Support library for text color behaviour of API 26

Question

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?

Answer 1

@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>