I am trying to understand the
ListView concept and how it works and I'm trying to create my own adapter which extends
ArrayAdapter for instance, there is the
notifyDataSetChanged() method which should be called after you've updated the array list which holds all your data, in order to refresh the
But I am creating my own subclass of
BaseAdapter. That method is not available to me, or is it? How do i implement this method? Basically, what does that method do exactly, maybe I'll understand then.
In case of the
ArrayAdapter i'm guessing it looks at what position the
ListView is currently displaying and it checks if it's the same one as in the
ArrayList after it was updated? Or...
It says that the method:
Notifies the attached observers that the underlying data has been changed and any View reflecting the data set should refresh itself.
But how exactly does it refresh itself?
Can someone explain please?
I've figured it out. I couldn't understand how the hell the adapter started and how did it know where to get the data from. When i extended the
BaseAdapter class, in the constructor of that class I initialized the list of items that I wanted to see in the
ListView. But I couldn't figure out how these values would be used and when.
So here's the thing !!! :
BaseAdapter there are some methods that need to be overridden. Among these, there is
ListView is created and whatnot, it calls
getCount(). If this returns a value different than 0 (I returned the size of the ArrayList which I've previously initialized in the constructor), then it calls
getView() enough times to fill the screen with items. For instance, I initialized the
ArrayList with 20 items. Because only 8 items initially fit on the screen,
getView() was called 8 times, each time asking for the position it required for me to return (more precisely it wanted to know how the row would look like in the list on that specific position, what data it needed to contain). If I scroll down the list,
getView() gets called over and over again, 'til I hit the end of the list, in my case 20 items / rows.
notifyDataSetChanged() does is ... when called, it looks at what items are displayed on the screen at the moment of its call (more precisely which row indexes ) and calls
getView() with those positions.
i.e. if you're displaying the first 8 items in the list (so those are the ones visible on the screen) and you add another item between the 2nd and 3rd item in the list and you call
getView() is called 8 times, with positions starting from 0 and ending with 7, and because in the
getView() method you're getting data from the
ArrayList then it will automatically return the new item inserted in the list alongside 7 out of the previous 8 (7 and not 8 because the last item went one position down, so it is not visible anymore), and the
ListView will redraw, or whatever, with these items.
Also, important to specify is that if you've implemented
getView() correctly, you'll end up recycling the items (the objects) already displayed (instead of creating new ones). See this video at around 12:00 minutes to see the correct way to implement
I've figured all this out by placing calls to
LogCat in every method and following what was going on.
Hope this helps someone who's just now starting to understand how
P.S. This example also helped me a lot to understand.
ListViews are not really used anymore. Android came out with the
RecyclerView which does the recycling of the views for you, but knowing the basics of a
ListView helps with understanding the
Here's a link for reference: https://developer.android.com/guide/topics/ui/layout/recyclerview
BaseAdapter can be "observed" by other classes. When you're calling method of
setAdapter() it's calls
registerDataSetObserver of adapter. So, adapter knows who are interested in new data.
You can check the source of
BaseAdapter here. It's pretty small.
notifyDataSetChanged is available for you and you're basically shouldn't override it (because it's not doing anything special, so you can just reuse it in your own class).
ListView displays some data stored in an
After you change the contents of the
ArrayList, you need to tell the list that the source of the data had changed and it needs to redraw itself to show the new data.
So, that is where
notifyDatasetChanged() comes in. It tells the
ListView that the data has been modified; and to show the new data, the
ListView must be redrawn.
As you are extending
BaseAdapter for your own for creating a subclass, you will get
notifyDataSetChanged() method as well.