canvas之自定义头像功能实现

写在最前：

前两天老大跟我说老虎官网上那个自定义头像的功能是flash实现的，没有安装过的还得手动去“允许”falsh的运行。所以让我用canvas实现一个一样的功能，嘿嘿，刚好最近也在研究canvas，所以欣然答应（其实，你没研究过难道就不答应么，哈哈哈哈哈~）

对了，大佬们记得给我的博客点赞，要星星哦~

成果展示：

Git地址:

github.com/ry928330/po…

功能说明：

拖拽左侧小方框，或者是鼠标放在小方框右下角，点击拉伸方框，方框覆盖部分的图片被自动截取下来，然后再在右侧的多个容器里面重绘。
输入宽高，自定义你需要订制的头像大小，目前只支持宽高相同的头像图片。

实现细节：

因为你要对图片所在的区域进行截图，所以你得制作一张canvas，盖在图片所在的区域。这里，我们给出了一个函数，根据传入的DOM里面元素的类名创建相同位置的canvas，盖在原来的DOM元素上面：

function createCanvasByClassName(tag) {var canvasInitialWidth = $('.' + tag).width();var canvasInitialHeight = $('.' + tag).height();var left = $('.' + tag).offset().left - $('.' + tag).parent('.portraitContainer').offset().left + 1;var top = $('.' + tag).offset().top - $('.' + tag).parent('.portraitContainer').offset().top + 1;//var left = $('.' + tag).offset().left + 1;//var top = $('.' + tag).offset().top + 1;clearCanvasObj.left = $('.' + tag).offset().left + 1;clearCanvasObj.top = $('.' + tag).offset().top + 1;// clearCanvasObj.left = left;// clearCanvasObj.top = top;var canvasElement = $('<canvas></canvas>');var randomNum = Math.floor(getRandom(0, 10000));clearCanvasObj.canvasId = randomNum;canvasElement.attr({id: 'canvas',width: canvasInitialWidth,height: canvasInitialHeight});canvasElement.css({position: 'absolute',top: top, left: left});//$('body').append(canvasElement);var appendEle = $('.portraitContainer').append(canvasElement);var canvas = document.getElementById('canvas');var ctx = canvas.getContext('2d');//ctx.fillStyle = "rgba(211,211,216,0.5)";ctx.clearRect(0, 0, canvasInitialWidth, canvasInitialHeight);ctx.fillStyle = "rgba(0,0,0, 0.4)";ctx.fillRect(0, 0, canvasInitialWidth, canvasInitialHeight);return canvas;
}复制代码

有了这张canvas你就可以在你图片所在区域肆意的操作了。首先，降整个区域画上一个浅黑色的阴影，然后再擦除初始小方框区域里面的颜色。然后给整个页面添加mousedown，mousemove，mouseup事件，他们所做的功能就跟你在页面中实现一个拖拽的功能类似，这里重点说下mousemove里面做的操作，代码如下：

