//绘制canvas 
<canvas style="width: 280px;height:380px" canvas-id="activityCode" ></canvas>
//下面两个方法是将base64图片转换成临时地址
function removeSave(FILE_BASE_NAME = 'tmp_base64src', format = 'jpg') {
    return new Promise((resolve) => {
        // 把文件删除后再写进，防止超过最大范围而无法写入
        const fsm = uni.getFileSystemManager(); //文件管理器
        const FILE_BASE_NAME = 'tmp_base64src';
        const filePath = `${wx.env.USER_DATA_PATH}/${FILE_BASE_NAME}.${format}`;
        fsm.unlink({
            filePath: filePath,
            success(res) {
                console.log('文件删除成功');
                resolve(true);
            },
            fail(e) {
                console.log('readdir文件删除失败：', e);
                resolve(true);
            }
        });
    })
}
function base64ToSave(base64data: any, FILE_BASE_NAME = 'tmp_base64src') {
    const fsm = uni.getFileSystemManager();
    return new Promise(async (resolve, reject) => {
        //format这个跟base64数据的开头对应
        const [, format, bodyData] = /data:image\/(\w+);base64,(.*)/.exec(base64data) || [];
        if (!format) {
            reject(new Error('ERROR_BASE64SRC_PARSE'));
        }
        await removeSave(FILE_BASE_NAME, format);
        const filePath = `${wx.env.USER_DATA_PATH}/${FILE_BASE_NAME}.${format}`;
        //const buffer = wx.base64ToArrayBuffer(bodyData);
        fsm.writeFile({
            filePath,
            data: bodyData,
            //data: base64data.split(";base64,")[1],
            encoding: 'base64',
            success() {
                resolve(filePath);
            },
            fail() {
                reject(new Error('ERROR_BASE64SRC_WRITE'));
            },
        });
    });
}

//开始绘制
//1.背景网络图片

function drawCanvas() {
    const ctx = uni.createCanvasContext('activityCode')
    uni.getImageInfo({
        src: '****',
        success:res=>{
              //背景图
            ctx.drawImage(res.path, 0, 0, 140, 190);   //插入图片
            //绘制文案
            ctx.setFontSize(9)
            ctx.fillStyle='#FFFFFF'
            ctx.setTextAlign('center')
            ctx.fillText('***', 70, 40)

            //绘制一个原型的图片
            circleImg(ctx)
            ctx.draw()

        },
        fail:res=>{
            console.log(`缓存失败：${res.errMsg}`);    
        }
    })
    
}
//2.绘制圆形图片
function circleImg(ctx) {
    ctx.beginPath()
    ctx.arc(70, 100, 34, 0, 2 * Math.PI)
    ctx.setFillStyle('#ffffff')
    ctx.fill()
    ctx.clip()
    //url就是上方保存的base64临时地址
    ctx.drawImage(url, 40, 70, 60, 60);
    
}
//保存图片
function saveImg() {
    uni.canvasToTempFilePath({
        canvasId: 'activityCode',
        success: function(res) {
            uni.saveImageToPhotosAlbum({
                filePath: res.tempFilePath,
                success: function () {
                    uni.showToast({
                        title: '保存成功',
                        icon: 'success'
                    })
                },
                fail: function () {
                    uni.showToast({
                        title: '保存失败',
                        icon: 'none'
                    })
                }
            })
        } 
    })
}