Develop Reference

Sorting recyclerview by recently accessed

I am doing a side project of making an app (with Java since I already know it). I have a recyclerview which loads some data via the room database library. The elements of the recyclerview are clickable.
My problem is I want the user to be able to sort the recyclerview so that the most recently accessed items go to the top.
My original idea was to assign the entities to have two variables - a String list_name which also serves as the id, and an Int order_of_access. Also, in my ViewModel I have a getAllLists method which returns a livedata list. I have an onChanged listener in the fragment activity which nicely updates the recyclerview when data is added/removed.
When the user adds a new list, it is assigned an order_of_access of the listsize (+1). But when the user deletes a group of lists, or clicks on a list, I want to update the order_of_access, say with an updateOrderAccess method.
Do you think this is the best way of doing what I want?
Where should I place updateOrderAccess and how would you recommend it be written? Since the method getAllLists returns livedata, it is tempting to put updateOrderAccess in an observer in the fragment (in onChanged) - but this will obviously create an infinite loop. It seems more in the correct philosophy to put it in the ViewModel, but then how would you suggest the updateOrderAccess method to be written? I'm having some trouble conceptualising what I need.
I hope the question is not too vague - I will update it if you need more details.

Where should I place updateOrderAccess and how would you recommend it
be written?
I am so sure that you must write it in the view model, as long as updateOrderAccess() is editing the list which is observable then you have andexpose by that the ui state then you have to put it in view model, and the observers will be notified ( in this case it is recycle view) and it will redraw the list in the order you offered.
note: do not you ever update the state(ui data) outside the state holder so you implement UDF (unidirectional Data Flow) pattern.
see the references below to read more about UDF so you never get confused where to declare your functions by letting the architicture lead you:
Guide to app architecture
ui layer
state holders and ui state
Do you think this is the best way of doing what I want?
i am not very sure that i got exactly what your app do, but it seems like you want to re-order the elements of recycle view depending on the ui event (click) or data change (deleting or adding new element), now you have two choices:
if the order is very importnat to you that much you want to keep it even if the app has been destroyed
then you have to add a field in the room entity represent the ordering (let us call it order) and whenever the user click on the recycle view you have to update the rooms field "order" which is "flow" or "liveData" or any observable type, that will tell the view model that there is a changing in the data, now the view model have to re-order the new data by the field "order" and pass it to the recycle view to show it.
if your app do not have to save the order changes after the app been destroyed
then you can simply do that:
create list which is called "orderedList" you will put the list items in it by the right order, and another list called "unorderlist" which have getAllLists
for the first case where the ordering is being changed by user click, you
can declare a function in viewModel then use it in the ui
controller (your activity or fragment), so whenever the list item is
clicked this function just re-order the orderedList elements ( which
is observable, so the changes reflect on the ui ) just by change the
clicked item position to the front of the list.
for the second case where the ordering changes by data changes like
add or delet a list item in the database, then you have to compare
the legnth of orderlist and unorderlist legnth, if unorderList is
longer then it is an add situation else it is a delete situation, in
adding case just add the last item of unorderList to the orderList,
else you have to check the deleted item and delete it from
orderList.

Too few items for RecyclerView on screen

I'm making an API call getData(forPage: Int): Response which returns a page-worth of data (10 items max) and thereIsMoreData: Boolean.
The recyclerView is implemented that by scrolling, the scroll listener automatically fetches more data using that API call:
val scrollListener = object : MyScrollListener() {
override fun loadMoreItems() {
apiFunctionForLoading(currentPage + 1)
}
}
The problem is that with longer screen devices that have more space for items (let's say 20), the RV receives 10 items and then doesn't allow scrolling, because there's no more items to scroll to. Without scrolling, more data cannot be loaded.
My naive solution:
load first set of data
if thereIsMoreData == true I load another page of data
now I have more data than the screen can display at once hence allowing scroll
Is there a more ellegant solution?

Android has this Paging Library now which is about displaying chunks of data and fetching more when needed. I haven't used it and it looks like it might be a bit of work, but maybe it's worth a look?
Codepath has a tutorial on using it and I think their stuff is pretty good and easy to follow, so maybe check that out too. They also have this older tutorial that's closer to what you're doing (handling it yourself) so there's that too.
I guess in general, you'd want your adapter to return an "infinite" number for getItemCount() (like Integer.MAX_VALUE). And then in your onBindViewHolder(holder, position) method you'd either set the item at position, or if you don't have that item yet you load in the next page until you get it.
That way your initial page will always have the right amount of content, because it will be full of ViewHolders that have asked for data - if there's more than 10, then item 11 will have triggered the API call. But actually handling the callback and all the updating is the tricky part! If you have that working already then great, but it's what the Paging library was built to take care of for you (or at least make it easier!)

An elegant way would be to check whether the view can actually scroll down:
recyclerView.canScrollVertically(1)
1 means downwards -> returns true if it is possible tro scroll down.
So if it returns false, your page is not fully filled yet.

