Develop Reference

How do I "Save State" of my Recyclerview ItemDecorations & Offsets?

I have a RecyclerView of items and a layoutManager that is of type StaggeredGridLayoutManager. I was in an interesting situation, I wanted my items staggered to look like this:
but my views are all the same size, so they would not stagger. To correct the problem I needed to add an offset at the start of the 2nd column. Since I was also creating my own custom decorator class I figured the best way to accomplish this was to just add an offset for the first right column item in my list using the getItemsOffsets method.
Here is the relevant code for my decorator class:
public class StampListDecoration extends RecyclerView.ItemDecoration {
...
#Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
// good example here: https://stackoverflow.com/questions/29666598/android-recyclerview-finding-out-first-and-last-view-on-itemdecoration/30404499#30404499
/**
* Special case. Te first right side item in the list should have an extra 50% top
* offset so that these equal sized views are perfectly staggered.
*/
if (parent.getChildAdapterPosition(view) == 1) {
/**
* We would normally do a outRect.top = view.getHeight()/2 to create a 50% top offset on the first right item in the list.
* However, problems would arise if we paused the app when the top right item was scrolled off screen.
* In this situation, when we re-inflated the recyclerview since the view was off screen
* Android would say the height of the view was zero. So instead I added code that
* looked for the height of the top most view that was visible (and would therefore
* have a height.
*
* see https://stackoverflow.com/questions/29463560/findfirstvisibleitempositions-doesnt-work-for-recycleview-android
* because as a staggeredGrid layout you have a special case first visible method
* findFirstVisibleItemPositions that returns an array of (notice the S on the end of
* the method name.
*/
StaggeredGridLayoutManager layoutMngr = ((StaggeredGridLayoutManager) parent.getLayoutManager());
int firstVisibleItemPosition = layoutMngr.findFirstVisibleItemPositions(null)[0];
int topPos = 0;
try {
topPos = parent.getChildAt(firstVisibleItemPosition).getMeasuredHeight()/2;
} catch (Exception e) {
e.printStackTrace();
}
outRect.set(0, topPos, 0, 0);
} else {
outRect.set(0, 0, 0, 0);
}
}
}
my problem is that these offsets are not getting saved to state when my activity pauses/resumes. So when I switch to another app and switch back, the right column in my RecyclerView slides back to the top...and I lose my stagger.
Can someone show me how to save my offset state? Where are offsets supposed to be saved? I was assuming the LayoutManager would save this information, and I'm saving the LayoutManager state, but that does not seem to be working.

Animate only last item of recycler view every time user scroll to end and make it visible

I want to animate appearance of only last item of a recyclerview. Every time scroll is done to end, view should be shown with animation.
Applying Item Animator animates when item is removed, added, deleted. But i did not get option to animate in on scroll.
I applied animation in onBindView, but onBindView is not always called and animation is not started. Moreover, in cases onBindView is called and user is performing slow scroll operation, animation had already started when view is actually visible to user.
What can be suitable way to apply this animation?
Thanks
Vibhor

I have tested this solution and It's working like a charm.
recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
#Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
LinearLayoutManager layoutManager = LinearLayoutManager.class.cast(recyclerView.getLayoutManager());
int lastItem=adapter.getItemCount()-1;
tryAnimation(layoutManager.findViewByPosition(lastItem));
}
});
public void tryAnimation(View view) {
Animation animation = AnimationUtils.loadAnimation(this, android.R.anim.slide_in_left);
if (view != null)
view.startAnimation(animation);
}
this solution can be optimized by taking advantage of dy

How do I make an infinitely looping RecyclerView while also being able to scroll to an item programmatically?

I want to make a RecyclerView that scrolls infinitely while also being able to scroll to an item programmatically.
At the moment I've made the RecyclerView loop infinitely using this hacky method: https://stackoverflow.com/a/31254146/7443375
i.e. Overriding my adapter
#Override
public int getCount() {
return Integer.MAX_VALUE;
}
Getting position of item in my adapter like so:
int positionInList = position % fragmentList.size();
And then initializing my RecyclerView's scroll position like so:
recyclerView.getLayoutManager().scrollToPosition(Integer.MAX_VALUE / 2);
However, in my Fragment that has the RecyclerView, I want to be able to scroll to a specific item in my list (i.e. item 3 out of a list of 10 items). When I call
recyclerView.getLayoutManager().scrollToPosition(2);
The list goes into an infinite scroll. I can't figure out how to go to the specific item itself.
Furthermore, how can I make sure that the specific item is centered in the screen? I am using LinearSnapHelper to snap the items in the center of the screen as I scroll, but LinearSnapHelper does not seem to work when setting positions programmatically.

