canvas上点击或涂抹（点击然后拖动鼠标）时，把点击区域变为透明，这样就可以看到第一个canvas上的内容，即实现了刮奖效果。

效果图

在这里插入图片描述

源代码

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>_直接复制网</title>
<meta name="viewport" content="width=device-width, initial-scale=1 ,user-scalable=no">
<script src="http://libs.baidu.com/jquery/2.1.1/jquery.min.js"></script>
<style>
#lotteryContainer {
    position:relative;
    width: 300px;
    height:100px;
}
#drawPercent {
    color:#F60;
}
</style>
</head>
<body>
<button id="freshBtn">刷新</button><label>已刮开 <span id="drawPercent">0%</span> 区域。</label>
<div id="lotteryContainer"></div>
<script>
function Lottery(id, cover, coverType, width, height, drawPercentCallback) {
    this.conId = id;
    this.conNode = document.getElementById(this.conId);
    this.cover = cover;
    this.coverType = coverType;
    this.background = null;
    this.backCtx = null;
    this.mask = null;
    this.maskCtx = null;
    this.lottery = null;
    this.lotteryType = 'image';
    this.width = width || 300;
    this.height = height || 100;
    this.clientRect = null;
    this.drawPercentCallback = drawPercentCallback;
}
Lottery.prototype = {
    createElement: function (tagName, attributes) {
        var ele = document.createElement(tagName);
        for (var key in attributes) {
            ele.setAttribute(key, attributes[key]);
        }
        return ele;
    },
    getTransparentPercent: function(ctx, width, height) {
        var imgData = ctx.getImageData(0, 0, width, height),
            pixles = imgData.data,
            transPixs = [];
        for (var i = 0, j = pixles.length; i < j; i += 4) {
            var a = pixles[i + 3];
            if (a < 128) {
                transPixs.push(i);
            }
        }
        return (transPixs.length / (pixles.length / 4) * 100).toFixed(2);
    },
    resizeCanvas: function (canvas, width, height) {
        canvas.width = width;
        canvas.height = height;
        canvas.getContext('2d').clearRect(0, 0, width, height);
    },
    drawPoint: function (x, y) {
        this.maskCtx.beginPath();
        var radgrad = this.maskCtx.createRadialGradient(x, y, 0, x, y, 30);
        radgrad.addColorStop(0, 'rgba(0,0,0,0.6)');
        radgrad.addColorStop(1, 'rgba(255, 255, 255, 0)');
        this.maskCtx.fillStyle = radgrad;
        this.maskCtx.arc(x, y, 30, 0, Math.PI * 2, true);
        this.maskCtx.fill();
        if (this.drawPercentCallback) {
            this.drawPercentCallback.call(null, this.getTransparentPercent(this.maskCtx, this.width, this.height));
        }
    },
    bindEvent: function () {
        var _this = this;
        var device = (/android|webos|iphone|ipad|ipod|blackberry|iemobile|opera mini/i.test(navigator.userAgent.toLowerCase()));
        var clickEvtName = device ? 'touchstart' : 'mousedown';
        var moveEvtName = device? 'touchmove': 'mousemove';
        if (!device) {
            var isMouseDown = false;
            document.addEventListener('mouseup', function(e) {
                isMouseDown = false;
            }, false);
        } else {
            document.addEventListener("touchmove", function(e) {
                if (isMouseDown) {
                    e.preventDefault();
                }
            }, false);
            document.addEventListener('touchend', function(e) {
                isMouseDown = false;
            }, false);
        }
        this.mask.addEventListener(clickEvtName, function (e) {
            isMouseDown = true;
            var docEle = document.documentElement;
            if (!_this.clientRect) {
                _this.clientRect = {
                    left: 0,
                    top:0
                };
            }
            var x = (device ? e.touches[0].clientX : e.clientX) - _this.clientRect.left + docEle.scrollLeft - docEle.clientLeft;
            var y = (device ? e.touches[0].clientY : e.clientY) - _this.clientRect.top + docEle.scrollTop - docEle.clientTop;
            _this.drawPoint(x, y);
        }, false);
        this.mask.addEventListener(moveEvtName, function (e) {
            if (!device && !isMouseDown) {