How to save the state of views held in dynamic viewpager

I have an enhanced loop, which will dynamically inflate however many layouts relevant to the number of values held in my array.
This works perfectly however, there is a method being called on each iteration, which also works but there is a big bug that I need help resolving.
Imagine there are 5 items in my array, therefore 5 layouts are inflated, in these layouts there is a little scratchcard type section on the layout.
Now if the user is on page 1, uses the scratchcard, then moves on to page 2, uses the scratchcard etc etc, it works fine.
But if the user is on page 1 and then goes to say, page 5 and then back to page 1 (basically in a random order), the scratchcard doesn't work.
From my understanding, the reason for this is that the method is being called an implemented on each iteration and the view is losing its state if the user scrolls back or scrolls in random orders.
Therefore I need a way to save the created view state in my viewpager.
Is this possible for my scenario? I have tried my best to find a solution, but cannot find something that feels relevant to my question.
Here is a snippet of the code in question. Thanks for any guidance or suggestions!
for (String x : array1) {
//loop out the number of layouts relative to the number of questions held in x
View current_layout = LayoutInflater.from(getActivity()).inflate(R.layout.question_fragment, null);
//use the pageAdapter to add the layout to the users view
pagerAdapter.addView(current_layout);
//call method to add functionality to the scratchcard
isCorrect(current_layout);
}
public void isCorrect(View current_layout) {
ScratchoffController controller1 = new ScratchoffController(getActivity())
.setThresholdPercent(0.40d)
.setTouchRadiusDip(getActivity(), 30)
.setFadeOnClear(true)
.setClearOnThresholdReached(true)
.setCompletionCallback(() -> {
})
.attach(current_layout.findViewById(R.id.scratch_view1), current_layout.findViewById(R.id.scratch_view_behind1));
ScratchoffController controller2 = new ScratchoffController(getActivity())
.setThresholdPercent(0.40d)
.setTouchRadiusDip(getActivity(), 30)
.setFadeOnClear(true)
.setClearOnThresholdReached(true)
.setCompletionCallback(() -> {
})
.attach(current_layout.findViewById(R.id.scratch_view2), current_layout.findViewById(R.id.scratch_view_behind2));
ScratchoffController controller3 = new ScratchoffController(getActivity())
.setThresholdPercent(0.40d)
.setTouchRadiusDip(getActivity(), 30)
.setFadeOnClear(true)
.setClearOnThresholdReached(true)
.setCompletionCallback(() -> {
})
.attach(current_layout.findViewById(R.id.scratch_view3), current_layout.findViewById(R.id.scratch_view_behind3));
ScratchoffController controller4 = new ScratchoffController(getActivity())
.setThresholdPercent(0.40d)
.setTouchRadiusDip(getActivity(), 30)
.setFadeOnClear(true)
.setClearOnThresholdReached(true)
.setCompletionCallback(() -> {
})
.attach(current_layout.findViewById(R.id.scratch_view4), current_layout.findViewById(R.id.scratch_view_behind4));
}

I ussually use ViewPager with Fragments and what you mention has happend to me when I try to keep references to the Fragment instances (in my case) outside of the viewpager.
This happens because the viewpager may create new instances of the Fragment it contains when you re-vist the tab in the way you mention. When this happens, the instance reference you hold outside of the viewpager is not anymore what the viewpager is showing.
In your case , according to this question, you have to oveeride instatiateItem and destroyItem. I think you can use these methods to save state restore state, and also you could update any external reference when instantiateItem is called.

Adding and Deleting between two seperate Array ListViews

I want to create a custom ListView.
Initially, the custom ListView has one array of data, but when user taps one of the list items, it's then removed from current array and added to another. If the user taps on the second array, the list item is then added back over into the first array.
Please suggest how to apply logic to do this.
Updates : I wants to use only one listview/recyclerview.
Following are screen shots..

Regarding the object switch - this is a simple transfer between lists, , just know beforehand if the insertion and removal is index based, e.g:
contacts.add(iLocation, ContactObject);
favorites.remove(iOtherLocation);
Regarding the ListView stuff, I would suggest converting to RecyclerView, let's build a general scenario:
You have a screen (Activity or Fragment) that holds one list (the implementation can be ListView or Recycler), and another screen that holds the other list.
In both of your lists you have adapters in which you implement the logic for the clicks on the objects in the lists.
The click transfers the object, either directly to the other list, OR to a temporary Object holder (because you might need it for other stuff), in which case you will need to pull that object from the other view, either way you remove it from the current one.
you switch to the other view, and refresh it.
An easy way to go -
Assuming the screens are the same, use only one Activity, holding a single RecyclerView, and handle 2 adapters, each for every list, the adapters allow you to handle the clicks easily, with an index for the object clicked, the click executes the info swap action,the Activity handles the visual swap Action.
a very general example would be:
//init everything obviously ;)
List<ContactObject> contacts;
List<ContactObject> favoritesContacts;
//the AdapteListener is an interface declared inside the adapter
mContactsRecyclerAdapter = new ContactsRecyclerAdapter(this, contacts,new ContactsRecyclerAdapter.AdapterListener()
{
#Override
public void cellClicked(int iIndex, ContactObject object)
{
favoritesContacts.add(iIndex, ContactObject);
contacts.remove(iIndex);
mContactsRecyclerAdapter.notifyDataSetChanged();
mFavoritesRecyclerAdapter.notifyDataSetChanged();
mRecyclerView.swapAdapter(mFavoritesRecyclerAdapter, false);
}
});
And vice-versa for the other adapter.
Hope this helps, comment if you have problems and I'll update.

