Develop Reference

How to avoid CollapsingToolbarLayout not being snapped or being "wobbly" when scrolling?

Background
Suppose you have an app you've created that has a similar UI as the one you can create via the wizard of "scrolling activity", yet you wish the scrolling flags to have snapping, as such:
<android.support.design.widget.CollapsingToolbarLayout ... app:layout_scrollFlags="scroll|exitUntilCollapsed|snap" >
The problem
As it turns out, on many cases it has issues of snapping. Sometimes the UI doesn't snap to top/bottom, making the CollapsingToolbarLayout stay in between.
Sometimes it also tries to snap to one direction, and then decides to snap to the other .
You can see both issues on the attached video here.
What I've tried
I thought it's one of the issues that I got for when I use setNestedScrollingEnabled(false) on a RecyclerView within, so I asked about it here, but then I noticed that even with the solution and without using this command at all and even when using a simple NestedScrollView (as is created by the wizard), I can still notice this behavior.
That's why I decided to report about this as an issue, here.
Sadly, I couldn't find any workaround for those weird bugs here on StackOverflow.
The question
Why does it occur, and more importantly: how can I avoid those issues while still using the behavior it's supposed to have?
EDIT: here's a nice improved Kotlin version of the accepted answer:
class RecyclerViewEx #JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyle: Int = 0) : RecyclerView(context, attrs, defStyle) {
private var mAppBarTracking: AppBarTracking? = null
private var mView: View? = null
private var mTopPos: Int = 0
private var mLayoutManager: LinearLayoutManager? = null
interface AppBarTracking {
fun isAppBarIdle(): Boolean
fun isAppBarExpanded(): Boolean
}
override fun dispatchNestedPreScroll(dx: Int, dy: Int, consumed: IntArray?, offsetInWindow: IntArray?, type: Int): Boolean {
if (mAppBarTracking == null)
return super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking!!.isAppBarIdle()
&& isNestedScrollingEnabled) {
if (dy > 0) {
if (mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
} else {
mTopPos = mLayoutManager!!.findFirstVisibleItemPosition()
if (mTopPos == 0) {
mView = mLayoutManager!!.findViewByPosition(mTopPos)
if (-mView!!.top + dy <= 0) {
consumed!![1] = dy - mView!!.top
return true
}
}
}
}
if (dy < 0 && type == ViewCompat.TYPE_TOUCH && mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
val returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (offsetInWindow != null && !isNestedScrollingEnabled && offsetInWindow[1] != 0)
offsetInWindow[1] = 0
return returnValue
}
override fun setLayoutManager(layout: RecyclerView.LayoutManager) {
super.setLayoutManager(layout)
mLayoutManager = layoutManager as LinearLayoutManager
}
fun setAppBarTracking(appBarTracking: AppBarTracking) {
mAppBarTracking = appBarTracking
}
fun setAppBarTracking(appBarLayout: AppBarLayout) {
val appBarIdle = AtomicBoolean(true)
val appBarExpanded = AtomicBoolean()
appBarLayout.addOnOffsetChangedListener(object : AppBarLayout.OnOffsetChangedListener {
private var mAppBarOffset = Integer.MIN_VALUE
override fun onOffsetChanged(appBarLayout: AppBarLayout, verticalOffset: Int) {
if (mAppBarOffset == verticalOffset)
return
mAppBarOffset = verticalOffset
appBarExpanded.set(verticalOffset == 0)
appBarIdle.set(mAppBarOffset >= 0 || mAppBarOffset <= -appBarLayout.totalScrollRange)
}
})
setAppBarTracking(object : AppBarTracking {
override fun isAppBarIdle(): Boolean = appBarIdle.get()
override fun isAppBarExpanded(): Boolean = appBarExpanded.get()
})
}
override fun fling(velocityX: Int, inputVelocityY: Int): Boolean {
var velocityY = inputVelocityY
if (mAppBarTracking != null && !mAppBarTracking!!.isAppBarIdle()) {
val vc = ViewConfiguration.get(context)
velocityY = if (velocityY < 0) -vc.scaledMinimumFlingVelocity
else vc.scaledMinimumFlingVelocity
}
return super.fling(velocityX, velocityY)
}
}

