About Kivy root

About app and root often used in Kivy

Kivy often uses variables such as app and root.

• app --Points to an instance of App --The target instance is basically one
• root --Points to the instance of the widget that is the root of the target --In the example below, the root of the Image is LargeImage

For example, use it like below.

main.kv

<LargeImage>:
    on_touch_down: app.changeScene()                   #Call changeeScene of MainApp when Widget is touched
    Image:
        allow_stretch: True
        source: root.path  

main.py

class LargeImage(Widget):
    def __init__(self, path):
        self.path = path
        super(LargeImage, self).__init__()

class MainApp(App):
    def build(self):
        self.root = LargeImage()
        return self.root

    def changeScene(self, scene, opt = None):
        pass

Where and when is root?

For example, if you have the file below The root of the upper Image is LargeImage, and the root of the lower Image is SmallImage.

main.kv

<LargeImage>:
    Image:
        source: root.path  

<SmallImage>:
    Image:
        source: root.path  

Now suppose you run the code below on the python file side.

main.py

l_img = LargetImage()
l_img.add_widget(SmallImage())         #Add SmallImage to LargeImage

In this case, the root of the Image on the SmallImage side remains SmallImage. Adding under LargeImage does not change root to LargeImage.

If you want to access LargeImage from the SmallImage side, Use root.parent instead of root.

Conversely, when accessing Small Image from the Large Image side It can be accessed with root.children [0].