If I understand correctly the ListView-Adapter loads 1 more view then currently visible on the screen. Lets say view 1 to 5, plus not visible but preloaded view 6. If the user scrolls down, view 1 is dismissed and view 6 gets displayed. I would like to know how to preload more views, like view 6 in the example, but instead of only one view e.g. three views.
I hope by increasing the preloaded-view-count to give the ListView items more time to compute the view / to have a smoother scroll experience.
Since there are so many topics with related key words, a short list what this question is not about:
Loading the ListView data model (getting the data from the Web/DB)
Loading more data when reaching the end of the ListView
It's not about the ViewHolder pattern (recycling views)
PS: Kind of hard to google this - all keywords that pop up in my mind are already taken by other topics...
The class that is responsible for controlling when to hide/show views is LayoutManager.
I'm not very familiar with the inner workings of that class, but I suppose you could create a custom LayoutManager that overrides some of the relevant parts to achieve what you need.
Related
I have a bit of a dilemma. I'll try to explain it as simply as I can.
My app has a page where I show a feed of posts through a recycler view (represented by the blue rectangles):
This is simple enough, as I've created a PostAdapter class for the recycler view where I handle all the child view logic.
Now I have another requirement where I need a page that shows a post (blue rectangle) at the top of the recycler view, and then the replies to that post below it (represented by the red rectangles):
Please remember that the blue and red rectangles are simplifications. Each of these views has dozens of child views and hundreds of lines of code of logic each.
I've created another class, ReplyAdapter, for the recycler view on this page, which handles all the logic for the replies.
My problem now becomes: how can I also show the post (blue rectangle) at the top of the recycler view?
I know that I can modify ReplyAdapter to allow for multiple types (as explained here), but because these are already two separate classes, each with their own complex logic, that's not really a possibility for me.
So my question is: is there a way to keep the logic of the two adapters separate while still being able to use both under the same recycler view? If so, how? If no, what should I do?
If your use case is exactly this - data from PostAdapter on top, data from ReplyAdapter below - then androidx.recyclerview.widget.ConcatAdapter may help. As its name implies, it concatenates adapters.
This question already has answers here:
Android Recyclerview vs ListView with Viewholder
(7 answers)
Closed 3 years ago.
I was reading about the difference b/w recyclerview and listview and found out that recyclerview is faster than listview.
I tried to search online but not found any satisfactory answer I know it is used ViewHolder pattern and Notifying adapter but what does it does intearlly so it is faster?
Recycler View you could say is an efficient way to create list of views.
If you have 1000 items like ur contact list , and If ur visible screen can show only 10 items at once, it will Create only 10+1 (or +2) Views and as u scroll , items/views that left will be reused (not create) to show new data.
Recycler View by default does this, where as List View by default doesn't do.
There are some differences between these two views.
ListView is a bit heavy and it has a lot of responsibilities. Whenever we have to handle the list, such as to configure it in some way, the only way to do this is through the ListView object or inside the adapter.
A lot of bad things in the ListView were fixed or changed in the RecyclerView. It’s more efficient by default, the layout is separated and we have more possibilities over the data set inside the adapter.
These are some crucial differences between ListView and RecyclerView:
1 ViewHolder
The ViewHolder pattern allows us to make our list scrolling act smoothly. It stores list row views references and, thanks to this, calling the findViewById() method only occurs a couple of times, rather than for the entire dataset and on each bind view.
The RecyclerView’s adapter forces us to use the ViewHolder pattern. The creating part (inflating the layout and finding views) and updating the views is split into two methods — onCreateViewHolder() and onBindViewHolder().
The ListView, on the other hand, doesn’t give us that kind of protection by default, so without implementing the ViewHolder pattern inside the getView() method, we’ll end with inefficient scrolling in our list.
2 LayoutManager
The LayoutManager takes responsibility for layouting row views. Thanks to this, RecyclerView doesn’t have to think about how to position the row view. This class gives us the opportunity to choose the way that we want to show the row views and how to scroll the list. For example, if we want to scroll our list vertically or horizontally, we can choose LinearLayoutManager. For grids, it is more suitable to choose the GridLayoutManager.
Previously, with the use of the ListView, we were only able to create a vertical-scrolling list, so it wasn’t that flexible. If we wanted grids on our list, we had to choose the other widget for that — GridView.