Update
I have changed the code slightly to address remaining issues - at least the ones that I can reproduce. The key update was to dispose of dy only when the AppBar is expanded or collapsed. In the first iteration, dispatchNestedPreScroll() was disposing of scroll without checking the status of the AppBar for a collapsed state.
Other changes are minor and fall under the category of clean up. The code blocks are updated below.
This answer addresses the question's issue regarding RecyclerView. The other answer I have given still stands and applies here. RecyclerView has the same issues as NestedScrollView that were introduced in 26.0.0-beta2 of the support libraries.
The code below is base upon this answer to a related question but includes the fix for the erratic behavior of the AppBar. I have removed the code that fixed the odd scrolling because it no longer seems to be needed.
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
MyRecyclerView.java
public class MyRecyclerView extends RecyclerView {
public MyRecyclerView(Context context) {
this(context, null);
}
public MyRecyclerView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyRecyclerView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
private AppBarTracking mAppBarTracking;
private View mView;
private int mTopPos;
private LinearLayoutManager mLayoutManager;
#Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
// Only dy < 0 will open the AppBar. Stop it from opening by consuming dy if needed.
mTopPos = mLayoutManager.findFirstVisibleItemPosition();
if (mTopPos == 0) {
mView = mLayoutManager.findViewByPosition(mTopPos);
if (-mView.getTop() + dy <= 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy - mView.getTop();
return true;
}
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2.
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
offsetInWindow[1] = 0;
}
return returnValue;
}
#Override
public void setLayoutManager(RecyclerView.LayoutManager layout) {
super.setLayoutManager(layout);
mLayoutManager = (LinearLayoutManager) getLayoutManager();
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
#SuppressWarnings("unused")
private static final String TAG = "MyRecyclerView";
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity
implements AppBarTracking {
private MyRecyclerView mNestedView;
private int mAppBarOffset;
private boolean mAppBarIdle = false;
private int mAppBarMaxOffset;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mNestedView = findViewById(R.id.nestedView);
final AppBarLayout appBar = findViewById(R.id.app_bar);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
#Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
appBar.post(new Runnable() {
#Override
public void run() {
mAppBarMaxOffset = -appBar.getTotalScrollRange();
}
});
findViewById(R.id.disableNestedScrollingButton).setOnClickListener(new OnClickListener() {
#Override
public void onClick(final View v) {
// If the AppBar is fully expanded or fully collapsed (idle), then disable
// expansion and apply the patch; otherwise, set a flag to disable the expansion
// and apply the patch when the AppBar is idle.
setExpandEnabled(false);
}
});
findViewById(R.id.enableNestedScrollingButton).setOnClickListener(new OnClickListener() {
#Override
public void onClick(final View v) {
setExpandEnabled(true);
}
});
mNestedView.setAppBarTracking(this);
mNestedView.setLayoutManager(new LinearLayoutManager(this));
mNestedView.setAdapter(new Adapter() {
#Override
public ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(
android.R.layout.simple_list_item_1,
parent,
false)) {
};
}
#SuppressLint("SetTextI18n")
#Override
public void onBindViewHolder(final ViewHolder holder, final int position) {
((TextView) holder.itemView.findViewById(android.R.id.text1)).setText("item " + position);
}
#Override
public int getItemCount() {
return 100;
}
});
}
private void setExpandEnabled(boolean enabled) {
mNestedView.setNestedScrollingEnabled(enabled);
}
#Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
#Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
#SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
What is happening here?
From the question, it was apparent that the layout was failing to snap the app bar closed or open as it should when the user's finger was not on the screen. When dragging, the app bar behaves as it should.
In version 26.0.0-beta2, some new methods were introduced - specifically dispatchNestedPreScroll() with a new type argument. The type argument specifies if the movement specified by dx and dy are due to the user touching the screen ViewCompat.TYPE_TOUCH or not ViewCompat.TYPE_NON_TOUCH.
Although the specific code that causes the problem was not identified, the tack of the fix is to kill vertical movement in dispatchNestedPreScroll() (dispose of dy) when needed by not letting vertical movement propagate. In effect, the app bar is to be latched into place when expanded and will not allowed to start to close until it is closing through a touch gesture. The app bar will also be latched when closed until the RecyclerView is positioned at its topmost extent and there is sufficient dy to open the app bar while performing a touch gesture.
So, this is not so much a fix as much as a discouragement of problematic conditions.
The last part of the MyRecyclerView code deals with an issue that was identified in this question dealing with improper scroll movements when nested scrolling is disabled. This is the part that comes after the call to the super of dispatchNestedPreScroll() that changes the value of offsetInWindow[1]. The thinking behind this code is the same as presented in the accepted answer for the question. The only difference is that since the underlying nested scrolling code has changed, the argument offsetInWindow is sometime null. Fortunately, it seems to be non-null when it matters, so the last part continues to work.
The caveat is that this "fix" is very specific to the question asked and is not a general solution. The fix will likely have a very short shelf life since I expect that such an obvious problem will be addressed shortly.

Looks like onStartNestedScroll and onStopNestedScroll calls can be reordered and it lead to "wobbly" snap. I made a small hack inside AppBarLayout.Behavior. Don't really want to mess up with all that stuff in activity as proposed by other answers.
#SuppressWarnings("unused")
public class ExtAppBarLayoutBehavior extends AppBarLayout.Behavior {
private int mStartedScrollType = -1;
private boolean mSkipNextStop;
public ExtAppBarLayoutBehavior() {
super();
}
public ExtAppBarLayoutBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
}
#Override
public boolean onStartNestedScroll(CoordinatorLayout parent, AppBarLayout child, View directTargetChild, View target, int nestedScrollAxes, int type) {
if (mStartedScrollType != -1) {
onStopNestedScroll(parent, child, target, mStartedScrollType);
mSkipNextStop = true;
}
mStartedScrollType = type;
return super.onStartNestedScroll(parent, child, directTargetChild, target, nestedScrollAxes, type);
}
#Override
public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, AppBarLayout abl, View target, int type) {
if (mSkipNextStop) {
mSkipNextStop = false;
return;
}
if (mStartedScrollType == -1) {
return;
}
mStartedScrollType = -1;
// Always pass TYPE_TOUCH, because want to snap even after fling
super.onStopNestedScroll(coordinatorLayout, abl, target, ViewCompat.TYPE_TOUCH);
}
}
Usage in XML layout:
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.ExtAppBarLayoutBehavior">
<!-- Put here everything you usually add to AppBarLayout: CollapsingToolbarLayout, etc... -->
</android.support.design.widget.AppBarLayout>
<!-- Content: recycler for example -->
<android.support.v7.widget.RecyclerView
app:layout_behavior="#string/appbar_scrolling_view_behavior" />
...
</android.support.design.widget.CoordinatorLayout>
It is very likely that the root cause of the problem in the RecyclerView. Do not have an opportunity to dig deeper now.

