How does the Android L contacts app collapse its toolbar?

Question

I've been trying to reproduce the way that the Contacts app on version 5.0 collapses the toolbar when the listview is scrolled.

Gallery of screenshots demonstrating the desired interaction Note the collapse of the toolbar in stages, where it displays search+last contact, fades last contact, collapses last contact, collapses search, leaving only the tabs.

So far, I have a toolbar sitting above a recyclerview in a LinearLayout, and the toolbar is used as an actionbar, not standalone.

I can't figure out how to intercept the touch event on the recyclerview and make it shrink the toolbar, and then return the scroll event to the recyclerview. I tried putting the entire thing in a scrollview, but then the recyclerview couldn't calculate it's height properly and displayed no content. I tried overriding onscroll on the recyclerview, and found that it will only notify me when a scroll event started, and provide me with the first visible card id.

The way that looks right, but I can't get working for the life of me, is this:

getSupportActionBar().setHideOnContentScrollEnabled(true);

Which returns:

 Caused by: java.lang.UnsupportedOperationException: Hide on content scroll is not supported in this action bar configuration.

Using a traditional actionbar, putting a toolbar below it, and setting hideoncontentscrollenabled also didn't work, scrolling never triggered the hide method on the actionbar.

-- edit -- I was able to get hideOnContentScrollEnabled working on a listview with a traditional actionbar, but the behavior is not the same as the contacts app. This is clearly not the method they used-- it simply triggers .hide() on the actionbar when a fling event occurs on a listview, which is notably different from the contacts app, which drags the toolbar along with the scroll event. -- /edit --

So I abandoned that route, and put fill_parent on the cardview height, and animated a collapse on the toolbar. But how do I trigger it so that it follows the touch event and then returns the touch event to the recyclerview?

activity_main.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    >

    <android.support.v7.widget.Toolbar
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:minHeight="?android:attr/actionBarSize"
        android:background="@color/colorPrimary"
        />

    <fragment android:name="me.myapplication.FragmentTab"
          android:id="@+id/tab_fragment"
          android:layout_width="match_parent"
          android:layout_height="wrap_content" />
</LinearLayout>

fragment_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center_horizontal"
    android:orientation="vertical"
    android:padding="8dp"
    android:background="#eeeeee"
    >

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="fill_parent"
        />

</LinearLayout>

styles.xml

...
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
...

MainActivity.java

Toolbar toolbar = (Toolbar)findViewById(R.id.toolbar);

// Disable the logo in the actionbar, as per material guidelines
toolbar.getMenu().clear();
toolbar.setTitle("My toolbar");
setSupportActionBar(toolbar);

Answer 1

I haven't investigated the source code yet but this guy seems to have made life easy yet enlightening.

https://github.com/ksoichiro/Android-ObservableScrollView

EDIT

Google has just released Android Design Library. Please take a look as it contains all the effects of collapsing toolbars and much more.