Question

I have a Custom component with a textField and a button, I want to onClick (of the Button), get the text from the textField and call another Component.

The error I get when I call mainLayout.addComponent(new A(textField.getValue()); is an
java.lang.IllegalStateException: Composition root must be set to non-null value before the com.example.vaadpro.

Answer 1

As the exception says, you have to call setCompositionRoot for your CustomComponent:

public class A extends CustomComponent {

  HorizontalLayout layout = new new HorizontalLayout(); 

  public A() {
    layout.add(new Label("Hello world!"));
    layout.add(new Button("Click me!"));

    setCompositionRoot(layout); // This is needed!
   }
}