I have a RecyclerView which has a header and a list of data below the header. I have two adapters for these two and are merged using ConcatAdapter. I want the header to be not recycled and I tried setting setIsRecyclable to false for the ViewHolder in the first adapter. However, it seems not to work and is always recycled. This is predictable because ConcatAdapter is also a child of the RecyclerView adapter. Is there any way that I can stop recycling a particular viewholder in a particular adapter while using ConcatAdapter?
I have watched a lot of videos on RecyclerView but I am very confused on whether the ViewHolder is an adapter that changes the view or does it really just represent each item that is displayed on the screen.
How can I understand the concept more?
Let's see if this helps.
The general job of any list-style view would be to display a long chain of views, each representing a piece of data, most likely from a list.
Now imagine we consider the simplest implementation, where it draws all those views when it is created and allows you to scroll through them. This is obviously very inefficient for performance as a long list would require a lot of processing all at once.
RecyclerView aims to solve this and only creates enough views to fit on the screen and when scrolling, changes the content of those views seamlessly to reflect more data.
Now these views are created initially as empty blueprints and the RecyclerView wraps them inside something called a ViewHolder, which can not only hold the view but also pointers to different parts of the view, which saves doing even more work every time new data is displayed. Then initially and when scrolling, the RecyclerView 'binds' the relevant data to view holders.
The job of the adapter is to tie this process together and has three methods that require you to provide a concrete implementation:
getItemCount - expected to return how many items there are in the full dataset
onCreateViewHolder - create a view holder representing a generic row
onBindViewHolder - bind data to a view holder, therefore updating the rows content when given the view holder and the position in the dataset that should be bound
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 am trying to understand the RecyclerView but I can't understand the difference between the Adapter and LayoutManager.
Can anyone explain it to me?
The adapter is used to create (and bind data to) views that correspond to each item in your dataset.
The layout manager is responsible for the layout of these views.
The adapter doesn't know how the views will be positioned and sized. This means you can swap the layout manager without having to change your adapter code e.g. switching from a LinearLayoutManager to GridLayoutManager.
Check out this answer for a deeper (though still high-level) explanation (scroll past the initial code block to the section about RecyclerView philosophy):
ListView to RecyclerView Migration for CustomView
So in Recycler View you need an Adapter to manage the data which will contain the Recycler view. Adapter will take the data it can come from internet or from internal database, than adapter fetches the dat when it gets the data without an error it comes back to adapter(e.g.adapter grabs data from dataset) then it shows the data in the View Holder in your card like a one cell in the Recycler View, View Holder is a class that manages the view and than that view appears in the Recycler View. The above image which i tried to draw shows that process for Horizontal Recycler
In my case i found these benifits when i working on them
Basically an Adapter is used to gather all corresponding data what you provided
to all views defined by you
after gathering all data it wants a layout to display thease views for this layoutmanager gives a flatform to show them .
I have a ListView which onItemClick selected item changes its layout, pops different buttons. If any other item is selected, the previous selected one returns to normal. My ListView adapter works fine but refreshing the whole list with notifyDataSetChanged() in my adapter takes too much time.
My problem is to refresh only the changed items in the ListView.
And also I would like to have suggestions for better scrolling performance.
try to implement View Holder Pattern it increases the performance of loading and scrolling of ListViews
Making ListView Scrolling Smooth | Android Developers
Using lists in Android (ListView) - Tutorial - Vogella
from the docs:
Your code might call findViewById() frequently during the scrolling of ListView, which can slow down performance. Even when the Adapter returns an inflated view for recycling, you still need to look up the elements and update them. A way around repeated use of findViewById() is to use the "view holder" design pattern.
you can define one method in adapter class which will return current item view. in onitemclick use this method to make changes in clicked item. You can define class view type class variable in activity and store previous view there...
ListView scrolling performance slows down when widgests like textviews, images are at the bottom of the layout hierarchy.
So for improving list performance one should design item xmls with minimum layout tree levels.