The kv Design Language
Up until this point we have been adding widgets to our window/app using python code. This is acceptable and works fine however there is a much easier and nicer way to do this.
Kivy has something called the kv design language. You think of it as a language similar to HTML and CSS where it is responsible for styling and adding elements to the display but does not handle any logic. In this tutorial I will show you how to create a .kv file and create the graphical part of your application from within that file.
Note: We still need our python script to handle the logic and load the application.
Creating a .kv File
There are a few conventions we need to follow when creating a .kv file.
Naming: The name of your .kv file must follow the rules below in order for python/kivy to be able to see and load the file.
1. It must be all lowercase
2. It must match with the name of your main class. (The one that has the build method)
3. If the name of your main class ends in "app" (lowercase or uppercase) you must not include "app" in your file name.
File Location: The file must be in the same directory as your python script.
File Extension: The file must be saved as type *All Files and end with .kv
My main classes name is MyApp. Therefore I will name my file my.kv.
If you have named your file correctly there is nothing else you need to do to ensure that it is linked with your python script.
Setting up The Python Script
Before continuing please import the following module from your python script.
The first thing we need to do to add widgets to our screen using .kv is set up an empty class in our python script. This class will simply inherit from Widget and will be what is used from within the kv file to add widgets.
Now ensure that your main classes build method returns an instance of MyGrid and we are ready to go.
Adding Widgets From a .kv File
Two important things to remember about .kv files:
- Capitals are Important
- Indentation is important
The first thing we do when writing in a .kv is declare the class we will be adding widgets to. In my case it's MyGrid.
The next step is to define the widgets that we want to add to that class. Each widget also has several properties associated with it that we can change.
You can see that the widgets that are inside of the MyGrid class are tabbed in once and the properties of those widgets are tabbed twice.
Creating a Form With a .kv File
The file below is the recreation of the form we made in tutorial 3, this time using a kv file. You can see that we have multiple grid layouts like before and that we add widgets to those layouts.
Notice that to make these widgets fill the entire screen we must change the size attribute. By making the size of the first grid layout root.width, root.height we are telling it to fill the size of the window dynamically.