Try this way..
recyclerView.post(new Runnable() {
#Override
public void run() {
new CountDownTimer(Integer.MAX_VALUE, 20 ) {
public void onTick(long millis) {
recyclerView.scrollBy(0, 20);
}
public void onFinish() {
}
}.start();
}
});

Parallax header effect with RecyclerView

I want to change my ListView I currently have over to use RecyclerView so I can make use of StaggeredGridLayoutManager but RecyclerView does not have the ability to add a header like ListView.
Usually with a ListView I set an empty view in the header and put the image below the listview and translate the bottom image with the scrolling of the list to create the Parallax effect.
So with out a header how can I create the same parallax effect with RecyclerView?

the easiest way to do it, is using below onScrollListener without relying on any library.
View view = recyclerView.getChildAt(0);
if(view != null && recyclerView.getChildAdapterPosition(view) == 0) {
view.setTranslationY(-view.getTop() / 2);// or use view.animate().translateY();
}
make sure your second viewHolder item has a background color to match the drawer/activity background. so the scrolling looks parallax.

So today I tried to archive that effect on a RecyclerView. I was able to do it but since the code is too much I will paste here my github project and I will explain some of the key points of the project.
https://github.com/kanytu/android-parallax-recyclerview
First we need to look at getItemViewType on the RecyclerView.Adapter class. This methods defines what type of view we're dealing with. That type will be passed on to onCreateViewHolder and there we can inflate different views. So what I did was: check if the position is the first one. If so then inflate the header, if not inflate a normal row.
I've added also a CustomRelativeLayout that clips the view so we don't have any trouble with the dividers and with the rows getting on top of the header.
From this point you seem to know the rest of the logic behind it.
The final result was:
EDIT:
If you need to insert something in adapter make sure you notify the correct position by adding 1 in the notifyItemChanged/Inserted method. For example:
public void addItem(String item, int position) {
mData.add(position, item);
notifyItemInserted(position + 1); //we have to add 1 to the notification position since we don't want to mess with the header
}
Another important edit I've done is the scroll logic. The mCurrentOffset system I was using didn't work with the item insertion since the offset will change if you add an item. So what I did was:
ViewHolder holder = findViewHolderForPosition(0);
if (holder != null)
((ParallaxRecyclerAdapter) getAdapter()).translateHeader(-holder.itemView.getTop() * 0.5f);
To test this I added a postDelayed Runnable, started the app, scrolled to the end, add the item in position 0, and scroll up again. The result was:
If anyone is looking for other parallax effects they can check my other repo:
https://github.com/kanytu/android-parallax-listview

For kotlin, you may config the recycler view as below
//setting parallax effects
recyclerView.addOnScrollListener(object :RecyclerView.OnScrollListener(){
override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
super.onScrolled(recyclerView, dx, dy)
val view = recyclerView?.getChildAt(0)
if (view != null && recyclerView?.getChildAdapterPosition(view) === 0) {
val imageView = view.findViewById(R.id.parallaxImage)
imageView.translationY = -view.top / 2f
}
}
})

This answer is for those curious about adding a parallax header to a GridLayoutManager or a StaggeredGridLayoutManager
You'll want to add the following code to your adapter in either onBindViewHolder or onCreateViewHolder
StaggeredGridLayoutManager.LayoutParams layoutParams =
(StaggeredGridLayoutManager.LayoutParams) holder.itemView.getLayoutParams();
layoutParams.setFullSpan(true);

keeping bottom element of ListView visible when keyboard appears

I have a LinearLayout containing both a ListView and an EditText. When the On-Screen keyboard is launched by touching the EditText, the ListView resizes so that only the top few elements remain visible.
The context that the ListView is being used in has the bottom few elements being more visually relevant than the top, and so I'd like for it to resize so that the bottom remains visible, rather than the top. Any pointers?
(Incidentally, the current fix I'm using involves using smoothScrollToPosition, but the laggy scroll behaviour makes this undesirable)

