Develop Reference

list adapter with sections

I already have a list adapter that works properly. But I want to divide the object in the list into sections according to the date they were created. Something like this:
I found something called "sectioned recycler view" but couldn't find any documentation on that. I read all the related questions, but they all are either outdated or use a third-party library. What's the native way of implementing this feature?

There are a couple of approaches you could use. First the easy one:
make the header part of your item layout, but with GONE visibility by default
in onBindViewHolder, decide whether the header should be VISIBLE or GONE
The logic there depends on what you want, but it could be as simple as
val visible = position == 0 || items[position].date != items[position - 1].date
Basically you just need to work out what the condition is that would cause an item to be in a different "group" than the previous item, and then if it's met, show the header over that item.
The approach MarkL is talking about is more complex, but it's also more robust - by having separate Item and Header elements, you can treat them differently, and even do stuff like having the header collapse/show its children, select them all etc. You can do that with the other approach, but it requires more code since you're not really treating things as groups, it's more of a trick when it comes to displaying stuff.
Basically, ignoring the how for now, you need a list of headers and items. A sealed class is a good way to represent that:
sealed class ListElement {
data class Header(val date: Date) : ListElement()
data class Item(val itemData: YourItem) : ListElement()
}
I've made Item a wrapper class that holds your actual data object inside, since that's probably coming from elsewhere and you can't define it as part of this sealed class hierarchy - so sticking it inside one of the subclasses like this allows you to do that.
So now you can have a List<ListElement> containing Headers and Items in display order. Since you've mentioned creating the ViewHolders in a comment I won't explain all that, but basically when you're getting the item type for a position, you just need to check is Header or is Item and then handle it from there.
As for creating that list, there are lots of ways to do it - you could use groupBy to generate a Map of dates to lists of items, map each of those entries to a list of Header, Item, Item..., and flatten the whole thing into a single list:
items.map { Item(it) }
.groupBy { it.itemData.date }
.entries
.flatMap { (date, items) -> listOf(Header(date)) + items }
The advantage with a map like this is it's an actual grouped structure, so you can keep it around to generate flat lists for display - e.g. hiding a group's contents by only including the header in the list.
Or you could just build the list yourself, similar to the logic I mentioned in the first example - if the date has changed from the previous item, insert a Header first:
val list = mutableListOf<ListElement>().apply {
for (item in items) {
// add a header if the date changed - this handles the first header
// in an empty list too (where lastOrNull returns null, so the date is null)
val previousItemDate = (lastOrNull() as? Item)?.itemData?.date
if (previousItemDate != item.date) add(Header(item.date))
add(Item(item))
}
}
Or you could use fold. Don't forget to sort stuff!

You could create 2 types of view holders:
header which holds the date
data container which holds the other information.
And then create a list of objects which contain something like this:
listToBind = (header, data, data, header, data, data)
For your case, where header & info is the same object, you can do something like this:
take your object you receive from backend (example)
YourObject(val header: String, val info:InfoObject)
create 2 display objects, both inheriting from a type that your adapter accepts (say - AdapterDisplayEntity)
HeaderDisplayEntity(val header: String): AdapterDisplayEntity
InfoDisplayEntity(val info: InfoObject): AdapterDisplayEntity
now you can use your list that contains these items and submit to your adapter.

Use nested recycler view for this instead. You can check the example here.
Best solution for this scenario so far.

If you are using Jetpack Compose you can use the stickyHeader() as documented in the documentation

Android LayoutPosition is -1 causing OutOfBoundsException

I've created an app that has a list of cards within a RecyclerView that each have functionality of their own. I wanted to have each card choose the next color from an array defined in my colors.xml. In order to accomplish this, within my ViewHolder initialization, I set the background color of the card using cardContainer.setBackgroundColor(colors[this.layoutPosition % colors.size]. This would make it so that the colors would be cycled when more cards are created. However, I seem to be encountering the issue where my layout position is negative despite there being a set number of cards (25) created at the beginning.
While trying to search around and find the cause, I read here that if you call notifyDataSetChanged() the adapterPosition will become -1. While I am not using adapterPosition here, I thought that maybe it would be a similar issue, however, I am not adding any additional data at the time of the creation of the list items.
My ViewHolder code can be seen below. This is where the issue arises, but if any additional code is necessary feel free to ask.
class ViewHolder(itemView : View, private val listener: HabitClickListener) : RecyclerView.ViewHolder(itemView) {
val habitTitle: TextView = itemView.habitTitle
val streak: TextView = itemView.dayCounter
val cardContainer: LinearLayout = itemView.cardContainer
private val decreaseCounterButton : Button = itemView.decreaseCounterButton
private val increaseCounterButton : Button = itemView.increaseCounterButton
init {
chooseCardColor() // Choose the color for each card from the available colors
itemView.setOnClickListener {
listener.onCardClick(this.layoutPosition)
}
decreaseCounterButton.setOnClickListener {
listener.onDecrease(this.layoutPosition)
}
increaseCounterButton.setOnClickListener {
listener.onIncrease(this.layoutPosition)
}
}
private fun chooseCardColor() {
val colors = itemView.resources.getIntArray(R.array.cardColors)
cardContainer.setBackgroundColor(colors[this.layoutPosition % colors.size])
}
}

