Explanation Of The Getview() Method Of An Arrayadapter

Could you explain the getView() method of an ArrayAdapter. I read the docs and it has three parameters: position: The position of the item within the adapter's data set of the ite

Solution 1:

Is is a cached copy of the last layout returned by getView()?

The convertView is the view of a row that left the screen(so it isn't the last view returned by the getView method). For example, the list is first shown, in this case convertView is null, no row view was previously built and left the screen. If you scroll down, row 0 will leave the screen(will not be visible anymore), when that happens the ListView may choose to keep that view in a cache to later use it(this makes sense, as the rows of a ListView generally have the same layout with only the data being different). The reason to keep some views in a cache and later use them is because the getView method could be called a lot of times(each time the user scrolls up/down and new rows appear on the screen). If each time the row view would need to be recreated this would have resulted in a lot of objects being created which is something to avoid. In your getView method you would check convertView to see if it is null. If it's null then you must build a new row view and populate it with data, if it isn't null, the ListView has offered you a previous view. Having this previous view means you don't need to build a new row layout, instead you must populate it with the correct data, as that cached view has the old data still attached to it(you would see a lot of questions on stackoverflow where users ask why the rows of their ListView are duplicating when they scroll down).

What does the parent parameter do. I haven't seen too many examples utilising this. Most of them simply reuse/inflate a row layout and return it.

It should be used to get the correct LayoutParams for the newly inflated/built row. For example, if you inflate a layout which has a RelativeLayout as the root and you don't use the parent to get the LayoutParams you could have some problems with the row layout. To take the parent in consideration you would use :

convertView = getLayoutInflater().inflate(R.layout.row_layout, parent, false);

Solution 2:

My understanding of convertView is that it's essentially views that have been recycled because they're not being used at the moment - for example, you scroll down the list, the ones at the top aren't on the screen, so they get passed into this parameter for use when you need a new view (so you don't have to create a whole new one while having unused ones sitting around idle). iOS has a similar method called dequeueReusableCellWithIdentifier. If each row of your listview has the same structure, it's safe to cast this to the appropriate type and just update the information in it - text, images, etc. It will be a View that was previously returned by a getView() call for the same list.

My best guess (and it is admittedly a guess) with parent is that it's the view that this adapter's list is a child of. It gives you a route back to the rendering system if you need a context, access to the resource system, to pass information to or receive information from the list's parent view.