AndreiBogdan AndreiBogdan - 11 days ago 4
Android Question

Android. How does notifyDataSetChanged() method and ListViews work?

I am trying to understand the

ListView
concept and how it works and I'm trying to create my own adapter which extends
BaseAdapter
. For
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
ListView
.

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?

Answer

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 !!! :

In the BaseAdapter there are some methods that need to be overridden. Among these, there is getCount().

When the 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.

What 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 notifyDataSetChanged() then 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 getView()

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 ListViews work.

P.S. This example also helped me a lot to understand.

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