Embedding external app into slicer

Embedding.py

def onConnectAppButton(self):
    import win32gui
    import win32con
    import time

    # Launch the application
    proc = qt.QProcess()
    proc.setProgram("/path/to/application")
    proc.startDetached()
    
    # Grab the splashscreen / startup window and minimize it
    splashScreenHandle = 0
    while splashScreenHandle == 0:
      splashScreenHandle = win32gui.FindWindow(None, 'App')
      time.sleep(.01)
    win32gui.ShowWindow(splashScreenHandle, win32con.SW_MINIMIZE)
    
    # Grab the actual window once available
    windowHandle=0
    while windowHandle == 0:
      windowHandle = win32gui.FindWindow(None, 'App: With More stuff in title')
      time.sleep(.01)

    # Embed into slicer application
    window  = qt.QWindow.fromWinId(windowHandle)
    windowContainer = slicer.util.mainWindow().createWindowContainer(window)
    slicer.util.findChild(slicer.util.mainWindow(), 'AppWindowFrame').layout().addWidget(windowContainer)

    # Show embedded application
    self.toggleCenterView()
    
def toggleCenterView(self):
    layoutVisible = slicer.util.findChild(slicer.util.mainWindow(), 'CentralWidgetLayoutFrame').visible
    appVisible = slicer.util.findChild(slicer.util.mainWindow(), 'AppWindowFrame').visible

    slicer.util.findChild(slicer.util.mainWindow(), 'CentralWidgetLayoutFrame').visible = not layoutVisible
    slicer.util.findChild(slicer.util.mainWindow(), 'AppWindowFrame').visible = not appVisible
    
def callThisInSetupOfModuleToCreateFrameForApp(self):
    centralWidget = slicer.util.findChild(slicer.util.mainWindow(), 'CentralWidget')
    AppWindowFrame = qt.QFrame()
    AppWindowFrame.objectName = 'AppWindowFrame'
    AppWindowFrame.visible = False
    AppWindowFrame.setLayout(qt.QVBoxLayout())
    centralWidget.layout().addWidget(AppWindowFrame)

Windows specific, Linux would use xterm for finding the window ID.

python package: pypiwin32

This example is also specific to an application that has a splashscreen / startup window that is not the one you want to embed

getting the window: https://doc.qt.io/qt-5/qwindow.html#fromWinId
embedding the window: https://doc.qt.io/qt-5/qwidget.html#createWindowContainer