function mousemoveFunc(event) {/* Act on the event */var nowMouseX = event.clientX - clearCanvasObj.left;var nowMouseY = event.clientY - clearCanvasObj.top;if (nowMouseX >= clearCanvasObj.xStart && nowMouseX <= clearCanvasObj.xStart + clearCanvasObj.width && nowMouseY >= clearCanvasObj.yStart && nowMouseY <= clearCanvasObj.yStart + clearCanvasObj.height) {clearCanvasObj.isCanvasArea = true;//clearCanvasObj.isRightCorner = false;imgContainerCanvas.style.cursor = 'move';} else if ((nowMouseX >= clearCanvasObj.xStart + clearCanvasObj.width - 10) && (nowMouseX <= clearCanvasObj.xStart+ clearCanvasObj.width + 10) && (nowMouseY >= clearCanvasObj.yStart + clearCanvasObj.height - 10) && (nowMouseY <= clearCanvasObj.yStart + clearCanvasObj.height + 10)) {clearCanvasObj.isCanvasArea = true;//clearCanvasObj.beginDraw = false;imgContainerCanvas.style.cursor = 'se-resize';} else {clearCanvasObj.isCanvasArea = false;//clearCanvasObj.isRightCorner = false;imgContainerCanvas.style.cursor = 'default';}var outerDomWidth = $(".imgContainer").width();var outerDomHeight = $(".imgContainer").height();var xDistance = event.clientX - clearCanvasObj.mouseX;var yDistance = event.clientY - clearCanvasObj.mouseY;//var outerCTX = canvas.getContext('2d');//移动小方框if (clearCanvasObj.beginDraw && clearCanvasObj.isCanvasArea && !clearCanvasObj.isRightCorner) {ry_CTX.fillStyle = clearCanvasObj.color;// console.log('1', clearCanvasObj.xStart, clearCanvasObj.yStart)ry_CTX.fillRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);//outerCTX.fillRect(0, 0, canvas.width, canvas.height);clearCanvasObj.xStart += xDistance;clearCanvasObj.yStart += yDistance;//判断方框是否达到边界if (clearCanvasObj.xStart <= 0) {clearCanvasObj.xStart = 0;}if (clearCanvasObj.yStart <= 0) {clearCanvasObj.yStart = 0;}if ((clearCanvasObj.xStart + clearCanvasObj.width) >= outerDomWidth) {clearCanvasObj.xStart = outerDomWidth - clearCanvasObj.width;}if ((clearCanvasObj.yStart + clearCanvasObj.height) >= outerDomHeight) {clearCanvasObj.yStart = outerDomHeight - clearCanvasObj.height;}// console.log('2', clearCanvasObj.xStart, clearCanvasObj.yStart)ry_CTX.clearRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);produceSmallPic(clearCanvasObj.xStart+clearCanvasObj.left, clearCanvasObj.yStart+clearCanvasObj.top, clearCanvasObj.width, clearCanvasObj.height, imageURL)clearCanvasObj.mouseX = event.clientX;clearCanvasObj.mouseY = event.clientY;}//拖拽小方框if (clearCanvasObj.isRightCorner) {ry_CTX.fillStyle = clearCanvasObj.color;ry_CTX.fillRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);var realDistance = Math.min(xDistance, yDistance)clearCanvasObj.width +=  realDistance;clearCanvasObj.height += realDistance;//拖动时边界条件的判断if (clearCanvasObj.xStart + clearCanvasObj.width >= outerDomWidth) {clearCanvasObj.width = outerDomWidth - clearCanvasObj.xStart;clearCanvasObj.height = outerDomWidth - clearCanvasObj.xStart;}if (clearCanvasObj.yStart + clearCanvasObj.height >= outerDomHeight) {clearCanvasObj.width = outerDomHeight - clearCanvasObj.yStart;clearCanvasObj.height = outerDomHeight - clearCanvasObj.yStart;}if (clearCanvasObj.width <= 10) {clearCanvasObj.width = 10;}if (clearCanvasObj.height <= 10) {clearCanvasObj.height = 10;}ry_CTX.clearRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);produceSmallPic(clearCanvasObj.xStart+clearCanvasObj.left, clearCanvasObj.yStart+clearCanvasObj.top, clearCanvasObj.width, clearCanvasObj.height, imageURL);clearCanvasObj.mouseX = event.clientX;clearCanvasObj.mouseY = event.clientY;}
}复制代码

函数里面，你需要注意拖拽的边界条件，一个是方框不能拖到图片所在DOM外的边界；另外一个就是当你鼠标放在小方框所在的区域改变鼠标的样式。方框在拖动的过程中，我们不断重绘方框移动的区域（也就是不断的画上阴影），然后在新的位置调用clearRect函数，重新擦出一个小方框出来。在拖拽或是拉伸的过程中，我们会不断调用produceSmallPic函数，在右边的容器（每个容器都是一个canvas）里面不断根据容器大小重绘出所需的头像。代码如下：

function produceSmallPic(imageURL,left, top, width, height) {var img = new Image();img.src = imageURL;var targetCtx = new Array();var targetCanvas = null;img.onload = function() {portraitGroupsArr.forEach(function(item, index) {targetCanvas = document.getElementById(item.class);targetCtx.push(targetCanvas.getContext('2d'));targetCtx[index].clearRect(0,0, item.width, item.height);targetCtx[index].drawImage(img, left - clearCanvasObj.left, top - clearCanvasObj.top, width, height, 0, 0 , item.width, item.height);})}
}复制代码

我们说下这个函数的作用，这里我们要注意一个参数imageURL，这个URL是由图片所在的DOM转化来的。因为你要把DOM所在的区域变成一张图片，这样你才能在利用drawImage函数截取你所需要的区域。所以我们先利用html2canvas库函数讲图片所在的DOM转化为canvas，这张canvas的内容是包含你所要截取的图片的，然后把这张canvas转化为图片取得图片地址imageURL，代码如下：

html2canvas(document.getElementById('imgContainer'), {onrendered: function(canvas) {var imageURL = canvasTransToImage(canavs);...}})
function canvasTransToImage(canvas) {var imageURL = canvas.toDataURL('image/png');return imageURL;
}复制代码

接着，你就可以便利右侧的canvas容器，讲图片重回到里面了，整个过程就这样结束，回头看来是不是很简单。

写在最后：

canvas的操作，要多多注意那些边界条件，什么时候该重绘什么时候该清除，这些是比较重要的。逻辑清晰了，canvas本身的API也就那么几个，操作起来也就没那么麻烦了，最后，谢谢大家查阅，写的不是很清楚，有不懂的可以一起讨论~