Edit The code has been updated to bring it more in line with the code for the accepted answer. This answer concerns NestedScrollView while the accepted answer is about RecyclerView.
This is an issue what was introduced in the API 26.0.0-beta2 release. It does not happen on the beta 1 release or with API 25. As you noted, it also happens with API 26.0.0. Generally, the problem seems to be related to how flings and nested scrolling are handled in beta2. There was a major rewrite of nested scrolling (see "Carry on Scrolling"), so it is not surprising that this type of issue has cropped up.
My thinking is that excess scroll is not being disposed of properly somewhere in NestedScrollView. The work-around is to quietly consume certain scrolls that are "non-touch" scrolls (type == ViewCompat.TYPE_NON_TOUCH) when the AppBar is expanded or collapsed. This stops the bouncing, allows snaps and, generally, makes the AppBar better behaved.
ScrollingActivity has been modified to track the status of the AppBar to report whether it is expanded or not. A new class call "MyNestedScrollView" overrides dispatchNestedPreScroll() (the new one, see here) to manipulate the consumption of the excess scroll.
The following code should suffice to stop AppBarLayout from wobbling and refusing to snap. (XML will also have to change to accommodate MyNestedSrollView. The following only applies to support lib 26.0.0-beta2 and above.)
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity implements AppBarTracking {
private int mAppBarOffset;
private int mAppBarMaxOffset;
private MyNestedScrollView mNestedView;
private boolean mAppBarIdle = true;
#Override
protected void onCreate(Bundle savedInstanceState) {
AppBarLayout appBar;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
final Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
appBar = findViewById(R.id.app_bar);
mNestedView = findViewById(R.id.nestedScrollView);
mNestedView.setAppBarTracking(this);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
#Override
public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
}
});
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
#Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
mNestedView.post(new Runnable() {
#Override
public void run() {
mAppBarMaxOffset = mNestedView.getMaxScrollAmount();
}
});
}
#Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
#Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
#Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_scrolling, menu);
return true;
}
#Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
#SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
MyNestedScrollView.java
public class MyNestedScrollView extends NestedScrollView {
public MyNestedScrollView(Context context) {
this(context, null);
}
public MyNestedScrollView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyNestedScrollView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
setOnScrollChangeListener(new View.OnScrollChangeListener() {
#Override
public void onScrollChange(View view, int x, int y, int oldx, int oldy) {
mScrollPosition = y;
}
});
}
private AppBarTracking mAppBarTracking;
private int mScrollPosition;
#Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
if (mScrollPosition + dy < 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy + mScrollPosition;
return true;
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2. (Not sure that this is a problem for 26.0.0-beta2 and later.)
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
Log.d(TAG, "<<<<offsetInWindow[1] forced to zero");
offsetInWindow[1] = 0;
}
return returnValue;
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
#SuppressWarnings("unused")
private static final String TAG = "MyNestedScrollView";
}

