How to customize the Action Bar subtitle Font?

Question

I have created an ActionBar (android.support.v7.widget.Toolbar) as below.

Answer 1

If you don't already have this namespace attribute in your layout file add it:

xmlns:app="http://schemas.android.com/apk/res-auto"

Update your Toolbar attributes in your layout file like this:

<android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/myToolbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:titleTextAppearance="@style/ToolbarTitleAppearance"
    app:subtitleTextAppearance="@style/ToolbarSubtitleAppearance"
    android:minHeight="?attr/actionBarSize">
</android.support.v7.widget.Toolbar>

Add styles like these:

<style name="ToolbarTitleAppearance" parent="@style/TextAppearance.Widget.AppCompat.Toolbar.Title">
    <item name="android:textSize">20dp</item>
    <!-- include other attributes you want to change: textColor, textStyle, etc -->
</style>

<style name="ToolbarSubtitleAppearance" parent="@style/TextAppearance.Widget.AppCompat.Toolbar.Subtitle">
    <item name="android:textSize">14dp</item>
</style>

Comments on this answer discuss whether the units for textSize should be dp or sp. When I originally wrote this answer, I looked at the Android source files and saw that dp was used. I recommend staying with that. As FrancescoDonzello explains in his comment, the size of the Toolbar is fixed and, unlike other widgets, will not expand to contain text enlarged by changes to the phone settings.