基于Vue如何实现移动端图片裁剪组件功能

小编给大家分享一下基于Vue如何实现移动端图片裁剪组件功能，希望大家阅读完这篇文章之后都有所收获，下面让我们一起去探讨吧！

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最近项目上要做一个车牌识别的功能。本来以为很简单，只需要将图片扔给后台就可以了，但是经测试后识别率只有20-40%。因此产品建议拍摄图片后，可以对图片进行拖拽和缩放，然后裁剪车牌部分上传给后台来提高识别率。刚开始的话还是百度了一下看看有没有现成的组件，但是找来找去都没有找到一个合适的，还好这个功能不是很着急，因此自己周末就在家里研究一下。

Demo地址：https://vivialex.github.io/demo/imageClipper/index.html

下载地址：https://github.com/vivialex/vue-imageClipper

因为移动端是用vue，所以就写成了一个vue组件，下面就说说自己的一些实现思路（本人技术有限，各位大神请体谅。另外展示的代码不一定是某个功能的完整代码），先看看效果：　

基于Vue如何实现移动端图片裁剪组件功能　

一、组件的初始化参数

1、图片img（url或者base64 data-url）

2、截图的宽clipperImgWidth

3、截图的高clipperImgHeight

props: {
  img: String, //url或dataUrl
  clipperImgWidth: {
    type: Number,
    default: 500
  },
  clipperImgHeight: {
    type: Number,
    default: 200
  }
}

　二、布局

在Z轴方向看主要是由4层组成。第1层是一个占满整个容器的canvas（称cCanvas）；第2层是一个有透明度的遮罩层；第3层是裁剪的区域（示例图中的白色方框），里面包含一个与裁剪区域大小相等的canvas（称pCanvas）；第4层是一个透明层gesture-mask，用作绑定touchstart，touchmove，touchend事件。其中两个canvas都会加载同一张图片，只是起始坐标不一样。为什么需要两个canvas？因为想做出当手指离开屏幕时，裁剪区域外的部分表面会有一个遮罩层的效果，这样能突出裁剪区域的内容。

_ratio(size) { return parseInt(window.devicePixelRatio * size); }, _initCanvas() { let $canvas = this.$refs.canvas, $pCanvas = this.$refs.pCanvas, clipperClientRect = this.$refs.clipper.getBoundingClientRect(), clipperWidth = parseInt(this.clipperImgWidth / window.devicePixelRatio), clipperHeight = parseInt(this.clipperImgHeight / window.devicePixelRatio); this.ctx = $canvas.getContext('2d'); this.pCtx = $pCanvas.getContext('2d'); //判断clipperWidth与clipperHeight有没有超过容器值 if (clipperWidth < 0 || clipperWidth > clipperClientRect.width) { clipperWidth = 250 } if (clipperHeight < 0 || clipperHeight > clipperClientRect.height) { clipperHeight = 100 } //因为canvas在手机上会被放大，因此里面的内容会模糊，这里根据手机的devicePixelRatio来放大canvas，然后再通过设置css来收缩，因此关于canvas的所有值或坐标都要乘以devicePixelRatio $canvas.style.width = clipperClientRect.width + 'px'; $canvas.style.height = clipperClientRect.height + 'px'; $canvas.width = this._ratio(clipperClientRect.width); $canvas.height = this._ratio(clipperClientRect.height); $pCanvas.style.width = clipperWidth + 'px'; $pCanvas.style.height = clipperHeight + 'px'; $pCanvas.width = this._ratio(clipperWidth); $pCanvas.height = this._ratio(clipperHeight); //计算两个canvas原点的x y差值 let cClientRect = $canvas.getBoundingClientRect(), pClientRect = $pCanvas.getBoundingClientRect(); this.originXDiff = pClientRect.left - cClientRect.left; this.originYDiff = pClientRect.top - cClientRect.top; this.cWidth = cClientRect.width; this.cHeight = cClientRect.height; }

