Variables in XML resources - pass values from parent to child

Question

I'm trying to reuse primitive shapes and compose much of my user interface using these declarative XML elements.

How to make a variable Android attribute?

But I do not want to create a separate XML file for each attribute value, and their permutations, and in the process duplicate much of the work.

For instance, I would like the consumer of this shape be able to define the android:radius value?

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
            android:startColor="#449def"
            android:endColor="#2f6699"
            android:angle="270"/>
    <stroke
            android:width="1dp"
            android:color="#2f6699"/>
    <corners
            android:radius="3dp"/>
</shape>

Set attribute from consuming XML parent?

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true" android:drawable="@drawable/shape_box_round_blue_uniform" />
    <!-- How to set the corner radius here? -->
    <item android:drawable="@drawable/shape_box_round_blue" />
</selector>

Solutions?

• If at all possible, I would like to not use any Java code-behind / avoid the need to create custom controls / classes
• Using dimension resources may be a fruitful avenue?

Answer 1

You can create

style attributes

, which I think is what you are looking for. They basically are variable attributes. E.g. you can use this in your themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- You can define attributes here -->
    <attr name="button_radius" format="reference" />
</resources>

Which defines a variable reference called: button_radius This can be used in Styles, or in layout xml files:

<!-- You can use them like so: -->
<style name="MyApp.Theme1" parent="android:Theme.Holo.Light">
    <item name="button_radius">12</item>
</style>

<style name="MyApp.Theme2" parent="android:Theme.Holo.Light">
    <item name="button_radius">36</item>
</style>

This way, by changing the theme, you can use the different values for your radius. Below is an UNTESTED example of how you would change your shape drawable:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:startColor="#449def"
        android:endColor="#2f6699"
        android:angle="270"/>
    <stroke
        android:width="1dp"
        android:color="#2f6699"/>
    <corners
        android:radius="?button_radius"/> <!-- NOTE the ?button_radius-->
</shape>

Users can simply apply a different style to use different attributes. I don't know if this example answers your question completele, but there's a lot you can do by declaring attributes in your themes. These attributes are dynamic references. For more info about the possibilities see this article