利用html2canvas实现移动端上传图片生成海报

摘要：所以需要在这里做一个判断。使用的内联样式时遇上的写法问题直接使用手机拍照得到的图片方向有问题

原文链接：链接描述
使用vue+html2canvas+exif-js
github地址
线上demo

上传图片

对图片进行操作：移动、放大、缩小

合成海报

html:

js

getPhoto () {
    var imageInput = document.querySelector("#image-input")
    var that = this
    imageInput.addEventListener("change", function (e) {
        reads = new FileReader()
        reads.readAsDataURL(this.files[0])
        reads.addEventListener("load", function (e) {
            that.imgUrl = this.result
            that.myImg.position.x = 0
            that.myImg.position.y = 0
            that.myImg.scale = 1
            var orientation
            that.previewImg.addEventListener("load", function () {
                Exif.getData(that.previewImg, function() { // 获取图像的数据
                Exif.getAllTags(this); // 获取图像的全部数据，值以对象的方式返回
                orientation = Exif.getTag(this, "Orientation"); // 获取图像的拍摄方向
                var rotateCanvas = document.createElement("canvas"),
                rotateCtx = rotateCanvas.getContext("2d");
                 // 针对图像方向进行处理
                switch (orientation) {
                    case 1 :
                        rotateCanvas.width = that.previewImg.width;
                        rotateCanvas.height = that.previewImg.height;
                        rotateCtx.drawImage(that.previewImg, 0, 0, that.previewImg.width, that.previewImg.height);
                        break;
                    case 6 : // 顺时针 90 度
                        rotateCanvas.width = that.previewImg.height;
                        rotateCanvas.height = that.previewImg.width;
                        rotateCtx.translate(0, 0);
                        rotateCtx.rotate(90 * Math.PI / 180);
                        rotateCtx.drawImage(that.previewImg, 0, -that.previewImg.height, that.previewImg.width, that.previewImg.height);
                        break;
                    case 8 :
                        rotateCanvas.width = that.previewImg.height;
                        rotateCanvas.height = that.previewImg.width;
                        rotateCtx.translate(0, 0);
                        rotateCtx.rotate(-90 * Math.PI / 180);
                        rotateCtx.drawImage(that.previewImg, -that.previewImg.width, 0, that.previewImg.width, that.previewImg.height);
                        break;
                    case 3 : // 180 度
                        rotateCanvas.width = that.previewImg.width;
                        rotateCanvas.height = that.previewImg.height;
                        rotateCtx.translate(0, 0);
                        rotateCtx.rotate(Math.PI);
                        rotateCtx.drawImage(that.previewImg, -that.previewImg.width, -that.previewImg.height, that.previewImg.width, that.previewImg.height);
                        break;
                    default :
                        rotateCanvas.width = that.previewImg.width;
                        rotateCanvas.height = that.previewImg.height;
                        rotateCtx.drawImage(that.previewImg, 0, 0, that.previewImg.width, that.previewImg.height);
                    }
                    var rotateBase64 = rotateCanvas.toDataURL("image/jpeg", 0.5);
                });
            })
        }) 
    })
}

对图片和相框绑定@touchstart @touchmove @touchend

getInitPosition (e) {
    event.preventDefault()
    if (this.imgUrl) {
        var length = e.touches.length
         if (length > 1) {
            let pointOne = e.touches[0]
            let pointTwo = e.touches[1]
            this.initTouchX = pointOne.clientX - pointTwo.clientX
            this.initTouchY = pointOne.clientY - pointTwo.clientY
        } else {
            var touches = e.touches[0]
            this.initTouchX = touches.clientX
            this.initTouchY = touches.clientY
        }   
    }
},
 getMovePosition (e) {
    event.preventDefault()
    if (this.imgUrl) {
        var length = e.touches.length
        if (length > 1) {
            let pointOne = e.touches[0]
            let pointTwo = e.touches[1]
            this.changeTouchX = pointOne.clientX - pointTwo.clientX
            this.changeTouchY = pointOne.clientY - pointTwo.clientY
            var scale = (this.changeTouchX - this.initTouchX) > (this.changeTouchY - this.initTouchY) ? (this.changeTouchX / this.initTouchX) : (this.changeTouchY / this.initTouchY) 
            scale *= this.myImg.lastScale
            this.myImg.scale = scale > 3 ? 3 : scale < 0.5 ? 0.5 : scale
        } else {
            var touches = e.touches[0]
            this.changeTouchX = touches.clientX - this.initTouchX
            this.changeTouchY = touches.clientY - this.initTouchY
            this.myImg.position.x = this.lastTouchX + (this.changeTouchX / this.myImg.scale)
            this.myImg.position.y = this.lastTouchY + (this.changeTouchY / this.myImg.scale)
        }   
    }
},
getLeavePosition (e) {
    this.myImg.lastScale = this.myImg.scale
    if (e.touches.length > 0) {
        var touches = e.touches[0]
        this.initTouchX = touches.clientX
        this.initTouchY = touches.clientY
    }
    this.lastTouchX = this.myImg.position.x
    this.lastTouchY = this.myImg.position.y
},

createPhoto () {
    if (this.imgUrl) {
        let photoBox = document.querySelector(".photo-box")
        newImgWidth = photoBox.style.offsetWidth
        let newImgHeight = photoBox.style.offsetHeight
        let scale = window.devicePixelRatio
        let that = this
        html2canvas(photoBox, {
            width: newImgWidth,
            height: newImgHeight,
            scale: scale,
            useCORS: true
        }).then(function (canvas) { 
            var dataUrl = canvas.toDataURL("image/jpg")
            localStorage.imgData = dataUrl
            that.$router.push({
                name: "share",
                params: {
                    storage: "imgData"
                }
            })
        })
    } else {
        alert("请上传图片")
    }
}

1) 在浏览器上阻止缩放的问题

在tounchmove时使用event.preventDefault()

2) 合成图片的清晰度

在html2canvas写参数时，scale = window.devicePixelRatio

3) 对图片进行缩放时，对距离的判断

这里需要区分两种情况：
- 两指缩放
- 一指移动  
在两指缩放后会有一个手放手另一个手继续操作的情况。这样在touchend时，e.touches还存在一个元素。所以需要在这里做一个判断。

4) 使用vue的:style="内联样式"时遇上的写法问题

:style="{transform:"scale("+ myImg.scale+ ") translate("+myImg.position.x+"px,"+myImg.position.y+"px)"}"

5）直接使用手机拍照得到的图片方向有问题