Please implements with custom view extend with Linearlayout
Custom view has 2 child Linearlayout in which will add with this custom view
First time add all the element in first Linearlayout and based on user action please remove from first Linearlayout and add it in another layout

setListAdapter(adapter_name) to change the list displayed on screen, but still appears information from the previous displayed list

I have an activity that extends ListActivity, a list of "concepts" (let's call this list "C") and an onItemClickListener defined for this list. Whenever I click a "concept", no matter which one, the app must display another list. I have the following code to change the displayed list:
if(position == 0) change_list("adapter1");
else if (position == 1) change_list("adapter2");
else if (position == 2) change_list("adapter3");
else if (position == 3) change_list("adapter4");
else if (position == 4) change_list("adapter5");
Where position is the position of the clicked element in C
The function change_list performs setListAdapter(parameter) depending on the parameter I pass.
If I click the first element of C (the first concept), a list related to the first concept must appear. However, after calling setListAdapter(adapter), the data related to this concept is displayed, and also part of the C's list data.
For example: let's suppose C has these concepts:
A B C D E
and I click "A", which would lead to display a list with the following data: {a1,a2}
That's the final result:
a1 a2 C D E
And then, when I interact with another element on screen or I scroll down the list, the "ghost" data disappears and only the correct data remains on screen, just like this:
a1 a2
To make things worse, when I want to display list C again, nothing strange happens. Everything is displayed correctly.
At any time incorrect data is stored where it doesn't have to. One function my app must allow is to generate a txt file , and the generated txt file contains exactly the data I introduced. No data is corrupted or duplicated. I also tried using notifyDataSetChanged() and other functions, but I didn't solve the problem.
EDIT :
Here goes the xml code used for the main list of the activity:
<ListView
android:id="#android:id/list"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:background="#FF0000"
android:layout_below="#+id/afegir"/>
And an example of code in which I determine which contents must be displayed on screen:
else if(comprovar_concepte_actiu() == 1){
pnt = mydbhandler.getStoredValues("despeses1");
pnt.moveToFirst();
if(pnt.moveToFirst()){
do{
adapter_mostrar.add(pnt.getString(pnt.getColumnIndex("nom")));
}while(pnt.moveToNext());
}
adapter_mostrar.notifyDataSetChanged();
}
Where comprovar_concepte_actiu() returns and integer that tells which concept has been clicked in the main list C and adapter_mostrar is the single adapter I'm using now, instead of using multiple adapters (which made me use setListAdapter)
At the beginning of the activity, I call this.setListAdapter(adapter_mostrar). That's all I have.
EDIT 2 :
https://www.dropbox.com/s/7twgy043lkxb2x5/conceptes.java?dl=0
Here is a link to my conceptes.java activity. Press CTRL+F once opened and search "this is where I call.. " and you will directly get to the function where the change of list displayed on screen starts
I haven't found a solution yet. Any idea will be totally appreciated

The problem here is that - when you set a new adapter - the old data is still drawn. In other words, there has been no command to "refresh" the listView. However, the new adapter will be commanded to draw its own views. What ultimately occurs is that the old items are still there, the new items are redrawn, but when scrolled away the new adapter won't redraw/recreate the old items.
The solution is to simply refresh the adapter. However, there are two ways to go about this:
Add a new adapter every time and use myListView.invalidateViews(); or something similar [This is probably the easiest solution to implement, although probably not the best in the long run]
Change the dataset of the adapter and use notifyDataSetChanged() [on the adapter]
The latter option is a far better idea. You should use a single adapter and simply change its data over time. Once its dataset is changed, then tell the adapter that such a thing happened so it refreshes. However, you should read more here on all the different thoughts and processes about it, rather than take my opinion on it.
Edit:
There's apparently some very nicely, thought out answers around. Here's another one, that tells you more specifically about the differences between these two:
Is there any difference between ListView.invalidateViews() and Adapter.notifyDataSetChanged()?
Edit2:
With the onClickListener in mind, invalidateViews() will most likely not work, as it'll probably still draw the old views to "finish" the click (ie, draw the highlighting).
Changing the data directly inside a single adapter and using Adapter.notifyDataSetChanged() is your best bet, as it'll know to redraw everything from a single adapter and use only the current data defined by this single adapter.
Best to leave the data specifics (and defining what to draw based off of that data) up to what actually knows the data, rather than a higher up container that knows nothing specific about the actual data.