Widget creation slows GUI thread

[MrGentleman]

Hi folks,

I am developing an application using Qt/embedded 4.8.

At runtime, depending on various events, widgets of type MainView (see simplified example code below) needs to be created and displayed in a tab view.
The situation I am now facing is, that the creation of an instance for MainView lasts approx. 1 second.
Because widgets may only be created in the GUI thread, this causes the thread to 'hang' for this second and the GUI is not responsive.

class MainView :  public QWidget {
   MainView() {
      a = new MyWidgetA(); b = new MyWidgetB(); c = new MyKeyboard();
      layout.add(a); layout.add(b); layout.add(c);
      setLayout(&layout);
   }
   MyWidgetA* a;
   MyWidgetB* b;
   MyKeyboard* c;
   QStackedLayout layout;
};

class MainView :  public QWidget {
   MainView() {
      a = new MyWidgetA(); b = new MyWidgetB(); c = new MyKeyboard();
      layout.add(a); layout.add(b); layout.add(c);
      setLayout(&layout);
   }
   MyWidgetA* a;
   MyWidgetB* b;
   MyKeyboard* c;
   QStackedLayout layout;
};

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The widgets which are created are e.g. a virtual keyboard which again consists of approx. 120 child widgets of type QPushButton....

Does anyone know a 'typical' pattern to avoid this problem? Or are there any other solutions - maybe 'speed-up widget creation'?

Thanks in advance, mike

[anda_skoa]

Have you measured which of the widgets is causing this?
Or are all three equally "bad" (each needing 330ms)?

Cheers,
_

[MrGentleman]

I have not yet measured each widget separately...

When the keyboard is populated with the buttons 3 pages in a stacked layout, on each page 4 lines
with each 10 keys on it; for every key a global style is used to set the background image.

Can generally be said that setStyleSheet() is an expensive operation on widgets?

thanks!

[anda_skoa]

I have not yet measured each widget separately...

You need to find out where you are losing the time, otherwise you spend optizing things that have had no impact anyway.

Can generally be said that setStyleSheet() is an expensive operation on widgets?

Likely depends on the complexity, but I haven't heard about it being a problem so far.

Cheers,
_

[MrGentleman]

Hi,

I have now measured the time required to create my pages.

MyWidgetA: 15316950 ns
MyWidgetB: 16899540 ns
MyWidgetC: 14246580 ns
MyWidgetD: 12074940 ns
MyWidgetE: 202042562 ns
MyWidgetF: 684948037 ns <- This widget contains the virtual keyboard
MyWidgetG: 7859490 ns
MyWidgetH: 7589730 ns


All these widgets are put into the QStackedLayout to display a single entity (which are created at run time)
When all these times are summed up, I get a delay of approx. nearly one second where the GUI is not responsive.
It is also remarkable, that the creation of the widget which contains the VK is approx. 4 times longer compared to the other ones..

Thanks and kind regards!

[anda_skoa]

You try a delayed creation approach.
E.g. only create the stacked widget pages when the tab is first activated.

Cheers,
_

[MrGentleman]

Thanks for the suggestion!

I have now implemented a mechanism which creates the MainView instances strictly sequential.

A background thread listens for events when to create main views. It then emits a signal to create the view, the GUI thread
listens for this signal and creates the view. While the GUI thread is creating the view, the thread waits until the GUI thread
notifies it via a signal that the main view has been created. Only then a new create request can be emitted.

This does actually not solve the problem that the creation of widgets takes much time, but all in all the GUI remains much
more responsive than creating all views at one time.

Thanks, best regards!