class RobotRemote : public QWidget
{
public:
    RobotRemote(QWidget * parent = nullptr);
    ~RobotRemote();
    bool Start();
signals:
    void signalUpdateUi();
public slots:

    void onUpdateUi();
private:
    Json::Value mJsonString;

}
RobotRemote::RobotRemote(QWidget * parent ) :QWidget(parent) 
{ 
    QObject::connect(
 
this,SIGNAL(signalUpdateUi()),this,SLOT(onUpdateUi()));
    mJsonString = Json::Value::null;
　　 Start();
}
bool RobotRemote::Start()
{
    // 載入地圖
    QString fileName = QFileDialog::getOpenFileName(this, tr("open file"), "",
                                                    tr("JsonFile(*.json);"));
    
 
if (!fileName.isEmpty())
    {
        mJsonString = UmJson::ReadFileToJson(fileName.toStdString());
        if(!mJsonString.empty())
            HandleGetMap(mJsonString);
        LOG(INFO)<<"OpenMap: "<<QThread::currentThread();
    }
}
void RobotRemote::onUpdateUi()
{
    if(!mJsonString.empty())
        HandleGetMap(mJsonString);
    LOG(INFO)
 
<<"AddLabel: "<<QThread::currentThread();

}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->centralWidget->setLayout(ui->gridLayout);

    mPRR = GetRobotRemoteInterface();

    if(mPRR==nullptr)
    {
        LOG(WARNING)<<"RobotRemote allocate object failed!\n";
        return;
    }

    ui->stackedWidget->setCurrentWidget(ui->mainpage);
    mPRR->setParent(ui->mainpage);
    mPRR->move(0,0); //to its parent
    mPRR->resize(840,678);//to its parent
}

    ui->stackedWidget->setCurrentWidget(ui->mainpage);
    mPRR->setParent(ui->mainpage);

Start()可以認為在建構函式中一開始就呼叫了，此時開啟一個檔案，mJsonString中有Json檔案中讀出來的內容。 用訊號觸發槽函式onUpdateUi()執行後，mJsonString內容是沒有變的，我認為這是符合我的期望的。

但是如果我改成：去掉

    mPRR->setParent(ui->mainpage);
    mPRR->move(0,0); //to its parent
    mPRR->resize(840,678);//to its parent

保留

 ui->stackedWidget->setCurrentWidget(ui->mainpage);

mJsonString竟然為空，中間沒有任何mJsonString清空或修改的操作，執行緒id也沒有變。

對Qt看來還不瞭解。繼續追。。。