python mah_button.py

Posted 2021-05-09

'''
Little demonstration of various kivy useful features:
   - loading kv code
   - adding dynamic parts to static parts using ids
   - binding events to python expressions on kv side
 
useful references:
   - http://kivy.org/docs/guide/lang.html#referencing-widgets
   - http://kivy.org/docs/api-kivy.uix.widget.html?highlight=ids#kivy.uix.widget.Widget.ids
   - http://kivy.org/docs/api-kivy.uix.widget.html?highlight=size%20hint#kivy.uix.widget.Widget.size_hint
   - http://kivy.org/docs/api-kivy.uix.screenmanager.html?highlight=screenmanager#kivy.uix.screenmanager
'''
 
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.screenmanager import Screen
 
from string import letters
 
kv = '''                    # write the string to a variable to pass it to the builder later
BoxLayout:                  
    # Define it as 'BoxLayout' http://kivy.org/docs/api-kivy.uix.layout.html
    orientation: 'vertical' # with the property vertical so it can be placed on the top of the screen 
    BoxLayout:  
        size_hint_y: None # Otherwise the parent would autoscale it to halfscreen.
        height: 30          #  height in pixel
        id: buttons         # The id of the button, we will later use it to integrate it.
    ScreenManager:          
        id: sm              # The id of the ScreenManager, we will later use it to integrate it.
 
<MahButton>:   
    on_press: app.root.ids.sm.current = self.text     # on press this button:
'''   #                          # --> the ScreenManager changes to the SM with the Title that is self.text
 
class MahButton(Button):        # Inherits Mahbutton with Button
    pass                        # Pass does nothing is useful as a placeholder when a statement is required
 
class MahApp(App): # The main application
    def build(self):
        self.root = Builder.load_string(kv)   # Execute the string kv as kivy code
        for l in letters:
            s = Screen(name=l)                # Creates a new screen
            s.add_widget(Label(text=l))       # Adds the letters to the screen
            self.root.ids.sm.add_widget(s)    # Adds the screen to the ScreenManager
                                              # -> which is located in self.root under the id ids.sm
            self.root.ids.buttons.add_widget( # Adds buttons as widgets
                MahButton(text=l))            # Adds the Button with the text
 
        return self.root                      # Return self.root so it can be run
 
if __name__ == '__main__':                    # Executes code in this file but excludes imported main()
    MahApp().run()                            # Runs the application.