                return false;
            }
            var docEle = document.documentElement;
            if (!_this.clientRect) {
                _this.clientRect = {
                    left: 0,
                    top:0
                };
            }
            var x = (device ? e.touches[0].clientX : e.clientX) - _this.clientRect.left + docEle.scrollLeft - docEle.clientLeft;
            var y = (device ? e.touches[0].clientY : e.clientY) - _this.clientRect.top + docEle.scrollTop - docEle.clientTop;
            _this.drawPoint(x, y);
        }, false);
    },
    drawLottery: function () {
        this.background = this.background || this.createElement('canvas', {
            style: 'position:absolute;left:0;top:0;'
        });
        this.mask = this.mask || this.createElement('canvas', {
            style: 'position:absolute;left:0;top:0;'
        });
        if (!this.conNode.innerHTML.replace(/[\w\W]| /g, '')) {
            this.conNode.appendChild(this.background);
            this.conNode.appendChild(this.mask);
            this.clientRect = this.conNode ? this.conNode.getBoundingClientRect() : null;
            this.bindEvent();
        }
        this.backCtx = this.backCtx || this.background.getContext('2d');
        this.maskCtx = this.maskCtx || this.mask.getContext('2d');
        if (this.lotteryType == 'image') {
            var image = new Image(),
                _this = this;
            image.onload = function () {
                _this.width = this.width;
                _this.height = this.height;
                _this.resizeCanvas(_this.background, this.width, this.height);
                _this.backCtx.drawImage(this, 0, 0);
                _this.drawMask();
            }
            image.src = this.lottery;
        } else if (this.lotteryType == 'text') {
            this.width = this.width;
            this.height = this.height;
            this.resizeCanvas(this.background, this.width, this.height);
            this.backCtx.save();
            this.backCtx.fillStyle = '#FFF';
            this.backCtx.fillRect(0, 0, this.width, this.height);
            this.backCtx.restore();
            this.backCtx.save();
            var fontSize = 30;
            this.backCtx.font = 'Bold ' + fontSize + 'px Arial';
            this.backCtx.textAlign = 'center';
            this.backCtx.fillStyle = '#F60';
            this.backCtx.fillText(this.lottery, this.width / 2, this.height / 2 + fontSize / 2);
            this.backCtx.restore();
            this.drawMask();
        }
    },
    drawMask: function() {
        this.resizeCanvas(this.mask, this.width, this.height);
        if (this.coverType == 'color') {
            this.maskCtx.fillStyle = this.cover;
            this.maskCtx.fillRect(0, 0, this.width, this.height);
            this.maskCtx.globalCompositeOperation = 'destination-out';
        } else if (this.coverType == 'image'){
            var image = new Image(),
                _this = this;
            image.onload = function () {
                _this.maskCtx.drawImage(this, 0, 0);
                _this.maskCtx.globalCompositeOperation = 'destination-out';
            }
            image.src = this.cover;
        }
    },
    init: function (lottery, lotteryType) {
        this.lottery = lottery;
        this.lotteryType = lotteryType || 'image';
        this.drawLottery();
    }
}
window.onload = function () {
    var lottery = new Lottery('lotteryContainer', '#CCC', 'color', 300, 100, drawPercent);
    lottery.init('http://www.baidu.com/img/bdlogo.gif', 'image');
    document.getElementById('freshBtn').onclick = function() {
        drawPercentNode.innerHTML = '0%';
        lottery.init(getRandomStr(10), 'text');
    }
    var drawPercentNode = document.getElementById('drawPercent');
    function drawPercent(percent) {
        drawPercentNode.innerHTML = percent + '%';
    }
}
function getRandomStr(len) {
    var text = "";
    var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    for( var i=0; i < len; i++ )
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    return text;
}
</script>
</body>
</html>