_loadImg() { if (this.imgLoading || this.loadImgQueue.length === 0) { return; } let img = this.loadImgQueue.shift(); if (!img) { return; } let $img = new Image(), onLoad = e => { $img.removeEventListener('load', onLoad, false); this.$img = $img; this.imgLoaded = true; this.imgLoading = false; this._initImg($img.width, $img.height); this.$emit('loadSuccess', e); this.$emit('loadComplete', e); this._loadImg(); }, onError = e => { $img.removeEventListener('error', onError, false); this.$img = $img = null; this.imgLoading = false; this.$emit('loadError', e); this.$emit('loadComplete', e); this._loadImg(); }; this.$emit('beforeLoad'); this.imgLoading = true; this.imgLoaded = false; $img.src = this.img; $img.crossOrigin = 'Anonymous'; //因为canvas toDataUrl不能操作未经允许的跨域图片，这需要服务器设置Access-Control-Allow-Origin头 $img.addEventListener('load', onLoad, false); $img.addEventListener('error', onError, false); } _initImg(w, h) { let eW = null, eH = null, maxW = this.cWidth, maxH = this.cHeight - this.actionBarHeight; //如果图片的宽高都少于容器的宽高，则不做处理 if (w <= maxW && h <= maxH) { eW = w; eH = h; } else if (w > maxW && h <= maxH) { eW = maxW; eH = parseInt(h / w * maxW); } else if (w <= maxW && h > maxH) { eW = parseInt(w / h * maxH); eH = maxH; } else { //判断是横图还是竖图 if (h > w) { eW = parseInt(w / h * maxH); eH = maxH; } else { eW = maxW; eH = parseInt(h / w * maxW); } } if (eW <= maxW && eH <= maxH) { //记录其初始化的宽高，日后的缩放功能以此值为基础 this.imgStartWidth = eW; this.imgStartHeight = eH; this._drawImage((maxW - eW) / 2, (maxH - eH) / 2, eW, eH); } else { this._initImg(eW, eH); } }

_drawImage(x, y, w, h) { this._clearCanvas(); this.imgX = parseInt(x); this.imgY = parseInt(y); this.imgCurrentWidth = parseInt(w); this.imgCurrentHeight = parseInt(h); //更新canvas this.ctx.drawImage(this.$img, this._ratio(x), this._ratio(y), this._ratio(w), this._ratio(h)); //更新pCanvas，只需要减去两个canvas坐标原点对应的差值即可 this.pCtx.drawImage(this.$img, this._ratio(x - this.originXDiff), this._ratio(y - this.originYDiff), this._ratio(w), this._ratio(h)); }, _clearCanvas() { let $canvas = this.$refs.canvas, $pCanvas = this.$refs.pCanvas; $canvas.width = $canvas.width; $canvas.height = $canvas.height; $pCanvas.width = $pCanvas.width; $pCanvas.height = $pCanvas.height; }

_initEvent() { let $gesture = this.$refs.gesture, scx = 0, scy = 0; let iX = this.imgX, iY = this.imgY; $gesture.addEventListener('touchstart', e => { if (!this.imgLoaded) { return; } let finger = e.touches[0]; scx = finger.pageX; scy = finger.pageY; iX = this.imgX; iY = this.imgY; }, false); $gesture.addEventListener('touchmove', e => { e.preventDefault(); if (!this.imgLoaded) { return; } let f1x = e.touches[0].pageX, f1y = e.touches[0].pageY; this._drawImage(iX + f1x - scx, iY + f1y - scy, this.imgCurrentWidth, this.imgCurrentHeight); }, false); }