Since the issue is still not fixed as of February 2020 (latest material library version is 1.2.0-alpha5) I want to share my solution to the buggy AppBar animation.
The idea is to implmenet custom snapping logic by extending AppBarLayout.Behavior (Kotlin version):
package com.example
import android.content.Context
import android.os.Handler
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import androidx.coordinatorlayout.widget.CoordinatorLayout
import com.google.android.material.appbar.AppBarLayout
import com.google.android.material.appbar.AppBarLayout.LayoutParams
#Suppress("unused")
class AppBarBehaviorFixed(context: Context?, attrs: AttributeSet?) :
AppBarLayout.Behavior(context, attrs) {
private var view: AppBarLayout? = null
private var snapEnabled = false
private var isUpdating = false
private var isScrolling = false
private var isTouching = false
private var lastOffset = 0
private val handler = Handler()
private val snapAction = Runnable {
val view = view ?: return#Runnable
val offset = -lastOffset
val height = view.run { height - paddingTop - paddingBottom - getChildAt(0).minimumHeight }
if (offset > 1 && offset < height - 1) view.setExpanded(offset < height / 2)
}
private val updateFinishDetector = Runnable {
isUpdating = false
scheduleSnapping()
}
private fun initView(view: AppBarLayout) {
if (this.view != null) return
this.view = view
// Checking "snap" flag existence (applied through child view) and removing it
val child = view.getChildAt(0)
val params = child.layoutParams as LayoutParams
snapEnabled = params.scrollFlags hasFlag LayoutParams.SCROLL_FLAG_SNAP
params.scrollFlags = params.scrollFlags removeFlag LayoutParams.SCROLL_FLAG_SNAP
child.layoutParams = params
// Listening for offset changes
view.addOnOffsetChangedListener(AppBarLayout.OnOffsetChangedListener { _, offset ->
lastOffset = offset
isUpdating = true
scheduleSnapping()
handler.removeCallbacks(updateFinishDetector)
handler.postDelayed(updateFinishDetector, 50L)
})
}
private fun scheduleSnapping() {
handler.removeCallbacks(snapAction)
if (snapEnabled && !isUpdating && !isScrolling && !isTouching) {
handler.postDelayed(snapAction, 50L)
}
}
override fun onLayoutChild(
parent: CoordinatorLayout,
abl: AppBarLayout,
layoutDirection: Int
): Boolean {
initView(abl)
return super.onLayoutChild(parent, abl, layoutDirection)
}
override fun onTouchEvent(
parent: CoordinatorLayout,
child: AppBarLayout,
ev: MotionEvent
): Boolean {
isTouching =
ev.actionMasked != MotionEvent.ACTION_UP && ev.actionMasked != MotionEvent.ACTION_CANCEL
scheduleSnapping()
return super.onTouchEvent(parent, child, ev)
}
override fun onStartNestedScroll(
parent: CoordinatorLayout,
child: AppBarLayout,
directTargetChild: View,
target: View,
nestedScrollAxes: Int,
type: Int
): Boolean {
val started = super.onStartNestedScroll(
parent, child, directTargetChild, target, nestedScrollAxes, type
)
if (started) {
isScrolling = true
scheduleSnapping()
}
return started
}
override fun onStopNestedScroll(
coordinatorLayout: CoordinatorLayout,
abl: AppBarLayout,
target: View,
type: Int
) {
isScrolling = false
scheduleSnapping()
super.onStopNestedScroll(coordinatorLayout, abl, target, type)
}
private infix fun Int.hasFlag(flag: Int) = flag and this == flag
private infix fun Int.removeFlag(flag: Int) = this and flag.inv()
}
And now apply this behavior to the AppBarLayout in xml:
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.AppBarBehaviorFixed">
<com.google.android.material.appbar.CollapsingToolbarLayout
app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">
<!-- Toolbar declaration -->
</com.google.android.material.appbar.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<!-- Scrolling view (RecyclerView, NestedScrollView) -->
</android.support.design.widget.CoordinatorLayout>
That is still a hack but it seems to work quite well, and it does not require to put dirty code into your activity or extend RecyclerView and NestedScrollView widgets (thanks to #vyndor for this idea).

Android LinearSnapHelper - how to increase scrolling/"snapping" speed?

I'm using a LinearSnapHelper to make items in my RecyclerView "snap" into place on the screen (my cards take up most of the screen, so I want them to snap into place and fill the screen on every swipe/fling/scroll).
I'm struggling with how to make the cards snap into place faster. I've tried creating a custom LinearLayoutManager (and editing the calculateSpeedPerPixel method in scrollToPosition or smoothScrollToPosition), as well as a custom RecyclerView (and editing the fling method). But nothing effects the speed that cards "snap" into place.
I suppose the issue is that I don't really understand how LinearSnapHelper "scrolls" the cards into position. It doesn't seem to use LinearLayoutManager's scrollToPosition or smoothScrollToPosition methods.
snapHelper = new LinearSnapHelper() {
#Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
View centerView = findSnapView(layoutManager);
if (centerView == null) {
return RecyclerView.NO_POSITION;
}
int position = layoutManager.getPosition(centerView);
int targetPosition = -1;
if (layoutManager.canScrollHorizontally()) {
if (velocityX < 0) {
targetPosition = position - 1;
} else {
targetPosition = position + 1;
}
}
if (layoutManager.canScrollVertically()) {
if (velocityY > 0) {
targetPosition = position + 1;
} else {
targetPosition = position - 1;
}
}
final int firstItem = 0;
final int lastItem = layoutManager.getItemCount() - 1;
targetPosition = Math.min(lastItem, Math.max(targetPosition, firstItem));
return targetPosition;
}
};
snapHelper.attachToRecyclerView(mRecyclerView);

As 郭玉龙 mentioned, SnapHelper call RecyclerView.smoothScrollBy() method. And it use default sQuinticInterpolator.
To change speed of snap you can do next:
public class SlowdownRecyclerView extends RecyclerView {
// Change pow to control speed.
// Bigger = faster. RecyclerView default is 5.
private static final int POW = 2;
private Interpolator interpolator;
public SlowdownRecyclerView(Context context) {
super(context);
createInterpolator();
}
public SlowdownRecyclerView(Context context, #Nullable AttributeSet attrs) {
super(context, attrs);
createInterpolator();
}
public SlowdownRecyclerView(Context context, #Nullable AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
createInterpolator();
}
private void createInterpolator(){
interpolator = new Interpolator() {
#Override
public float getInterpolation(float t) {
t = Math.abs(t - 1.0f);
return (float) (1.0f - Math.pow(t, POW));
}
};
}
#Override
public void smoothScrollBy(int dx, int dy) {
super.smoothScrollBy(dx, dy, interpolator);
}
Or you can implement your own interpolator.

The speed of snapping scroll is affected by RecyclerView.smoothScrollBy().
Here's the snippet of source code.
Override this function to increase or decrease the speed of snapping scroll.

