【报Bug】app使用canvas绘制图片后，输出的图片为空白

示例代码：

<template>  
    <view >  
        <!-- <view class="title">请在下面输入签名：</view> -->  
        <canvas class="mycanvas" canvas-id="mycanvas" @touchstart="touchstart" @touchmove="touchmove" @touchend="touchend"></canvas>  
        <view class="footer">  
            <view class="left" @click="finish">保存到相册</view>  
            <view class="right" @click="clear">清除</view>  
        </view>  
    </view>  
</template>  

<script>  
    var x = 20;  
    var y =20;  
    export default{  
        data(){  
            return {  
                ctx:'',         //绘图图像  
                points:[]       //路径点集合   
            }  
        },  
        onReady() {  
            this.ctx = uni.createCanvasContext("mycanvas",this);   //创建绘图对象  

            //设置画笔样式  
            this.ctx.lineWidth = 4;  
            this.ctx.lineCap = "round"  
            this.ctx.lineJoin = "round"  
        },  
        methods:{  
            //触摸开始，获取到起点  
            touchstart:function(e){  
                let startX = e.changedTouches[0].x;  
                let startY = e.changedTouches[0].y;  
                let startPoint = {X:startX,Y:startY};  
                this.points.push(startPoint);  
                //每次触摸开始，开启新的路径  
                this.ctx.beginPath();  
            },  

            //触摸移动，获取到路径点  
            touchmove:function(e){  
                let moveX = e.changedTouches[0].x;  
                let moveY = e.changedTouches[0].y;  
                let movePoint = {X:moveX,Y:moveY};  
                this.points.push(movePoint);       //存点  
                let len = this.points.length;  
                if(len>=2){  
                    this.draw();                   //绘制路径  
                }  

            },  

            // 触摸结束，将未绘制的点清空防止对后续路径产生干扰  
            touchend:function(){                     
                this.points=[];  
            },  

            /* ***********************************************  
            #   绘制笔迹  
            #   1.为保证笔迹实时显示，必须在移动的同时绘制笔迹  
            #   2.为保证笔迹连续，每次从路径集合中区两个点作为起点（moveTo）和终点(lineTo)  
            #   3.将上一次的终点作为下一次绘制的起点（即清除第一个点）  
            ************************************************ */  
            draw: function() {  
                let point1 = this.points[0]  
                let point2 = this.points[1]  
                this.points.shift()  
                this.ctx.moveTo(point1.X, point1.Y)  
                this.ctx.lineTo(point2.X, point2.Y)  
                this.ctx.stroke()  
                this.ctx.draw(true)  
            },  

            //清空画布  
            clear:function(){  
                let that = this;  
                uni.getSystemInfo({  
                    success: function(res) {  
                        let canvasw = res.windowWidth;  
                        let canvash = res.windowHeight;  
                        that.ctx.clearRect(0, 0, canvasw, canvash);  
                        that.ctx.draw(true);  
                    },  
                })  
            },  

            //完成绘画并保存到本地  
            finish:function(){  
                uni.canvasToTempFilePath({  
                  canvasId: 'mycanvas',  
                  success: function(res) {  
                      console.log(res)  
                    let path = res.tempFilePath;  
                    uni.saveImageToPhotosAlbum({  
                        filePath:path,  
                    })  
                  }   
                })  
            }  
        },  
    }  
</script>  

<style>  
    .title{  
        height:50upx;  
        line-height: 50upx;  
        font-size: 16px;  
    }  
    .mycanvas{  
        width: 100%;  
        height: calc(100vh - 200upx);  
        background-color: #ECECEC;  
    }  
    .footer{  
        font-size: 16px;  
        height: 150upx;  
        display: flex;  
        justify-content: space-around;  
        align-items: center;  
    }  
    .left,.right{  
        line-height: 100upx;  
        height: 100upx;  
        width: 250upx;  
        text-align: center;  
        font-weight: bold;  
        color: white;  
        border-radius: 5upx;  
    }  
    .left{  
        background: #007AFF;  
    }  
    .right{  
        background:orange;  
    }  
</style>

操作步骤：

绘制图形，保存图片，图片为空白

预期结果：

绘制图形，保存图片，图片为绘制的内容

实际结果：

绘制图形，保存图片，图片为空白