3 ItemDecoration
A duty of the ItemDecoration is simple in theory – add some decorations for the list row views – but in practice, it’s that simple to implement if we want to create a custom one. In this case, we should extend the ItemDecoration class and implement our solution. For example, the RecyclerView list has no dividers between rows by default and it’s consistent with the Material Design guidelines. However, if we want to add a divider for some reason, we can use DividerItemDecoration and add it to the RecyclerView. In case we use the ListView, we have to figure out rows decorations by ourselves. There is no helper class like ItemDecoration for this widget.
4 ItemAnimator
The last but not least component of RecyclerView that I want to mention is ItemAnimator. As we can expect, it’s handling row views animations like list appearance and disappearance, adding or removing particular views and so on. By default, RecyclerView’s list animations are nice and smooth. Of course, we can change that by creating our own ItemAnimator, which is also not that easy. To make it easier, we should extend the SimpleItemAnimator class and implement the methods that we need (just add animations to a ViewHolder’s views). To be honest, implementing animations on the ListView was a pain. Again, we had to figure out how we want to handle them.
5 Notifying adapter
We have a couple of cool notifiers on the RecyclerView’s adapter. We are still able to use notifyDataSetChanged() but there are also ones for particular list elements, like notifyItemInserted(), notifyItemRemoved() or even notifyItemChanged() and more. We should use the most appropriate ones for what is actually happening, so the proper animations will fire correctly.
Using ListView, we were able to use just notifyDataSetChanged() on the adapter and then had to handle the rest ourselves, again.
Because of ViewHolder Pattern.
Thats was the simplest answer. Now for some details.
What recycler view does is what it's name indicates "Recycle", yes it recycles items, and it does it with the help of ViewHolder Pattern.
By Using ViewHolder we do-not need to call findViewByID() every time we go through getView()method. The reference for all rows are stored in-memory. This increases the performance significantly, as findViewByID()is a heavy process.
Hope this clears your confusion.
I have a rather complicated List with nested RecyclerViews. I get it that nested RecyclerViews aren't the best solution, but in my case it is one of few solutions that create structured code and meet the requirements. I have attached an image of the structure. You can take telegram as an example to improve your understanding of the structure. Basically I have an outer RecyclerView RV-1 with Items RV-1-Item and an inner RecyclerView RV-2 with Items RV-2-Item. So far so good, my problem is that the outer RecyclerView recycles views as intended, but if one of the RV-1-Items comes into view, all ViewHolders of RV-2 are created (That means that sometimes more than 100 ViewHolders are created). To sum it all up my question is how to force the inner RecyclerView RV-2 to recycle ViewHolders as well.
I know that the inner RecyclerView RV-2 has to have a hight of wrap_content because it depends in the count of the inner items, also i cannot set setHasFixedHeigth(true) (and I don't know if it would help) because during runtime new RV-2-Items can be added into RV-2. I also tried to set setNestedScrollingEnabled(false) on RV-2 because I read a lot about it online but it didn't help me either.
So basically this is how I configure
RV-1
layoutManager = LinearLayoutManager(context)
isNestedScrollingEnabled = false
RV-2
setHasFixedSize(true)
layoutManager = LinearLayoutManager(context).apply {
reverseLayout = true
}
In addition to that I have some ItemDecorators but they only create the space between the items, so they shouldn't have to do anything with the problem.
To sum it all up:
The outer RV-1 recycles it ViewHolders as intended but the inner RV-2 creates all ViewHolders at once, even if they are not on screen. I assume that this is the case because RV-2 has a height of wrap_content and when the layout_height need to be measured it needs to create all views. THE QUESTION: Is there a way to force RV-2 to recycle its views?
EDIT:
Also I am using a shared RecycledViewPool between all RV-2 RecyclerViews but that isn't really related to the problem, because even if the ViewHolders are shared between the RecyclerViews, an RV-2 RecyclerView shouldn't create ViewHolders that aren't visible when it is initialised.
EDIT 2:
A lot of comments and related questions say that two vertical nested RecyclerViews isn't a possible thing in android, in case all visitors of this question think the same my question is: How would you implement such a structure. It is obvious that I could make a single view which has a IM (Round Image View) and RV-2-Item and just make the IM invisible when it isn't needed. In my opinion this somehow makes the structure more complicated. Furthermore a requirement is that the IM on the left side of RV-1-Item must have the ability to move up and down in RV-1-Item, which is obviously easier with my current structure.
