参考文档：https://developers.weixin.qq.com/minigame/dev/document/render/canvas/Canvas.toTempFilePathSync.html

核心代码：

var canvas = cc.game.canvas;
    var width  = cc.winSize.width;
    var height  = cc.winSize.height;
    canvas.toTempFilePath({
        x: 0,y: 0,width: width,height: height,destWidth: width,destHeight: height,success (res) {
            //.可以保存该截屏图片
            console.log(res)
            wx.shareAppMessage({
                imageUrl: res.tempFilePath
            })
        }
    })


    // let tempFilePath = canvas.toTempFilePathSync({
    // x: 0,
    // y: 0,
    // width: width,
    // height: height,
    // destWidth: width,
    // destHeight: height
    // })
    // wx.shareAppMessage({
    // imageUrl: tempFilePath
    // })