Recently, I have some need to add ripple effect which
was introduced following Material Design into a button that already has
background. The background is a selector. And I’ve found two
solution.

1. Use foreground

android:foreground=”?android:attr/selectableItemBackground” 

2. Use default attr background

• Ripples
  contained within the view: 
  android:background="?attr/selectableItemBackground"
• Ripples that
  extend beyond the view’s bounds: 
  android:background="?attr/selectableItemBackgroundBorderless"

So, use this selector in api 21 directory.

<ripple xmlns:android="http://schemas.android.com/apk/res/android" android:color="#DDDDDD">
    <item>
        <selector >
            <item android:state_pressed="true"
                  android:drawable="@color/theme_background_tab"/>
            <item android:state_activated="true"
                android:drawable="@color/theme_background_tab"/>
            <item android:drawable="@color/theme_white"/>
        </selector>

    </item>
</ripple>

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and in the normal directory

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true"
          android:drawable="@color/theme_background_tab"/>
    <item android:state_activated="true"
        android:drawable="@color/theme_background_tab"/>
    <item android:drawable="@color/theme_white"/>
</selector>

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This will let the android below api 21 have the desired
effect I wanted.

Also, I got this useful material:Using ?attr: instead of ?android:attr references the support library, so is available back to
API 7.

The Ripple Effect need API 21+, and the offical site tell us how to create custom effect
and color. Also, I found some great project in Android Arsenal, like RippleLayout and RippleEffect.Some effect is really beautiful.