EDIT 3: (My last one I promise)
The Problem I have shown can be solved by using the approche I explain in my EDIT 2, even if it isn't the best solution it would work. But the issue is that I have an even more complex screen where this approche wouldn't work anymore because I have three nested RecyclerViews. I could get that number down to two with the approche of EDIT 2 but I would still be left with two nested RecyclerViews and I cannot think of a workaround that could solve the problem of the remaining two nested RecyclerViews. I attached an image of the even more complex screen which contains a the interface of the app with marked sections to help you to understand the structure.
(Not quite an answer to your specific question in solving "how to not get the RecyclerView to create all items at once", but something that most likely will fix your specific problem by not using nested recyclerviews at all)
I would suggest (in a quite similar way as already suggested in this answer), to flatten your feed into one recyclerview
(No matter how much you tweak your nested recyclerview architecture, imho it will never be as performant than having just one recyclerview, and as you don't need nested scrolling (I guess), just one recycler view should be your best option).
I would propose to not think of your feed in the way your data is structured, but in a way you want to show it and how it can be split into smaller items which are "look alikes" / consist of the same things.
From your screenshot I would see for example the following items / view types for each chat item:
Chat header (the thing with the icon and the text "New Group")
the user badge (the picture with the text "Jürgen")
a message item (one bubble of text, so e.g. in your screenshot at the bottom there would actually be 3 of those items, one for each message)
The section with the date and the action/reply items.
Those items are way smaller than a whole chat item, and therefore can be faster created / recycled.
For each of those items, create a view-type and a view-holder, and treat them as seperate recycler-view items.
The recyclerview will, when the getItemViewType method is correctly used, create / prepare the correct type of view for the position you need.
For this to work, the adapter needs to add some logic, as your data most likely will be structured something like
a list of chats, and each chat has a name and some messages to display
and we need it as
the first 6 elements are for the first chat, where the first position
is the header, the second the user badge, the next 3 items are message
items and then we need an action item.
So you basically need to calculate how many recyclerview items you will need to show each single chat-item, which could be a calculation along the lines:
1 chat header item + 1 user badge item + 3 message items + 1 action/reply item = 6
This calculation needs to be performed for each chat item of your data list separately.
So if you only have this single chat item in your list of data to display, you actually need to tell the adapter to create 6 items (by returning in this case 6 at getItemViewCount()).
Then, you need to tell the adapter using the getItemViewType(position: Int) function, at which position of the recyclerview which type of view the adapter needs to prepare.
So there you again need some logic to say that e.g. on position 0 the chat header for the first chat item should be, at position 1 the user badge for the first chat item, at position 2-4 message items should be, on position 5 the action item and at position 6 the chat header for the second chat should be and so on
(again, the logic then needs to be in place for all chat items, and it can get really messy / complicated, as to calculate each chat items view types for a position, e.g. all prior chat element view counts need to be recalculated, too (in order to know at which recycler-view position your current chat item starts)).
As this tends to blow your adapter up, I would suggest (if you don't already do so), to get some manager / delegate architecture in there.
So e.g. have a delegate for each view type, and a manager which calculates the number of recyclerview items / view types needed for each chat item.
Just for reference:
Some time ago we had a situation similar to yours
(a recycler-view with a design similar to a social media feed, which should show the first n comments in the feed and we displayed the comments for each feed item (which was a recyclerview item) with another recyclerview in the item) and also after some troubles with performance which we could not manage to resolve just flattened the recyclerview, and never had performance troubles again.
A lot of comments and related questions say that two vertical nested RecyclerViews isn't a possible thing in android
This is not true; whoever says this is not a thing has not done it and thinks it's not possible. It is possible, albeit with complications, side-effects, and most likely, the annoyance of your users when they tap around trying to scroll up/down and the wrong touch interceptor wins.
Why is this a problem?
On iOS, when you try to do something that the platform devs didn't think it was good, most people and other devs scream at you: don't fight the framework!!!.
On Android, we see the craziest Java (and now Kotlin) implementations of things that makes you wonder what are we -developers- learning at school and what are we teaching?! and yet nobody says anything (for the most part) :p
The truth is, you're trying to design a complicated user interaction and data transformation, and yet, your attempt is biased by trying to use the data "as you have it" (which implies dealing with these two different RV/Adapters), as opposed to do what one should do: transform the data for presentation.
This leads me to the next question:
How would you implement such a structure.
Well, for starters, I don't know how your data looks like, nor where it's coming from; I don't know what your users can do with your data, outside of the obvious scrolling.
I also don't know how your data wants to be presented, aside from your mock up.
But I do know the situation very well. A list of things, which also contain their own list of things.