I wound up doing this by adding a ScrollListener to my RecycleView, and then creating a custom LinearLayoutManager and custom smoothScrollToPosition method.
final CustomLinearLayoutManager mLayoutManager = new CustomLinearLayoutManager(getActivity());
mRecyclerView.setLayoutManager(mLayoutManager);
mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
private boolean scrollingUp;
#Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
scrollingUp = dy < 0;
}
#Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
int visiblePosition = scrollingUp ? mLayoutManager.findFirstVisibleItemPosition() : mLayoutManager.findLastVisibleItemPosition();
int completelyVisiblePosition = scrollingUp ? mLayoutManager
.findFirstCompletelyVisibleItemPosition() : mLayoutManager
.findLastCompletelyVisibleItemPosition();
if (visiblePosition != completelyVisiblePosition) {
recyclerView.smoothScrollToPosition(visiblePosition);
return;
}
}
});

I achieved this using a library https://github.com/rubensousa/GravitySnapHelper
you can also override findTargetSnapPosition to get pager like scroll
tweek the scrollMsPerInch to increase / decrease speed
val snapHelper : GravitySnapHelper = GravitySnapHelper(Gravity.CENTER)
// the lower the higher the speed, default is 100f
snapHelper.scrollMsPerInch = 40f
snapHelper.attachToRecyclerView(binding?.mRecyclerView)

Actually you can modify the LinearSnapHelper and SnapHelperClass by simply copy/paste the existing code the only thing you will do is to set MILLISECONDS_PER_INCH on SnapHelper as you want and then use simply use the LinearSnapHelper you created

Custom CoordinatorLayout.Behavior and RecyclerView scroll issue

I have a CoordinatorLayout with two children, a View that acts as header and a RecyclerView:
<android.support.design.widget.CoordinatorLayout
android:id="#+id/coordinator"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:id="#+id/view"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:gravity="center_horizontal"
app:layout_behavior="some.package.AlphaBehavior">
<ImageView
android:id="#+id/header_iv"
style="#style/some_style"/>
<TextView
android:id="#+id/header_retails_tv"
style="#style/some_style_tv"
android:text="#string/some_text"/>
</LinearLayout>
<android.support.v7.widget.RecyclerView
android:id="#+id/recyclerview"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:clipToPadding="false" />
</android.support.design.widget.CoordinatorLayout>
I set a padding dynamically to RecyclerView with the size of header and I set the clipToPadding to false, so the RecyclerView is displayed below the header and when the user makes scroll up, the RecyclerView is shown above the header view.
I made a custom CoordinatorLayout.Behavior in order to accomplish a fade out of the view when the user scrolls up the list and a fade in when the header have to be visible again, the AlphaBehavior:
public class AlphaBehavior extends CoordinatorLayout.Behavior {
private float alpha = 1.0f;
private float scrolly = 0.f;
private int headerSize = 0;
private Animation defaultFadeInAnimation;
public AlphaBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
defaultFadeInAnimation = AnimationUtils.loadAnimation(context, android.R.anim.fade_in);
}
public void setHeaderSize(int headerSize) {
this.headerSize = headerSize;
}
#Override
public boolean layoutDependsOn(CoordinatorLayout parent, View child, View dependency) {
return dependency instanceof RecyclerView;
}
#Override
public void onNestedScroll(CoordinatorLayout coordinatorLayout, View child, View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
scrolly += dyConsumed;
Log.d(Constants.TAG, dyConsumed + "/" + dyUnconsumed + "/" + scrolly);
float totalScrollY = ((RecyclerView)target).computeVerticalScrollOffset();
Log.d(Constants.TAG, "totalScrollY:" + totalScrollY);
alpha = (headerSize - totalScrollY) / headerSize;
if (alpha < 0.f) alpha = 0.f;
if (alpha > 1.0f) alpha = 1.f;
if (dyConsumed < 0 && totalScrollY > headerSize) {
alpha = 0.f;
}
Log.d(Constants.TAG, "alpha:" + alpha);
child.setAlpha(alpha);
}
#Override
public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, View child, View target) {
int pos = ((LinearLayoutManager)((RecyclerView)target).getLayoutManager()).findFirstCompletelyVisibleItemPosition();
Log.d(Constants.TAG, "pos:" + pos);
if (pos == 0 && child.getAlpha() == 0.f) {
child.startAnimation(defaultFadeInAnimation);
}
}
// overriding this in case we don't the other events are not called
#Override
public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout, View child, View directTargetChild, View target, int nestedScrollAxes) {
return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL;
}
}
But I'm facing an issue: if the user make scroll very fast the events of the behaviour are not calling correctly. The scrollY member is not well-related to the total scroll and the totalScrollY member (obtained from computing the scroll from RecyclerView) is not correct. Even I tried to find the firstCompletelyVisibleItem in the onStopNestedScroll event, but it is returning the position 2 or 3 when the recyclerView achieves the start of the list.

