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;
}
}
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);
})
}
}
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：