Case: The List of List
It is doable; you can have a list and inside said list, have another list. I've done it. I've seen it done by others. I've used it. I also never liked the idea of having this "small" scrollable thing, fighting to see who scrolls first when I tap "the wrong place".
I would not do this. If the inner-list is big (say more than 3 items per outer item), I would not present it as scrollable content.
What I would do (considering the things I do not know about your problem) is to have a single list displaying all the content properly flattened.
This has a issue with your content:
What if the inner-lists are super long, wouldn't this cause them all to be displayed? YES, and that's why I wouldn't do it this way if the data (as you described) can have 100 items. An options is to display the 3 first items with a "more" link to now open the inner-list "full screen"; this is 10 times better than the nested list from a user's PoV and from the technical aspect of it.
Another alternative, is to keep this single long list (RV-1) and let users "expand" the list to launch another full-screen list depicting the contents of RV-2, in a separate window. This is even better.
The time you'll spend implementing this and getting rid of the mess of code you probably have right now, will make you wonder why didn't you suggest this in the first place.
If this is something you absolutely cannot do, then I cannot offer you much more advice, for now you're tied to unknown to me business/product rules. Ultimately, the price will be paid by the users of your app, when they have to scroll that nightmare :)
Take a Step Back
Let me be clear, I am not criticizing you or your solution; I'm merely pointing out that, in my experience, this "pattern" you have here is not a good user experience.
Format your data for presentation, not the other way around. Your data should be properly shaped so it can be properly presented with the tools you have.
You're fighting against the tools Android is giving you; you're giving a RecyclerView (and its adapter) a lot of new problems to deal with when it already has a lot going on.
Think about it: RecyclerViews have to do a lot of things; Adapters must also conform to a few interfaces, ensure things are dispatched as soon as possible, calculate Diffs (if using a ListAdapter<T,V>), etc. Activities/Fragments? They have a lot on their plates dealing with ... well "Android"; now you're asking all these components to also handle a complicated scenario of scrolling content, touch recognition, event handling, view inflation, etc.
All this, while expecting each view to take 16ms or less (to stay above 60 FPS scrolling speed, your view/viewHolder should not take more than 16ms to do all it needs.
Instead, I'm asking you to take a step back, grab the data you have, compose it, transform it, map it, and create the data structure that can better serve the components you have (a RV + Adapter + a simple View).
Good luck :)
I want to put a list of items I am extracting from a firebase database. I was using RecyclerView for this but now I came to know that we can create Buttons, TextViews etc programmatically. I was wondering if there are any benefits of recyclerview or any special cases where it is preferable to use recyclerview.
Use RecyclerView, it is memory efficient. It does create view when it is visible and destroys it when it is not visible anymore, this way you can have infinite list without using a lot of resources.
Not bad thing to maintain view dynamically, But you need to know full concept of recycler view before taking the decision because there are lot's of aspect depend on when you are deciding to any component
Key aspects,
- Why you are using view?
- Is it sequential?
- All the data in the list are the same type?
- Also, How many records in the list if there is only tow-three record then no need to take the recyclerview.
Ultimately recyclerview is the optimized view for list out the item as a list,
Also, we suggest if your most of the data same type then take recyclerview don't do at dynamically, Because name self saying how recycler view (Recycle) will work
I have a Custom Adapter for the ListView. The Layout has three images, some text. When loading the Listview, it takes a while, because of the images. Its something like Posts.
Same app in iOS is loading very fast, I think that UITableView works different than Listview.
Is there a way, in place to load all posts, only load for example 3 posts and when the user scrolls the Listview down, load the next 3, scroll down, load the next 3 and so on. This could give a better performance.
Normally, android listview work that way. Let's say there are 5 views that user can reach at the moment. ListView creates 9 views and when user scrolls it loads the bottom ones. You can think it this way. Your main problem is how are you loading your images and create the custom view. There is a common pattern for custom adapters which handles the fast recycling views(ViewHolder pattern). You should checkout the link for ViewHolder pattern. https://www.javacodegeeks.com/2013/09/android-viewholder-pattern-example.html
It probably loads slow because the images should be resized every time, if you could save thumbnails it would go much faster.
Have you tried to use Recycler View instead? As long as I know using the Recycler View is the best practice nowadays. You can find a very good tutorial here:
http://www.vogella.com/tutorials/AndroidRecyclerView/article.html
You use the Recycler View almost the same way you do with List View: adapter, viewHolder, etc. It's good to mention though that you need to pay special attention to the use of the LayoutManager since the Recycler View it itself doesn't "know how to draw" the stuff on the screen.