canvas.toTempFilePath({

                x: 0,

                y: 0,

                width: canvas.width,

                height: canvas.height,

                destWidth: canvas.width,

                destHeight: canvas.height,

                success (res) {

                    console.log(res);

                    //不能用 ："saveFile:fail tempFilePath file not exist"

                    const fs = wx.getFileSystemManager();

                    fs.saveFile({

                        tempFilePath: res.tempFilePath, // 传入一个本地临时文件路径

                        success(res) {

                          console.log(res.savedFilePath) // res.savedFilePath 为一个本地缓存文件路径

                        },

                        fail(res){