近年來由於行動裝置對HTML5的較好支持，經常有活動用刮獎的效果，最近也在看H5方面的內容，就自己實現了一個，現分享出來跟大家交流。 #原理很簡單，就是在刮取區增加兩個
canvas，第二個canvas用來顯示塗層，可以用一張圖片或用純色填充，第二個canvas覆蓋在第一個canvas上面。 canvas上點擊或塗抹（點擊然後拖曳滑鼠）時，把點擊區域變成透明，這樣就可以看到第一個canvas上的內容，即實現了刮鬍效果。 （1）定義Lottery類別對參數解釋一下：function Lottery(id, cover, coverType, width, height, drawPercentCallback) {
this.conId = id;
this.conNode = document.getElementById(this.conId);
this.cover = cover || '#CCC';
this.coverType = coverType || 'color';
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;
}登入後複製
id：刮獎容器的id
getBoundingClientRect() 值
（2）新增二個canvas到刮獎容器，並取得2d上下文
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');事件，後面介紹。
，如果是圖片直接用canvas的drawImage就可以了，如果是string，要先用白色填充，然後在上下左右居中的地方繪製字符串，代碼如下：
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);
}
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();
}#即我們要把maskCtx的 globalCompositeOperation 設定為 destination-out ，詳細的用法請參考上面給出的連結。
因此，繪製第二個canvas的程式碼如下：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;
}keydown 和 mousemove事件，另外PC-WEB方式下，要給document綁定一個mouseup事件，用來判斷滑鼠是否按下。程式碼如下：
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);
}
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);
}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));
}
}在很多時候，我們還需要知道用戶塗抹了多少然後進行下一步交互，如當用戶塗抹了80%後，才允許下一張顯示。
這個百分比要如何計算呢？其實很簡單，我們可以用getImageData方法到畫布上指定矩形的像素數據，由於每個像素都是用rgba表示的，而塗抹過的區域是透明的，所以我們只需要判斷alpha通道的值就可以知道是否透明。程式碼如下:
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);
}init: function (lottery, lotteryType) {
this.lottery = lottery;
this.lotteryType = lotteryType || 'image';
this.drawLottery();
}以上是HTML5 Canvas實戰之刮獎效果的實例詳情的詳細內容。更多資訊請關注PHP中文網其他相關文章！