_initEvent() { let $gesture = this.$refs.gesture, cClientRect = this.$refs.canvas.getBoundingClientRect(), scx = 0, //对于单手操作是移动的起点坐标，对于缩放是图片距离两手指的中点最近的图标。 scy = 0, fingers = {}; //记录当前有多少只手指在触控屏幕 //one finger let iX = this.imgX, iY = this.imgY; //two finger let figuredistance = 0, pinchScale = this.imgScale; $gesture.addEventListener('touchstart', e => { if (!this.imgLoaded) { return; } if (e.touches.length === 1) { let finger = e.touches[0]; scx = finger.pageX; scy = finger.pageY; iX = this.imgX; iY = this.imgY; fingers[finger.identifier] = finger; } else if (e.touches.length === 2) { let finger1 = e.touches[0], finger2 = e.touches[1], f1x = finger1.pageX - cClientRect.left, f1y = finger1.pageY - cClientRect.top, f2x = finger2.pageX - cClientRect.left, f2y = finger2.pageY - cClientRect.top; scx = parseInt((f1x + f2x) / 2); scy = parseInt((f1y + f2y) / 2); figureDistance = this._pointDistance(f1x, f1y, f2x, f2y); fingers[finger1.identifier] = finger1; fingers[finger2.identifier] = finger2; //判断变换中点是否在图片中，如果不是则去离图片最近的点 if (scx < this.imgX) { scx = this.imgX; } if (scx > this.imgX + this.imgCurrentWidth) { scx = this.imgX + this.imgCurrentHeight; } if (scy < this.imgY) { scy = this.imgY; } if (scy > this.imgY + this.imgCurrentHeight) { scy = this.imgY + this.imgCurrentHeight; } } }, false); $gesture.addEventListener('touchmove', e => { e.preventDefault(); if (!this.imgLoaded) { return; } this.maskShowTimer && clearTimeout(this.maskShowTimer); this.maskShow = false; if (e.touches.length === 1) { let f1x = e.touches[0].pageX, f1y = e.touches[0].pageY; this._drawImage(iX + f1x - scx, iY + f1y - scy, this.imgCurrentWidth, this.imgCurrentHeight); } else if (e.touches.length === 2) { let finger1 = e.touches[0], finger2 = e.touches[1], f1x = finger1.pageX - cClientRect.left, f1y = finger1.pageY - cClientRect.top, f2x = finger2.pageX - cClientRect.left, f2y = finger2.pageY - cClientRect.top, newFigureDistance = this._pointDistance(f1x, f1y, f2x, f2y), scale = this.imgScale + parseFloat(((newFigureDistance - figureDistance) / this.imgScaleStep).toFixed(1)); fingers[finger1.identifier] = finger1; fingers[finger2.identifier] = finger2; if (scale !== pinchScale) { //目前缩放的最小比例是1，最大是5 if (scale < this.imgMinScale) { scale = this.imgMinScale; } else if (scale > this.imgMaxScale) { scale = this.imgMaxScale; } pinchScale = scale; this._scale(scx, scy, scale); } } }, false); $gesture.addEventListener('touchend', e => { if (!this.imgLoaded) { return; } this.imgScale = pinchScale; //从finger删除已经离开的手指 let touches = Array.prototype.slice.call(e.changedTouches, 0); touches.forEach(item => { delete fingers[item.identifier]; }); //迭代fingers，如果存在finger则更新scx,scy,iX,iY，因为可能缩放后立即单指拖动 let i, fingerArr = []; for(i in fingers) { if (fingers.hasOwnProperty(i)) { fingerArr.push(fingers[i]); } } if (fingerArr.length > 0) { scx = fingerArr[0].pageX; scy = fingerArr[0].pageY; iX = this.imgX; iY = this.imgY; } else { this.maskShowTimer = setTimeout(() => { this.maskShow = true; }, 300); } //做边界值检测 let x = this.imgX, y = this.imgY, pClientRect = this.$refs.pCanvas.getBoundingClientRect(); if (x > pClientRect.left + pClientRect.width) { x = pClientRect.left } else if (x + this.imgCurrentWidth < pClientRect.left) { x = pClientRect.left + pClientRect.width - this.imgCurrentWidth; } if (y > pClientRect.top + pClientRect.height) { y = pClientRect.top; } else if (y + this.imgCurrentHeight < pClientRect.top) { y = pClientRect.top + pClientRect.height - this.imgCurrentHeight; } if (this.imgX !== x || this.imgY !== y) { this._drawImage(x, y, this.imgCurrentWidth, this.imgCurrentHeight); } }); }, _scale(x, y, scale) { let newPicWidth = parseInt(this.imgStartWidth * scale), newPicHeight = parseInt(this.imgStartHeight * scale), newIX = parseInt(x - newPicWidth * (x - this.imgX) / this.imgCurrentWidth), newIY = parseInt(y - newPicHeight * (y - this.imgY) / this.imgCurrentHeight); this._drawImage(newIX, newIY, newPicWidth, newPicHeight); }, _pointDistance(x1, y1, x2, y2) { return parseInt(Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2))); }

_clipper() { let imgData = null; try { imgData = this.$refs.pCanvas.toDataURL(); } catch (e) { console.error('请在response header加上Access-Control-Allow-Origin，否则canvas无法裁剪未经许可的跨域图片'); } this.$emit('sure', imgData); }

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基于Vue如何实现移动端图片裁剪组件功能

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