I will try to simplify this further, you should use the getAdapterPosition of ViewHolder
In recyclerview, storing the data and displaying the data are two separate things(Notice how you can use different managers(LinearLayoutManager, GridLayoutManager) to present the data in a different way.When some data changes in recyclerview, it notifies the ui to change what is shown in the screen. Even though it is really small, there is a delay between the change in the content of recyclerview and change in layout, that's why these two behave differently.
My information in this may be outdated but also don't just use the position variable as it can be inconsistent when another element is added/deleted to recyclerview due to how onBindViewHolder()(existing variables position wasn't updated when a new element is added/deleted) behaves. Instead use getAdapterPosition().
Edit: Quick fix if you don't want to deal with viewHolder gimmicks.
Add a new field to your custom object which decides what color it should be. Then make this calculation in your fragment/activity by looking at the index of your object in the list instead of doing the calculation in the viewHolder. Now you can set the color you want inside the viewHolderby looking at your object's new field. Of course you should be careful when adding/deleting a new object when you do this, but same holds true when you do it via viewHolder

ViewHolder attribute changes affects Objects attribute

private fun turnOnAllItems() {
items.forEachIndexed { index, item ->
val viewHolder = recyclerView.findViewHolderForAdapterPosition(index)
as SwitchableItemViewHolder
viewHolder.switchButton.isChecked = false
}
}
What this does, is it also changes list items object values isEnabled to false. Looks weird to me, as I actually change viewHolder attribute. Why is this happening? How to avoid this?

I strongly believe that you are doing it the wrong way. RecyclerView is meant to display already modified data, meaning that you have a set of it.
Let's say, 10 tables in restaurant, and at some point table #4 becomes available for new customer and you want to indicate that.
A good approach would be to modify your list of tables somewhere outside RCV, even fragment or activity will do, and then just graphically update (all or just one) item by means of RCV.
Here's a little article I made to illustrate how to properly use RecyclerView, hope it will help you

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.

AlertDialog vs Spinner vs ListView

I've got what I thought was a simple android UI design problem but I've been going around in circles for a couple of days. I have a REST service that I'm downloading XML from and displaying the XML in a form in an android app. I have a web page built and am mimicking this with android, same options, same URLs being sent to the REST service whether from android or the web pages. With HTML I can easily create checkbox groups and radiobutton/dropdowns for various id/display items, so for instance, I can display a planet option as:
<select name="planet"><option value="0">Mercury</option></select>
I wanted to do something similar in android where I had a pair of values, one an id and the other the user-friendly text to display. So I decided to create an adapter using android.util.Pair:
public class PairView extends Pair<String, String> {
public PairView(String first, String second) {
super(first, second);
}
public String toString() {
return second;
}
}
public class PairAdapter extends ArrayAdapter<PairView> {
}
So now I can put my id in pair.first and what to display to the user in pair.second.
My problem comes in that some of these options will be single-selects and some will be multi-selects. In html, that's not an issue, just use a checkbox group for multi, and radio buttons/dropdowns for single selects. In android however, it seems it's not so straight forward. I tried using Spinners for the adapters, but Spinner seems to only allow single selection. AlertDialog.Builder allows for single and multi-selections, but curiously I don't see an option for using an adapter for the multi-selection, just for single selections.
I guess what I really want is a consistent look for all my options, with radio buttons displayed for single selections and checkboxes displayed for multi selections, via an adapter so I can get the id's from the Pair for the items selected.
What approach should I use? A custom spinner with code added for multi-selections? AlertDialog.Builder and somehow make it use an adapter for multi-selections? Just create a plain Alert and wrap a ListView in it? Another option that is (hopefully) simpler?
I feel like I'm missing something very basic here.

I had a similar situation in an app I was making so would share what I opted for. I had different type of questions and depending on that I removed and added things in my activity. For radio buttons I used with elements in it. For multiple choice questions I wanted a checkbox based view so I added an empty within my layout and in code added CheckBox(s) to it.
As for the caption and value, for radio buttons and checkboxes you can set display text by setText and add any object/value as a tag. So what I used to do was something like this:
CheckBox option = new CheckBox(MyActivity.this);
option.setText("Option 1");
option.setTag(10);
Later on when you get the selected option, you can simply get its tag and use its value.
This is just one way of doing it which I found simple. Hope this helps