vue下canvas绘制矩形

起因：根据项目需求本人写了一个绘制矩形的组件。
功能：在图片中绘制矩形，根据图片大小进行自适应展示，获取图片矩形坐标。
思路：首先定义一个固定大小的DIV，DIV标签中有监测鼠标变化的四个事件mousedown，mousemove，mouseup，mouseleave。
第二在DIV标签内有img，canvas两个标签，一个负责图片展示,一个负责绘制矩形。
其中img与DIV标签的大小相当，canvas是根据DIV标签position定位的以保证鼠标事件在图片上绘制矩形不会有偏差。

以下就是组件的全部代码和实现效果展示

<template>
    <div id="customPositionDiv">
        <div style=" 1460px; height:740px;  background-color: #c0c0c0; 
                    margin:0 auto ; display:-webkit-box; 
                    -webkit-box-align:center; -webkit-box-pack:center; ">
            <div @mousedown="mousedown" @mousemove="mousemove"
                 @mouseup="mouseup" @Mouseleave="Mouseleave" :style="imgstyle">
                <img :src="imgSrc" :style="imgstyle">
                <canvas ref="table" :width="canvasWidth" :height="canvasHeight" :style="canvasstyle"></canvas>
            </div>
        </div>
        <div style=" 1450px;z-index: inherit;text-align: right ;margin:10px 0 0 0">
            <span slot="footer" class="dialog-footer">
                <el-button @click="customClose">取 消</el-button>
                <el-button type="primary" @click="customQuery">确 定</el-button>
            </span>
        </div>
    </div>

</template>
<style lang="scss">

</style>
<script>

    import vue from 'vue';

    export default {
        name: 'canvasDraw',
        props: ['imgSrc'],
        data() {
            return {

                //  customPositionShow:false, //自定义位置
                //   showclose:false,
                startX: '',  //画画开始的X坐标
                startY: '',  //画画开始的Y坐标
                endX: '',    //画画结束的X坐标
                endY: '',    //画画结束的Y坐标
                isMouseDownInCanvas: '', //鼠标是否按下
                customcxt: '',      // cxt
                customR '',    //原图与展示图片的宽度比
                customRheight: '',   //原图与展示图片的高度比
                imgstyle: '',        //根据图片大小自适应样式
                canvasstyle: '',     //根据图片大小canvas自适应样式 居中显示
                canvasWidth: '',     //根据图片大小自适应canvas宽
                canvasHeight: '',    //根据图片大小自适应canvas高
                DivWidth: 1460,      //最大宽度
                DivHeight: 740,      //最大高度
            };
        },
        watch: {
            'imgSrc': function () {
                this.show();
            },

        },
        mounted() {
            this.show();
        },

        methods: {
            //取消时返回组件调用处所需的数据
            customClose() {
                this.customcxt.clearRect(0, 0, this.DivWidth, this.DivHeight);
                this.$emit('custom', { 'type': 1, 'data': '' });
            },
            //确定时返回组件调用处所需的数据
            customQuery() {
                this.customcxt.clearRect(0, 0, this.DivWidth, this.DivHeight);
                //根据绘制进行图片裁剪

                //获取矩形框Left，Width'
                let cLeft = 0;
                let cWidth = 0;
                if (this.startX > this.endX) {
                    cLeft = this.endX;
                    cWidth = this.startX - this.endX;
                } else {
                    cLeft = this.startX;
                    cWidth = this.endX - this.startX;
                }

                //获取矩形框Top，Height
                let cTop = 0;
                let cHeight = 0;
                if (this.startY > this.endY) {
                    cTop = this.endY;
                    cHeight = this.startY - this.endY;
                } else {
                    cTop = this.startY;
                    cHeight = this.endY - this.startY;
                }

                var oMark = [];
                oMark['offsetLeft'] = parseInt(cLeft / this.customRwidth);
                oMark['offsetTop'] = parseInt(cTop / this.customRheight);
                oMark['offsetWidth'] = parseInt(cWidth / this.customRwidth);
                oMark['offsetHeight'] = parseInt(cHeight / this.customRheight);

                this.$emit('custom', { 'type': 2, 'data': oMark });
            },

            // dialog展示自定义矩形框画板，
            // 计算img与canvas标签自适应图片的大小
            show() {
                vue.nextTick(_ => {
                    let customCanvas = this.$refs.table;// canvas显示层
                    this.customcxt = customCanvas.getContext("2d");
                    let img = new Image();
                    img.src = this.imgSrc;
                    let that = this;
                    img.onload = function () {

                        let canvasleft = 0;
                        let canvastop = 0;
                        let WrH = img.width / img.height;             //图片宽高比
                        let RWrH = that.DivWidth / that.DivHeight;    //放置图片DIV的宽高比
                        let aa = 0;
                        // 根据宽高比大小判断确定自适应的宽和高
                        if (RWrH > WrH) {
                            aa = that.DivHeight / img.height;
                            that.canvasHeight = that.DivHeight;
                            that.canvasWidth = img.width * aa;
                            canvasleft = (that.DivWidth - that.canvasWidth) / 2
                        } else {
                            aa = that.DivWidth / img.width;
                            that.canvasHeight = img.height * aa;
                            that.canvasWidth = that.DivWidth;
                            canvastop = (that.DivHeight - that.canvasHeight) / 2
                        }
                        that.imgstyle = ' position: relative;  ' + that.canvasWidth 
                                      + ' px; height:' + that.canvasHeight + 'px'; //img浮动定位居中显示
                        that.customRwidth = that.canvasWidth / img.width; //原图与展示图片的宽高比
                        that.customRheight = that.canvasHeight / img.height;

                        that.canvasstyle = 'position: absolute;left: ' + canvasleft 
                                        + '; top: ' + canvastop + ';' //canvas浮动定位

                    };
                })

            },
            //鼠标按下时执行
            mousedown(e) {
                this.isMouseDownInCanvas = true;
                // 鼠标按下时开始位置与结束位置相同 
                // 防止鼠标在画完矩形后 点击图画形成第二个图形
                this.endX = e.offsetX;
                this.endY = e.offsetY;
                this.startX = e.offsetX;
                this.startY = e.offsetY;
                this.mousemove(e)

            },
            //鼠标移动式时执行
            mousemove(e) {
                if (this.isMouseDownInCanvas) { // 当鼠标有按下操作时执行

                    this.endX = e.offsetX;
                    this.endY = e.offsetY;
                    let wwidth = this.endX - this.startX;
                    let wheigth = this.endY - this.startY;

                    // 清除指定区域的所有像素
                    this.customcxt.clearRect(0, 0, this.DivWidth, this.DivHeight);
                    this.customcxt.strokeStyle = " #00ff00"; //矩形框颜色
                    this.customcxt.lineWidth = "2";  //矩形框宽度
                    this.customcxt.strokeRect(this.startX, this.startY, wwidth, wheigth);  //绘制矩形

                }
            },
            //鼠标松开时执行
            mouseup(e) {
                this.isMouseDownInCanvas = false;
            },

            Mouseleave(e) {
                this.isMouseDownInCanvas = false
            },
        },
    }
</script>