Finally, I solved it using an OnScrollListener instead of using a CoordinatorLayout.Behavior and it is working like a charm. I put the code, maybe is useful for someone:
The custom onScrollListener to hide a view:
public class HideViewOnScrollListener extends RecyclerView.OnScrollListener {
private float alpha = 1.f;
private float scrolly = 0.f;
private int heightViewToHide;
private final View viewToHide;
public HideViewOnScrollListener(View viewToHide) {
this.viewToHide = viewToHide;
heightViewToHide = viewToHide.getHeight();
if (heightViewToHide == 0) {
ViewTreeObserver viewTreeObserver = viewToHide.getViewTreeObserver();
viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
#Override
public void onGlobalLayout() {
heightViewToHide = viewToHide.getHeight();
if (heightViewToHide > 0)
viewToHide.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
});
}
}
#Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
scrolly += dy;
alpha = (heightViewToHide - scrolly) / heightViewToHide;
if (alpha < 0.f) alpha = 0.f;
if (alpha > 1.0f) alpha = 1.f;
if (dy < 0 && scrolly > heightViewToHide) {
alpha = 0.f;
}
viewToHide.setAlpha(alpha);
}
}
And you can add to a RecyclerView that way:
recyclerView.addOnScrollListener(new HideViewOnScrollListener(viewToHide));

ItemTouchHelper with RecyclerView in NestedScrollView: Drag scrolling not work

I have implemented ItemTouchHelper like descriped in this articel:
https://medium.com/#ipaulpro/drag-and-swipe-with-recyclerview-b9456d2b1aaf#.k7xm7amxi
All works fine if the RecyclerView is a child of the CoordinatorLayout.
But if the RecyclerView is a child of NestedScrollView in CoordinatorLayout, the drag scrolling not working anymore.
Draging an item and move it to the top or bottom of the screen, the RecyclerView not scrolling like it do if its not a child of NestedScrollView.
Any ideas?

You have to disable the nestedScrolling for the recyclerView:
recyclerView.setIsNestedScrollingEnabled(false);

I have run into this same problem and I spent nearly a whole day to solve it.
Precondition:
First of all, my xml layout looks like this:
<CoordinatorLayout>
<com.google.android.material.appbar.AppBarLayout
...
</com.google.android.material.appbar.AppBarLayout>
<NestedScrollView>
<RecyclerView/>
</NestedScrollView>
</CoordinatorLayout>
And to make the scrolling behavior normal, I also let the nestedScrolling for the RecyclerView disabled by: RecyclerView.setIsNestedScrollingEnabled(false);
Reason:
But with ItemTouchHelper I still cannot make the Recyclerview auto scroll as expected when I drag the item in it. The reason why IT CANNOT SCROLL is in the method scrollIfNecessary() of ItemTouchHelper:
boolean scrollIfNecessary() {
RecyclerView.LayoutManager lm = mRecyclerView.getLayoutManager();
if (mTmpRect == null) {
mTmpRect = new Rect();
}
int scrollY = 0;
lm.calculateItemDecorationsForChild(mSelected.itemView, mTmpRect);
if (lm.canScrollVertically()) {
int curY = (int) (mSelectedStartY + mDy);
final int topDiff = curY - mTmpRect.top - mRecyclerView.getPaddingTop();
if (mDy < 0 && topDiff < 0) {
scrollY = topDiff;
} else if (mDy > 0) {
final int bottomDiff = curY + mSelected.itemView.getHeight() + mTmpRect.bottom
- (mRecyclerView.getHeight() - mRecyclerView.getPaddingBottom());
if (bottomDiff > 0) {
scrollY = bottomDiff;
}
}
}
if (scrollY != 0) {
scrollY = mCallback.interpolateOutOfBoundsScroll(mRecyclerView,
mSelected.itemView.getHeight(), scrollY,
mRecyclerView.getHeight(), scrollDuration);
}
if (scrollY != 0) {
mRecyclerView.scrollBy(scrollX, scrollY);
return true;
}
return false;
}
Reason 1: when nestedScrolling for the RecyclerView is set to false, actually the effective scrolling object is the NestedScrollView, which is the parent of RecyclerView. So RecyclerView.scrollBy(x, y) here does not work at all!
Reason 2: mRecyclerView.getHeight() is much bigger than NestedScrollView.getHeight(). So when I drag the item in RecyclerView to bottom, the result of scrollIfNecessary() is also false.
Reason 3: mSelectedStartY does not seem like the expected value when in our case. Because we need to calculate the scrollY of NestedScrollView in our case.
Therefore, we need to override this method to fullfill our expectation. Here comes the solution:
Solution:
Step 1:
In order to override this scrollIfNecessary()(This method is not public), you need to new a class under a package named the same as ItemTouchHelper's. Like this:
Step 2:
Besides overriding scrollIfNecessary(), we also need to override select() in order to get the value of mSelectedStartY and the scrollY of NestedScrollView when starting draging.
public override fun select(selected: RecyclerView.ViewHolder?, actionState: Int) {
super.select(selected, actionState)
if (selected != null) {
mSelectedStartY = selected.itemView.top
mSelectedStartScrollY = (mRecyclerView.parent as NestedScrollView).scrollY.toFloat()
}
}
Notice: mSelectedStartY and mSelectedStartScrollY are both very important for scrolling the NestedScrollView up or down.
Step 3:
Now we can override scrollIfNecessary(), and you need to pay attention to the comments below:
public override fun scrollIfNecessary(): Boolean {
...
val lm = mRecyclerView.layoutManager
if (mTmpRect == null) {
mTmpRect = Rect()
}
var scrollY = 0
val currentScrollY = (mRecyclerView.parent as NestedScrollView).scrollY
// We need to use the height of NestedScrollView, not RecyclerView's!
val actualShowingHeight = (mRecyclerView.parent as NestedScrollView).height
lm!!.calculateItemDecorationsForChild(mSelected.itemView, mTmpRect!!)
if (lm.canScrollVertically()) {
// The true current Y of the item in NestedScrollView, not in RecyclerView!
val curY = (mSelectedStartY + mDy - currentScrollY).toInt()
// The true mDy should plus the initial scrollY and minus current scrollY of NestedScrollView
val checkDy = (mDy + mSelectedStartScrollY - currentScrollY).toInt()
val topDiff = curY - mTmpRect!!.top - mRecyclerView.paddingTop
if (checkDy < 0 && topDiff < 0) {// User is draging the item out of the top edge.
scrollY = topDiff
} else if (checkDy > 0) { // User is draging the item out of the bottom edge.
val bottomDiff = (curY + mSelected.itemView.height + mTmpRect!!.bottom
- (actualShowingHeight - mRecyclerView.paddingBottom))
if (bottomDiff > 0) {
scrollY = bottomDiff
}
}
}
if (scrollY != 0) {
scrollY = mCallback.interpolateOutOfBoundsScroll(
mRecyclerView,
mSelected.itemView.height, scrollY, actualShowingHeight, scrollDuration
)
}
if (scrollY != 0) {
...
// The scrolling behavior should be assigned to NestedScrollView!
(mRecyclerView.parent as NestedScrollView).scrollBy(0, scrollY)
return true
}
...
return false
}
Result:
I can just show you my work through the Gif below:

