How to reference style attributes from a drawable?

I want to have 2 selectable themes for my application. In order to do that, I defined some attributes, like this:

 <attr format="color" name="item_background" />

Then, I created both themes, like this:

  <style name="ThemeA">
     <item name="item_background">#123456</item>
 </style>

 <style name="ThemeB">
     <item name="item_background">#ABCDEF</item>
 </style>

This method works great, allowing me to create and modify several themes easily. The problem is that it seems that it can be used only in Views, and not in Drawables.

For example, referencing a value from a View inside a layout works:

 <TextView android:background="?item_background" />

But doing the same in a Drawable doesn't:

 <shape android:shape="rectangle">
     <solid android:color="?item_background" />
 </shape>

I get this error when running the application:

    java.lang.UnsupportedOperationException: Can't convert to color: type=0x2

If instead of ?item_background I use a hardcoded color, it works, but that doesn't allow me to use my themes. I also tried ?attr:item_background, but the same happens.

How could I do this? And why does it work in Views but not in Drawables? I can't find this limitation anywhere in the documentation...

In my experience it is not possible to reference an attribute in an XML drawable.
In order to make your theme you need to:

• Create one XML drawable per theme.

• Include the needed color into you drawable directly with the @color tag or #RGB format.

  http://schemas.android.com/apk/res/android">

Make an attribute for your drawable in attrs.xml.

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <!-- Attributes must be lowercase as we want to use them for drawables -->
   <attr name="my_drawable" format="reference" />
</resources>

Add your drawable to your theme.xml.

<style name="MyTheme" parent="@android:style/Theme.NoTitleBar">
   <item name="my_drawable">@drawable/my_drawable</item>
</style>

Reference your drawable in your layout using your attribute.

<TextView android:background="?my_drawable" />

Starting with lollipop (API 21) this feature is supported, see https://code.google.com/p/android/issues/detail?id=26251

However, if you're targeting devices without lollipop, don't use it, as it will crash, use the workaround in the accepted answer instead.

Although it's not possible to reference style attributes from drawables on pre-Lollipop devices, but it's possible for color state lists. You can use AppCompatResources.getColorStateList(Context context, int resId) method from Android Support Library. The downside is that you will have to set those color state lists programmatically.

Here is a very basic example.

color/my_color_state.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android">
  <item android:state_checked="true" android:color="?colorControlActivated" />
  <item android:color="?colorControlNormal" />
</selector>

A widget that needs a color state list:

<RadioButton
  android:id="@+id/radio_button"
  android:text="My Radio" />

And the most important:

ColorStateList csl = AppCompatResources.getColorStateList(context, R.color.my_color_state);    
RadioButton r = (RadioButton) findViewById(R.id.radio_button);
r.setTextColor(csl);

Well, not the most elegant or shortest way, but this is what Android Support Library does to make it work on older versions (pre-Lollipop) of Android.

Unfortunately, the similar method for drawables doesn't work with style attributes.

I answered the same question in https://stackoverflow.com/a/59467269/3841352 but i will post it here as well:

I encountered the same problem and as of 2019 it hasn't been resolved so you can't have an attribute referenced in a selector as a drawable. I will share the solution I got for the problem as I don't see it posted in here. I found it in the last comment of the bug report.

The workaround is basically create a drawable resource that will be the one referring the attribute value.

To illustrate your case the solution would be instead of:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="?attr/colorPrimary" android:state_enabled="true" android:state_window_focused="false"/>
    <item android:drawable="?attr/colorPrimaryDark" android:state_pressed="true"/>
    <item android:drawable="@android:color/darker_gray" android:state_enabled="false"/>
    <item android:drawable="?attr/colorPrimary"/>
</selector>

you would replace the ?attr/* for a drawable resource:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/colorPrimaryDrawable" android:state_enabled="true" android:state_window_focused="false"/>
    <item android:drawable="@drawable/colorPrimaryDarkDrawable" android:state_pressed="true"/>
    <item android:drawable="@android:color/darker_gray" android:state_enabled="false"/>
    <item android:drawable="@drawable/colorPrimaryDrawable"/>
</selector>

The drawables would be defined as:

drawable/colorPrimaryDrawable

<shape xmlns:android="http://schemas.android.com/apk/res/android"android:shape="rectangle">
    <solid android:color="?attr/colorPrimary" />
</shape>

drawable/colorPrimaryDarkDrawable

<shape xmlns:android="http://schemas.android.com/apk/res/android"android:shape="rectangle">
    <solid android:color="?attr/colorPrimaryDark" />
</shape>

Hope it helps!!

As @marmor stated this is now supported on API 21. But for those us who need to support legacy versions of Android, you can use this feature. Using the v7 support library you can still use it on apps with minimum SDK level all the way down to 7.

The AppCompatImageView in the v7 Android Support Library has a bug free implementation of this feature. Simply replace your usages of ImageView with AppCompatImageView.