When to use "?android" or "@android"?

Question

I am a newbie and trying to understand the following XML code:

Looking at the documentation at developer.android.com, it says "starStyle" is a constant in R.attr and

---

public static final int starStyle Since: API Level 1

Default Star style.

Must be a reference to another resource, in the form "@[+][package:]type:name" or to a theme attribute in the form "?[package:][type:]name".

It seems to say that there are 2 syntax I can declare. 1) "@[+][package:]type:name" 2) "?[package:][type:]name".

If there are 2 syntax, what is the correct one for "@[+][package:]type:name" ?

I tried ""@android:attr/starStyle" but I didn't get a "star" checkbox even though the application compiled ok.

Answer 1

The @-syntax is used for resources that are defined in your project or the Android framework. The ?-syntax is used for resources in the current theme.

For starStyle, which should be defined in the theme, you want:

"?android:attr/starStyle"

See here for some elaboration on theme resources.

Answer 2

Android provides a shortcut to enable you to

use styles from the currently applied theme.

To do this, use ?android: rather than @ as a prefix to the resource you want to use.

example:

<EditText
android:id="@+id/myEditText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@android:string/url"
android:textColor="?android:textColor"
/>

This technique enables you to create styles that change if the current theme changes, without you modifying each individual style resource.