Support library for text color behaviour of API 26

API 26 introduces an advanced color calculation for ?textColorPrimary based on ?colorForeground. It makes use of states, primaryContentAlpha and disabledAlpha.

sdk/platforms/android-26/data/res/color/text_color_primary.xml:

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="false"
        android:alpha="?attr/disabledAlpha"
        android:color="?attr/colorForeground"/>
    <item android:alpha="?attr/primaryContentAlpha"
        android:color="?attr/colorForeground"/>
</selector>

On API 23 it falls back to white text by means I failed to figure out.

Is there a support library I could apply to get the color calculation of API 26 for older devices?

@eugen-pechanec is hinting that the attributes primaryContentAlpha and scondaryContentAlpha are missing, IMHO below API 26. Should we call this a bug or a missing back port? Don't know.

The consequence is that you can't use the setting ?attr/colorForeground as a default to automatically create all foreground colours out of the box. You basically have two options, either not to use it to to do a manual back port.

Disable `colorForground`

Instead of generating the colours from ?attr/colorForeground you set the attributes android:textColorPrimary and android:textColorSecondary directly. This will be the best choice in most cases.

Backport `colorForground`

If you plan to use a lot of different themes, you want to enable the feature to set defaults for all text colours in a central place. Then you have to implement the behaviour of API 26 in your root theme.

root theme:

    <!-- workaround to port back API 26+ behaviour -->

    <!-- below 26 these two attributes are missing in the android namespace -->
    <item name="primaryContentAlpha">1.0</item> 
    <item name="secondaryContentAlpha">.85</item>
    <!-- works below 26 -->
    <item name="android:disabledAlpha">.4</item>
    <!-- use my own files to connect my custom attributes -->
    <item name="android:textColorPrimary">@color/text_color_primary</item>
    <item name="android:textColorSecondary">@color/text_color_secondary</item>

app/src/main/res/color/text_color_primary.xml:

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="false" android:alpha="?android:disabledAlpha" android:color="?android:attr/colorForeground" />
    <item android:alpha="?attr/primaryContentAlpha" android:color="?android:attr/colorForeground" />
</selector>

app/src/main/res/color/text_color_secondary.xml:

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="false" android:alpha="?android:disabledAlpha" android:color="?android:colorForeground"/>
    <item android:alpha="?secondaryContentAlpha" android:color="?android:colorForeground"/>
</selector>