Develop Reference

Prefix in android style files item names

I am new to android and have a question about the styles.xml file.
Each item for example this
<item name="android:background">#color/black</item>
works both with "android:background" and with just "background".
What is the android: prefix there for?

When creating your own styles, you should always extend an existing style from the framework or support library so that you maintain compatibility with platform UI styles. To extend a style, specify the style you want to extend with the parent attribute. You can then override the inherited style attributes and add new ones.
For example, you can inherit the Android platform's default text appearance and modify it as follows:
<style name="GreenText" parent="#android:style/TextAppearance">
<item name="android:textColor">#00FF00</item>
</style>
However, you should always inherit your core app styles from the Android Support Library.
But if you don't inherit them from the android support library then too things will work the same!
Source: Explanation text and code taken from: https://developer.android.com/guide/topics/ui/look-and-feel/themes
You can read more about Styles and Themes from the above doc, from where I took the example to explain you!

TextAppearance.A inherits from TextAppearance.A.B doesn't cause a circular reference in android styles?

I know that
we have not to use parent attribute. We prefix one style to another
style separating by a period(.)
so in this style, does it have a circular inheritance?
<style name="TextAppearance.A" parent="TextAppearance.A.B">
<item name="android:textAlignment">viewStart</item>
<item name="android:gravity">start</item>
</style>
TextAppearance.A.B inherits from TextAppearance.A because of android dots' syntax.
but TextAppearance.A inherits from TextAppearance.A.B because of android paretn syntax.
Is it really a problem?

Technically As per Android Documentation I dont think this is possible,
Because this will lead to duplication of style, If you refer to same as Diamond Problem it will be one of those, also android prevents you from inheriting from more than one style.
Further Imagine if you have one attribute which is defined in style A also in Style B, it will be a problem at compile time that which attribute to choose from both.
For More Details please refer to android documentation
https://developer.android.com/guide/topics/ui/look-and-feel/themes

can we inherit `style_one.style_two.style_three` with having `style_one` and no `style_one.style_two` in android style? [duplicate]

I know that
we have not to use parent attribute. We prefix one style to another
style separating by a period(.)
so in this style, does it have a circular inheritance?
<style name="TextAppearance.A" parent="TextAppearance.A.B">
<item name="android:textAlignment">viewStart</item>
<item name="android:gravity">start</item>
</style>
TextAppearance.A.B inherits from TextAppearance.A because of android dots' syntax.
but TextAppearance.A inherits from TextAppearance.A.B because of android paretn syntax.
Is it really a problem?

Technically As per Android Documentation I dont think this is possible,
Because this will lead to duplication of style, If you refer to same as Diamond Problem it will be one of those, also android prevents you from inheriting from more than one style.
Further Imagine if you have one attribute which is defined in style A also in Style B, it will be a problem at compile time that which attribute to choose from both.
For More Details please refer to android documentation
https://developer.android.com/guide/topics/ui/look-and-feel/themes

How can I call the style of a new Library as parent in style.xml?

I am building a new library for Android and a demo project that I am using to test the library.
The library that I made has a style called Light and in the demo I am applying this style in Java using setTheme(R.style.Light) but I would like to replace it calling the theme directly in style.xml. What I have right now in the demo/res/values/style.xml is
<style name="AppTheme" parent="Theme.AppCompat.NoActionBar">
and I would like to replace it with something similar to:
<style name="AppTheme" parent="myLibrary.Light">
I have tried with parent="#style/Light" but it's not working.
Does anyone know how I can do it?

To inherit styles from a library or your own project, declare the parent style name without the #android:style/ part. For example, the following example inherits text appearance styles from the support library:
<style name="GreenText" parent="TextAppearance.AppCompat">
<item name="android:textColor">#00FF00</item>
</style>
For more details

I found the solution,
My problem was that in my library I had different styles depending on the version of Android. For each version of Android my main theme was referred to different parents by mistake. I just changed the parents in my library making sure that they are referring to the same one and it fixed my problem. Now in my app I can refer to the theme created in my library without crashing the application.

Override NumberPicker styling

Given an application which is themed via AppCompat (#style/Theme.AppCompat) and a layout using the framework's NumberPicker, is there a way to change its selectionDividersDistance[1]? Is this override even possible via XML, this is, without resorting to java reflection for example?
[1] selectionDividersDistance's styleable declaration)

You can use this library for doing it https://github.com/SimonVT/android-numberpicker. instead of styling and for styling
Override default NumberPicker style via activity theme:
<style name="AppTheme" parent="#style/Holo.Theme">
<item name="numberPickerStyle">#style/CustomNPStyle</item>
</style>
<style name="CustomNPStyle" parent="#style/Holo.NumberPicker">
<item name="selectionDivider">#drawable/custom_np_sd</item>
</style>
And apply #style/AppTheme as activity theme.

Answering my own question: it is not possible to change selectionDividersDistance, when using the framework's NumberPicker, solely by using XML to override it, since the attribute is not declared publicly by the Android framework, as seen here.
Here is a bit more context about how public attributes work, and a thorough explanation by Edward Falk about how resources are mapped