This is the solution that works for me.
Create 2 custom classes
1> LockableScrollView
public class LockableScrollView extends NestedScrollView {
// true if we can scroll (not locked)
// false if we cannot scroll (locked)
private boolean mScrollable = true;
public LockableScrollView(#NonNull Context context) {
super(context);
}
public LockableScrollView(#NonNull Context context, #Nullable AttributeSet attrs) {
super(context, attrs);
}
public LockableScrollView(#NonNull Context context, #Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public void setScrollingEnabled(boolean enabled) {
mScrollable = enabled;
}
#Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
// Don't do anything with intercepted touch events if
// we are not scrollable
if (ev.getAction() == MotionEvent.ACTION_MOVE) {// if we can scroll pass the event to the superclass
return mScrollable && super.onInterceptTouchEvent(ev);
}
return super.onInterceptTouchEvent(ev);
}
}
2>LockableRecyclerView extends RecyclerView
public class LockableRecyclerView extends RecyclerView {
private LockableScrollView scrollview;
public LockableRecyclerView(#NonNull Context context) {
super(context);
}
public LockableRecyclerView(#NonNull Context context, #Nullable AttributeSet attrs) {
super(context, attrs);
}
public LockableRecyclerView(#NonNull Context context, #Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public void setScrollview(LockableScrollView lockedscrollview) {
this.scrollview = lockedscrollview;
}
#Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_MOVE) {
scrollview.setScrollingEnabled(false);
return super.onInterceptTouchEvent(ev);
}
scrollview.setScrollingEnabled(true);
return super.onInterceptTouchEvent(ev);
}
#Override
public boolean onTouchEvent(MotionEvent e) {
if (e.getAction() == MotionEvent.ACTION_MOVE) {
scrollview.setScrollingEnabled(false);
return super.onTouchEvent(e);
}
scrollview.setScrollingEnabled(true);
return super.onTouchEvent(e);
}
}
Use this views instead of NestedScrollView and RecyclerView in xml
in kotlin file set
recyclerView.setScrollview(binding.scrollView)
recyclerView.isNestedScrollingEnabled = false
ItemTouchHelper(object :
ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.UP) {
override fun onMove(
#NonNull recyclerView: RecyclerView,
#NonNull viewHolder: RecyclerView.ViewHolder,
#NonNull target: RecyclerView.ViewHolder
): Boolean {
return false
}
override fun onSwiped(#NonNull viewHolder: RecyclerView.ViewHolder, direction: Int) {
// when user swipe thr recyclerview item to right remove item from favorite list
if (direction == ItemTouchHelper.UP) {
val itemToRemove = favList[viewHolder.absoluteAdapterPosition]
}
}
}).attachToRecyclerView(binding.recyclerView)

android:descendantFocusability="blocksDescendants"
add in NestedScrollView and add
android:focusableInTouchMode="true"
in child layout it look like below
<androidx.core.widget.NestedScrollView
android:descendantFocusability="blocksDescendants">
<androidx.constraintlayout.widget.ConstraintLayout
android:focusableInTouchMode="true">
</androidx.constraintlayout.widget.ConstraintLayout>
</androidx.core.widget.NestedScrollView>
check this github repo
https://github.com/khambhaytajaydip/Drag-Drop-recyclerview

Expand appbarlayout when recyclerview is scrolled/fling to top

I implemented a collapsingtoolbar layout with a recyclerview as shown in the sample code attached. My issue is that, when I fling the list downward, it does not go all the way to the top.
What happens is that, the scrolling stops right at the point where the AppBarLayout is supposed to end.
The effect that I want is upon flinging the list downward, the list will go all the way to the top AND reveal/expand the AppBarLayout
My minSdk is 14. Any help or suggestion is greatly appreciated.
<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.AppBarLayout>
<android.support.design.widget.CollapsingToolbarLayout
app:layout_scrollFlags="scroll|exitUntilCollapsed">
<LinearLayout
app:layout_collapseMode="parallax">
//some elements
</LinearLayout>
</android.support.design.widget.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<android.support.v7.widget.RecyclerView
app:layout_behavior="#string/appbar_scrolling_view_behavior"/> //value android.support.design.widget.AppBarLayout$ScrollingViewBehavior
<android.support.v7.widget.Toolbar
app:popupTheme="#style/AppTheme.PopupOverlay"
app:layout_collapseMode="parallax" />

