hellogl2 example: Decouple mainwindow from window

The circular dependency cannot be ported to the corresponding Python
example.

Task-number: PYSIDE-2206
Pick-to: 6.6 6.5
Change-Id: I031b3fffdd7bd677d2fc55e132975a65f66ad128
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

examples/opengl/hellogl2/mainwindow.cpp

@@ -23,7 +23,7 @@ MainWindow::MainWindow()
 void MainWindow::onAddNew()
 {
     if (!centralWidget())
-        setCentralWidget(new Window(this));
+        setCentralWidget(new Window);
     else
         QMessageBox::information(this, tr("Cannot Add New Window"),
                                  tr("Already occupied. Undock first."));

examples/opengl/hellogl2/window.cpp

@@ -1,9 +1,8 @@
 // Copyright (C) 2016 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
-#include "glwidget.h"
 #include "window.h"
-#include "mainwindow.h"
+#include "glwidget.h"
 #include <QSlider>
 #include <QVBoxLayout>
 #include <QHBoxLayout>
@@ -11,9 +10,18 @@
 #include <QPushButton>
 #include <QApplication>
 #include <QMessageBox>
+#include <QMainWindow>
 
-Window::Window(MainWindow *mw)
-    : mainWindow(mw)
+static QMainWindow *findMainWindow()
+{
+    for (auto *w : QApplication::topLevelWidgets()) {
+        if (auto *mw = qobject_cast<QMainWindow *>(w))
+            return mw;
+    }
+    return nullptr;
+}
+
+Window::Window()
 {
     glWidget = new GLWidget;
 
@@ -77,7 +85,8 @@ void Window::dockUndock()
 
 void Window::dock()
 {
-    if (!mainWindow->isVisible()) {
+    auto *mainWindow = findMainWindow();
+    if (mainWindow == nullptr || !mainWindow->isVisible()) {
         QMessageBox::information(this, tr("Cannot Dock"),
                                  tr("Main window already closed"));
         return;

examples/opengl/hellogl2/window.h

@@ -10,14 +10,13 @@ QT_FORWARD_DECLARE_CLASS(QSlider)
 QT_FORWARD_DECLARE_CLASS(QPushButton)
 
 class GLWidget;
-class MainWindow;
 
 class Window : public QWidget
 {
     Q_OBJECT
 
 public:
-    Window(MainWindow *mw);
+    Window();
 
 protected:
     void keyPressEvent(QKeyEvent *event) override;
@@ -35,7 +34,6 @@ private:
     QSlider *ySlider;
     QSlider *zSlider;
     QPushButton *dockBtn;
-    MainWindow *mainWindow;
 };
 
 #endif