I am trying to run a QApplication on button click from Main window.
However I am getting this error:
QCoreApplication::exec: The event loop is already running
My code inside button click is this:
Qt Code: 
  1. if (!g_thread_supported ())
  2.       g_thread_init (NULL);
  3.  
  4.     gst_init (NULL, NULL);
  5.     QApplication app(NULL, NULL);
  6.     app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));
  7.  
  8.     // prepare the pipeline
  9.  
  10.     GstElement *pipeline = gst_pipeline_new ("xvoverlay");
  11.     GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
  12.     GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
  13.     GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
  14.     //assert(jpegdec);
  15.     gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
  16.     gst_element_link_many(src,jpegdec, sink);
  17.  
  18.     g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
  19.     // prepare the ui
  20.  
  21.     QWidget window;
  22.     window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
  23.     window.move(0,400);
  24.     window.setFixedSize(320, 200);
  25.     window.show();
  26.  
  27.     WId xwinid = window.winId();
  28.     gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
  29.  
  30.     // run the pipeline
  31.  
  32.     GstStateChangeReturn sret = gst_element_set_state (pipeline,
  33.         GST_STATE_PLAYING);
  34.     if (sret == GST_STATE_CHANGE_FAILURE) {
  35.       gst_element_set_state (pipeline, GST_STATE_NULL);
  36.       gst_object_unref (pipeline);
  37.       // Exit application
  38.       QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
  39.     }
  40.  
  41.     int ret = app.exec();
  42.  
  43.     window.hide();
  44.     gst_element_set_state (pipeline, GST_STATE_NULL);
  45.     gst_object_unref (pipeline);
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I am able to run this code,if I put it after
Qt Code: 
  1. ui->setupUi(this);
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in the MainWindow

How can I resolve this problem?