Referencing widgets

Sometimes, it is necessary to access or reference other widgets in a specific widget tree. In the Kv language, there is a way to do it using IDs.

Getting ready

This recipe will use two common widgets just for reference. The Button and TextInput fields are very common widgets.

How to do it…

This recipe is as follows:

1. Make a rule
2. Establish the ID
3. Call the ID:

   <MyWidget>:
       Button: 
           id: f_but 
       TextInput: 
           text: f_but.state 

How it works…

Let's see the code; this is the first line:

<MyWidget>:

This is the name of the widget we will use, which is a clickable text input. The second line is:

Button:

This defines the button. The third line is:

id: f_but

This gives the button an ID of f_but, which we will use to reference the button. The fourth line is:

TextInput:

This defines the text input. The fifth line is:

text: f_but.state

This is the definition of the text that is in the text input where we are referencing the state of the button. It says that if you do not click the button, the text in the text input is normal, and if you click the button, the text in the text input is shown.

There's more…

An ID is limited in scope to the rule it is declared in, so in the preceding code, f_but cannot be accessed outside the <MyWidget> rule; that is, if we have a second <MyWidget2>, we are not able to reference f_but in <MyWidget2>.

Also ID is a weakref module for the widget and not the widget itself. As a consequence, storing the ID is not sufficient to keep the widget from being garbage collected. To demonstrate:

<MyWidget>:
    label_widget: label_widget.__self__ 
    Button: 
        text: 'Add Button' 
        on_press: root.add_widget(label_widget) 
    Button: 
        text: 'Remove Button' 
        on_press: root.remove_widget(label_widget) 
    Label: 
        id: label_widget 
        text: 'widget'

If we do not use ID.__self__ or in this case label_widget.__self__ just label_widget, we are going to get an error: ReferenceError: weakly-referenced object no longer exists.

See also

If you want to get more details about widgets, see the recipes in Chapter 4, Widgets.