I just solved a similar issue this morning, and thought I'd post my result here for the benefit of future searchers. My issue was that scrolling to the bottom wasn't helping since I was calling it before the view actually changed size. The solution? Wait until it does change size by using a GlobalLayoutListener
Steps:
1) implement the following method in the activity holding the listview
public void scrollToBottom(){
//I think this is supposed to run on the UI thread
listView.setSelection(mAdapter.getCount() - 1);
}
2) create the following class
public class OnGlobalLayoutListenerWithScrollToBottom implements OnGlobalLayoutListener{
private boolean scroll;
private OnScrollToBottomListener listener;
public interface OnScrollToBottomListener{
public void scrollToBottom();
}
public OnGlobalLayoutListenerWithScrollToBottom(OnScrollToBottomListener listener){
this.listener = listener;
}
#Override
public void onGlobalLayout() {
if(scroll){
listener.scrollToBottom();
scroll = false;
}
}
/**
* Lets the listener know to request a scroll to bottom the next time it is layed out
*/
public void scrollToBottomAtNextOpportunity(){
scroll = true;
}
};
3) In your activity, implement the interface from this class. Then, in your activity, create an instance of this OnGlobalLayoutListener and set it as the listener for your listView
//make sure your class implements OnGlobalLayoutListenerWithScrollToBottom.OnScrollToBottomListener
listViewLayoutListener = new OnGlobalLayoutListenerWithScrollToBottom(this);
listView.getViewTreeObserver().addOnGlobalLayoutListener(listViewLayoutListener);
4) In your activity, before you make changes that will affect the size of list view, such as showing and hiding other views or adding stuff to the listview, simply let the layout listener know to scroll at the next opportunity
listViewLayoutListener.scrollToBottomAtNextOpportunity();

You might achieve this with setSelectionFromTop() and by overriding onSizeChanged() in a custom listview. In your layout, you should have a RelativeLayout has a parent container and place the listview above the edittext.
By creating your own listview and overriding onSizeChange(), you will be able to get the last visible item's position before the listview resizing and get its new height, in order to finally set the "previous" position of the list with an offset of its height.
How it works: Your list will place the previous last visible item at its top and you will add the pixels to scroll it at its bottom, just above your edittext.
To override the method and display it with an offset, do as follows:
#Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
// get last visible item's position before resizing
int lastPosition = super.getLastVisiblePosition();
// call the super method to resize the listview
super.onSizeChanged(w, h, oldw, oldh);
// after resizing, show the last visible item at the bottom of new listview's height
super.setSelectionFromTop(lastPosition, (h - lastItemHeight));
}
lastItemHeight is a little workaround, because I didn't find how to get the last item's height before that onSizeChanged is called. Then, in the case of your listview contains many types of items (without the same height), I prefer to get the selected height when an event occurs, just before the SoftKeyboard opens up.
So in the custom listview, you have this global variable:
int lastItemHeight = 0;
And in the activity (or fragment, whatever), you update this value in OnClickListener:
edittext.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// set a new Thread
listview.post(new Runnable() {
#Override
public void run() {
// get last visible position of items
int lastPosition = listview.getLastVisiblePosition() - 1;
// if the view's last child can be retrieved
if ( listview.getChildAt(lastPosition) != null ) {
// update the height of the last child in custom listview
listview.lastItemHeight = listview.getChildAt(lastPosition).getHeight();
}
}
});
}
});
Note: there is another possible solution but you have to set android:stackFromBottom="true" on the listview, which stackes its content from the bottom. Instead, this solution here can display a specific item without forcing the content to start from the bottom, with the default listview's behaviour.
Second note: (just in case) don't forget to add android:windowSoftInputMode="adjustResize" in the manifest.

You should add this attribute in the manifest for your activity and pick the correct value for it (probably you will pick "adjustResize"):
<activity android:name="package.yourActivity" android:windowSoftInputMode="adjustResize"/>