How to call a user method after show method in qt c++

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:

    MainWindow(QWidget *parent = nullptr);
    void foo();
    ~MainWindow();

private slots:
    void on_pushButton_1_clicked();

private:
   Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

main.cpp

#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);


    MainWindow 
    w.show();       //It display blank window without any buttons or content
    w.foo();       //It execute before the window is rendered 

    return a.exec(); 
}

w.show() invokes MainWindow constructor. I would like to call function foo after the window is rendered completely. So, that I can perform actions on it. But after w.show() it looks like image1 and then calls foo which can't do anything because there is no interface. I removed w.foo() and set the break point at w.show(). While debugging at w.show() it look like below image1 and after return a.exec() it looks like image2 with full interface. The code for all these functions are written in mainwindow.cpp and gui in mainwindow.ui.

image1

image2

EDIT foo function

void MainWindow::foo()
{
    ui->statusbar->clearMessage();
    ec.pause();
    ec.start();   //start thread
    ec.wait();    //wait until thread ends
    new TreeOperations(); 
}