Added the ability to track the closure of the file manager without selecting content

[Neizvestnyj]

When clicking the close button, on Linux on_selection was not called. Same - #666

from textwrap import dedent

from plyer import filechooser

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import ListProperty
from kivy.uix.button import Button


class FileChoose(Button):
    '''
    Button that triggers 'filechooser.open_file()' and processes
    the data response from filechooser Activity.
    '''

    selection = ListProperty([], allownone=True)

    def choose(self):
        '''
        Call plyer filechooser API to run a filechooser Activity.
        '''
        filechooser.open_file(on_selection=self.handle_selection)

    def handle_selection(self, selection):
        '''
        Callback function for handling the selection response from Activity.
        '''
        self.selection = selection

    def on_selection(self, *a, **k):
        '''
        Update TextInput.text after FileChoose.selection is changed
        via FileChoose.handle_selection.
        '''
        print(self.selection)
        App.get_running_app().root.ids.result.text = str(self.selection)


class ChooserApp(App):
    '''
    Application class with root built in KV.
    '''

    def build(self):
        return Builder.load_string(dedent('''
            <FileChoose>:
            BoxLayout:
                BoxLayout:
                    orientation: 'vertical'
                    TextInput:
                        id: result
                        text: ''
                        hint_text: 'selected path'
                    FileChoose:
                        size_hint_y: 0.1
                        on_release: self.choose()
                        text: 'Select a file'
        ''')
                                   )


if __name__ == '__main__':
    ChooserApp().run()