I had similar problem and I used a simple trick to expand AppBarLayout when RecyclerView fling to top (you need to have support library >= 23.x.x)
mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
#Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
if (newState == RecyclerView.SCROLL_STATE_IDLE) {
int firstVisiblePosition = linearLayoutManager.findFirstCompletelyVisibleItemPosition();
if (firstVisiblePosition == 0) {
mAppBarLayout.setExpanded(true, true);
}
}
}
});

You can fully expand or collapse the App Bar with the setExpanded() method. One implementation could involve overriding dispatchTouchEvent() in your Activity class, and auto-collapsing/expanding your App Bar based on whether it is collapsed past the halfway point:
#Override
public boolean dispatchTouchEvent(MotionEvent event) {
if (event.getAction() == MotionEvent.ACTION_UP) {
float per = Math.abs(mAppBarLayout.getY()) / mAppBarLayout.getTotalScrollRange();
boolean setExpanded = (per <= 0.5F);
mAppBarLayout.setExpanded(setExpanded, true);
}
return super.dispatchTouchEvent(event);
}
In respect to automatically scrolling to the last position on a fling, I have put some code on GitHub that shows how to programmatically smooth scroll to a specific location that may help. Calling a scroll to list.size() - 1 on a fling for instance could replicate the behaviour. Parts of this code by the way are adapted from the StylingAndroid and Novoda blogs:
public class RecyclerLayoutManager extends LinearLayoutManager {
private AppBarManager mAppBarManager;
private int visibleHeightForRecyclerView;
public RecyclerLayoutManager(Context context) {
super(context);
}
#Override
public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
View firstVisibleChild = recyclerView.getChildAt(0);
final int childHeight = firstVisibleChild.getHeight();
int distanceInPixels = ((findFirstVisibleItemPosition() - position) * childHeight);
if (distanceInPixels == 0) {
distanceInPixels = (int) Math.abs(firstVisibleChild.getY());
}
//Called Once
if (visibleHeightForRecyclerView == 0) {
visibleHeightForRecyclerView = mAppBarManager.getVisibleHeightForRecyclerViewInPx();
}
//Subtract one as adapter position 0 based
final int visibleChildCount = visibleHeightForRecyclerView/childHeight - 1;
if (position <= visibleChildCount) {
//Scroll to the very top and expand the app bar
position = 0;
mAppBarManager.expandAppBar();
} else {
mAppBarManager.collapseAppBar();
}
SmoothScroller smoothScroller = new SmoothScroller(recyclerView.getContext(), Math.abs(distanceInPixels), 1000);
smoothScroller.setTargetPosition(position);
startSmoothScroll(smoothScroller);
}
public void setAppBarManager(AppBarManager appBarManager) {
mAppBarManager = appBarManager;
}
private class SmoothScroller extends LinearSmoothScroller {
private static final int TARGET_SEEK_SCROLL_DISTANCE_PX = 10000;
private final float distanceInPixels;
private final float duration;
public SmoothScroller(Context context, int distanceInPixels, int duration) {
super(context);
this.distanceInPixels = distanceInPixels;
float millisecondsPerPx = calculateSpeedPerPixel(context.getResources().getDisplayMetrics());
this.duration = distanceInPixels < TARGET_SEEK_SCROLL_DISTANCE_PX ?
(int) (Math.abs(distanceInPixels) * millisecondsPerPx) : duration;
}
#Override
public PointF computeScrollVectorForPosition(int targetPosition) {
return RecyclerLayoutManager.this
.computeScrollVectorForPosition(targetPosition);
}
#Override
protected int calculateTimeForScrolling(int dx) {
float proportion = (float) dx / distanceInPixels;
return (int) (duration * proportion);
}
}
}
Edit:
AppBarManager in the above code snippet refers to an interface used to communicate with the AppBarLayout in an Activity. Collapse/expand app bar methods do just that, with animations. The final method is used to calculate the number of RecyclerView rows visible on screen:
AppBarManager.java
public interface AppBarManager {
void collapseAppBar();
void expandAppBar();
int getVisibleHeightForRecyclerViewInPx();
}
MainActivity.java
public class MainActivity extends AppCompatActivity implements AppBarManager{
#Override
public void collapseAppBar() {
mAppBarLayout.setExpanded(false, true);
}
#Override
public void expandAppBar() {
mAppBarLayout.setExpanded(true, true);
}
#Override
public int getVisibleHeightForRecyclerViewInPx() {
if (mRecyclerFragment == null) mRecyclerFragment =
(RecyclerFragment) getSupportFragmentManager().findFragmentByTag(RecyclerFragment.TAG);
int windowHeight, appBarHeight, headerViewHeight;
windowHeight = getWindow().getDecorView().getHeight();
appBarHeight = mAppBarLayout.getHeight();
headerViewHeight = mRecyclerFragment.getHeaderView().getHeight();
return windowHeight - (appBarHeight + headerViewHeight);
}