Cascade the value of a custom attribute from a parent view to a child view?

Question

How do I "cascade" the value of a custom attribute from a parent view to its child view?

This is easiest to explain using an example:

<com.example.CustomLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    app:percent="35" >

    <com.example.CustomView
        android:id="@+id/customView1"
        app:percent="how-to-get-app:percent-value-here???"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</com.example.CustomLayout>

Here, CustomLayout extends LinearLayout. I have defined a custom attribute "percent" in attrs.xml using <declare-styleable> element. As you can see, I set percent to 35 in the XML for CustomLayout.

I now want to pass in the same value to CustomView (which extends View) and which I will include in CustomLayout. I am unable to find a way to do this in XML (it is easy to do this in code though).

I tried the following:

app:percent="@attr/percent"

app:percent="?attr/percent"

Both of these (expectedly) fail with NumberFormatException at TypedArray#getInt().

So, any ideas on how to get this to work?

Answer 1

Although the idea comes a bit late and the method is not straight forward, I think it's still worth sharing. We can put custom attributes into a theme, so the attributes can be passed to all child views from the parent view where the theme is used (i.e., the attributes exist within the View Group).

Example as follows:

<integer name="percentage">35</integer>

<style name="CustomTheme" parent="suitable theme for your case">
    <item name="percent">@integer/percentage</item>
</style>

<com.example.CustomLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:theme="@style/CustomTheme" >

    <com.example.CustomView
        android:id="@+id/customView1"
        app:percent="?attr/percent" <!--Note: that's how it refers to the value,
        however, re-assign the attribute value here is meaningless as attribute percent
        should exist in all child views now. You can retrieve its value via
        Theme.obtainStyledAttributes(R.style.CustomTheme, new int[] {R.attr.percent})
        in every child view-->
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</com.example.CustomLayout>