Appunti Kivy

This is the kivyexample.py file:

#!/usr/bin/env python
from kivy.app import App
#kivy.require("1.8.0")
from kivy.uix.floatlayout import FloatLayout
 
class MyLayout(FloatLayout):
    pass
 
class KivyExampleApp(App):
    def build(self):
        return MyLayout()
 
if __name__ == "__main__":
    KivyExampleApp().run()

In the code above we derived a class named KivyExampleApp from the Kivy's App class. The Kivy code will automatically search for a file called kivyexample.kv, and it will parse it using the KV language. The name of the file is derived by the class name, making it all lowercase and removing the leftmost App part (if it exists). So this is the kivyexample.kv file:

<MyButton@Button>:
    font_size: 40
    color: 0,1,0,1
    size_hint: 0.3, 0.2

<MyLayout>:
    MyButton:
        text: "Pos 0, 0"
        pos_hint: {'x': 0, 'y': 0}
    MyButton:
        text: "Center x-y"
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
    MyButton:
        text: "Pos right-top"
        pos_hint: {'right': 0.9, 'top': 0.3}
    MyButton:
        text: "Pos 0.5, 1.0"
        pos_hint: {'right': 0.5, 'top': 1.0}

In the Python code, the App constructor will return the MyLayout class, which is derived from the Kivy's FloatLayout class. The original class is an empty widget, but we added several buttons using the KV language.

Notice that MyLayout is derived from FloatLayout declaring it into the Python code, whereas the MyButton class is derived from the original Button class using the KV language, via the @Button construct. The result should be something like this:

We used a KV file to define the graphic interface of our App, but it is possible to do that using only Python code, thus having only a single file. But doing as seen above has several advantages. First of all you can try to keep the graphical presentation and the code logic as much separate as possibile. Another advantage is that resizing the app's window, will resize all its contents automatically (if you are smart enough not to use absolute values); otherwise you have to bind the window-resize event to a function which should adjust each widget size and position.

In general our App will return a class derived from the more generic Widget class, not the FloatLayout or others layouts classes. So the KV file will look like this:

<MyWidget>:
    FloatLayout:
        size: root.size
        MyButton:
            text: "Pos 0, 0"
            pos_hint: {'x': 0, 'y': 0}

We requested that the FloatLayout expands to all the parent (root) size, so the example will give the same output as in the previous paragraph.

kivywindowexample.py

#!/usr/bin/env python
from kivy.app import App
#kivy.require("1.8.0")
from kivy.uix.widget import Widget
 
class MyWidget(Widget):
    pass
 
class KivyWindowExampleApp(App):
    def build(self):
        return MyWidget()
 
if __name__ == "__main__":
    KivyWindowExampleApp().run()

kivywindowexample.kv

<MyButton@Button>:
    font_size: 28
    color: 0,1,0,1
    size_hint: 0.3, 0.2

<MyWidget>:
    BoxLayout:
        size: root.size
        orientation: 'vertical'
        ActionBar:
            pos_hint: {'top': 1}
            ActionView:
                use_separator: True
                ActionPrevious:
                    title: "MyGame"
                    with_previous: False
                ActionOverflow:
                    ActionButton:
                        text: "Exit"
                    ActionButton:
                        text: "About"
                ActionButton:
                    text: "New Game"
        FloatLayout:
            MyButton:
                text: "Pos 0, 0"
                pos_hint: {'x': 0, 'y': 0}
            MyButton:
                text: "Center x-y"
                pos_hint: {'center_x': 0.5, 'center_y': 0.5}
            MyButton:
                text: "Pos right-top"
                pos_hint: {'right': 0.9, 'top': 0.3}
            MyButton:
                text: "Pos 0.5, 1.0"
                pos_hint: {'right': 0.5, 'top': 1.0}

Like a plain Python program, it is possible for a Kivy app to produce some logging using the Kivy logger. The output will go, by default, to a file created into the app home directory, into the files/app/.kivy/logs/ directory.

from kivy.logger import Logger, LOG_LEVELS
# Set the loglevel. The Android log file will be create into
# [app_home]/files/app/.kivy/logs/
Logger.setLevel(LOG_LEVELS['debug'])
Logger.info('Informative message')

Without root privileges it is not possibile for another Android app (e.g. a file browser) to access the log directory. The Kivy app home will be into the adopted SD card, under something like this:

/mnt/expand/[UUID]/user/0/[fully.qualified.app.name]

In older Android versions (e.g. Android 8) the app home directory will be something like:

/data/data/[fully.qualified.app.name]

Every time that the Kivy environment is initializated, a new log file will be created; the filename will be something like kivy_YY-MM-DD_N.txt, where YY-MM-DD is the date and N is a progressive integer starting from zero.