How do I call a function when the screen is rotated?

Question

Problem

I am building in app (with Kivy) which renders some custom widgets to the screen. Unfortunately, their positions (relative to other widgets) changes when the tablet on which I am running the app is rotated. Now I have built a function that repositions them, and I would like to call this function when the tablet is rotated. How can I implement this?

My attempts so far...

I have seen from the Kivy documentation that the Window class has an on_rotate event, and have tried to incorporate this into my program in several different ways.

First, I tried implementing the callback after the if __name__ == '__main__' statement. Something like:

if __name__ == '__main__':
     app = MainApp()
     Window.on_rotate = lambda: app.reposition_widgets()
     app.run()

That did nothing. In vain I also tried:

if __name__ == '__main__':
     app = MainApp()
     Window.on_rotate(app.reposition_widgets)
     app.run()

I then attempted to bind the widgets to on_rotate events, something like:

class CustomWidget(Widget):
     def __init__(self, **kwargs)
          super().__init__(**kwargs)
          self.bind(on_rotate = self.reposition_widgets)

That did nothing. Neither did:

class CustomWidget(Widget):
     def __init__(self, **kwargs)
          super().__init__(**kwargs)
     
     on_rotate = reposition_widgets

My final attempt was to create a Window class in the accompanying kv file and the Python file, then specify the on_rotate event from there. Something like:

#kv file
Window:
     on_rotate: app.reposition_widgets()

That didn't work either.

Possible work around

To be honest, rotating the screen is not all that useful for my app, and so it wouldn't be all that bad if I just disabled screen rotation. However, I was not able to find a good way of doing so in the documentation. Do you know how I would go about doing this?
Binding the widgets to the on_pos event - or something similar. Something like:

class CustomWidget:
     
     def reposition_widgets():
         # Code here

     on_pos = reposition_widgets

The problem with this is that the widgets move about a lot, which means that the function gets called a lot - causing problems elsewhere.

Let me know if you would like to see more code. I have over 1000 lines spread over several files, and felt that just copy-and-pasting wouldn't be particularly useful.

I never used kivy, but try `Window.on_rotate(app.reposition_widgets)` note the absence of parenthesis. You called `reposition_widgets` instead of passing it as a callback. — DeepSpace, Nov 09 '20 at 16:40
Thanks for the suggestion! I've just tried what you've suggested and unfortunately it hasn't worked — jda5, Nov 09 '20 at 16:44
Check this [answer to a similar question](https://stackoverflow.com/questions/39430824/how-to-detect-screen-rotation-on-android-in-kivy). — John Anderson, Nov 09 '20 at 19:34
That dupe looks promising, although it only has a hint at an answer. @JacobStrauss, If you get it working due to the dupe, you could self-answer, and then flag the original as a dupe to this one. — SiHa, Nov 10 '20 at 11:36

score 0 · Accepted Answer · answered Nov 24 '20 at 15:57

I was able to solve this issue using the on_size event.

class CustomWidget(Widget):
    
    def reposition_widgets(self):
        # Reposition logic here

    on_size = reposition_widgets

This worked quite well actually. I added an additional parameter in the reposition_widgets function to differentiate between when I was repositioning because of on_size and when I was repositioning because I was calling it (see example below). However, this is not always necessary.

class CustomWidget(Widget):
    
    def reposition_widgets(self, rotation=False):
        if rotation:
            # When function is called due to on_size
        else:
            # When function is called in code

    on_size = lambda self, *args: self.reposition_widgets(rotation=True)

How do I call a function when the screen is rotated?

1 Answers1