need explanation for android layout syntax

Question

here is a line in layout file

 <TextView 
        android:id="@+id/username"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="@string/username"
        android:textAppearance="?android:attr/textAppearanceMedium"> 

in about context what is the exact meaning of

android:textAppearance="?android:attr/textAppearanceMedium" 

is that related to "?[package:][type:]name" , if yes then what that refers to ?

Answer 1

?android:attr/textAppearanceMedium is referring to the android.R.attr.textAppearanceMedium This is the way to reference to external package resource content.

Here's an article mentioned this.