Android L's Ripple Effect - Touch Feedback for Buttons - Using XML

UPDATE Material Components:

With the Material Components Library it is very easy to apply a ripple.
Just use the MaterialButton and the app:rippleColor attribute:

<com.google.android.material.button.MaterialButton
    app:rippleColor="@color/my_selector"
    ../>

With a selector like this:

<selector xmlns:android="http://schemas.android.com/apk/res/android">

  <item android:alpha="..." android:color="?attr/colorOnPrimary" android:state_pressed="true"/>
  <item android:alpha="..." android:color="?attr/colorOnPrimary" android:state_focused="true" android:state_hovered="true"/>
  <item android:alpha="..." android:color="?attr/colorOnPrimary" android:state_focused="true"/>
  <item android:alpha="..." android:color="?attr/colorOnPrimary" android:state_hovered="true"/>
  <item android:alpha="..." android:color="?attr/colorOnPrimary"/>

</selector>

Old answer
You can do something like this:

<Button
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:background="@drawable/ripple"
      
    />

Where the ripple.xml is:

<ripple xmlns:android="http://schemas.android.com/apk/res/android" 
                      android:color="?android:colorControlHighlight">
        <item android:id="@android:id/mask">
            <shape android:shape="oval">
                <solid android:color="?android:colorAccent" />
            </shape>
        </item>
 </ripple>

Just put ?attr/selectableItemBackground in the background of button for API 21+ , like below:

<Button
        android:layout_width="match_parent"
        android:layout_height="70dp"
        android:background="?attr/selectableItemBackground"
        android:text="Button" />

For lollipop(API>21) make file as btn_ripple_effect.xml in drawable and put below code

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

<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:color="?android:colorAccent"
    tools:targetApi="lollipop">
    <item android:drawable="@color/cancel_btn_clr" /> <!-- default -->
    <item android:id="@android:id/mask">
        <shape android:shape="rectangle">
            <solid android:color="?android:colorAccent" />
        </shape>
    </item>
</ripple>

For pre lollipop (API<21)make file as btn_ripple_effect.xml in drawable-v21 folder and put below code

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_pressed="true">
        <shape>
            <solid android:color="@color/colorAccent"></solid>
        </shape>
    </item>

    <item>
        <shape>
            <solid android:color="@color/cancel_btn_clr"></solid>
        </shape>
    </item>

</selector>

Slight addition to above answer: Note that the mask color is not used in any way.

You can do more complicated things with ripple as well. For example, if you wanted a border on your ripple button you can use it like a layer-list.

<ripple
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:color="?android:colorControlHighlight">

    <!-- Note: <ripple> acts like a layer-list -->
    <item android:id="@android:id/mask">
        <shape android:shape="oval">
            <!-- This color is not displayed in any way -->
            <solid android:color="@android:color/black" />
        </shape>
    </item>

    <!-- This is the border -->
    <item>
        <shape android:shape="rectangle">
            <corners android:radius="3dp"/>
            <!-- Use your border color in place of #f00 -->
            <stroke android:width="1dp" android:color="#f00"/>
        </shape>
    </item>
 </ripple>

Note that the element with id @android:id/mask is only used to show where the ripple effect will stop at. If you wanted it to cover the whole button, you could change the android:shape to be rectangle. You can imagine doing many more interesting things with this as well!

Also make sure to have a backup drawable for devices that aren't 21 yet or the app will crash on old devices.

The best way to use this in android:foreground, because it allows you use own background also.

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

Example:

<android.support.v7.widget.AppCompatButton
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:foreground="?android:attr/selectableItemBackground"
    android:background="@color/button.normal"
    android:textColor="@color/white"/>