Building an infinitely scrollable calendar-like view in Android

Question

This is not a code problem, I interpret the guidelines as that being OK.

I've been researching a way of building an infinitely scrolling calendar-like view in Android, but I've reached an impasse.

Right now my dilemma is that most of the similar views available have their children placed relative each other in a recurring style. With this I mean:

item 4 comes after item 3, which comes after item 2, and there is constant padding/margin between all items.

What I need is a way to produce an infinitely long scrollable view that may, or may not, contain items. The items should be placed at variable positions within the view. The best way I can describe a similar looking view is a one-day calendar-like view that is infinitely scrollable.

So far my best two bets are using the new RecyclerView with a custom LayoutManager (this seems very complex and still not perfectly documented by Google though). I like this approach because, among other things, it is optimized for displaying large sets in a limited view.

My other solution would be to build a completely custom View. However, with that solution I loose the adapter unless I build a container view (which is probably more complex than building a layout manager).

How would you go about solving such a problem? Tips are appreciated, I don't need code examples, just ideas which path is the best to solve this problem.

Thanks.

Apologies if I've misunderstood the guidelines

Edit: How I resolved this problem My first solution to use RecyclerView with a special Decorator seemed promising, but it remained a "hack" so we decided not to go for that solution since we were afraid of the complications that it would create down the line.

To solve the problem I went with a SurfaceView instead of an Adapter, this means having to rewrite all the adapter-functionality for my SurfaceView but it seemed to be the best way of solving this issue of very custom drawing and layout managing for my use-case.

It still would be nice to build a custom Viewgroup that can handle this kind of layout problems.

Answer 1

ListView and ListAdapter are based on a fixed list, so the current infinite-scrollers just keep adding more and more data to the end of the list.

But what you want is scroller similar to Google's Calendar app which has a bi-directional infinite scroller. The problem with using ListView and ListAdapter in this case is that if you add data to the front of the list, the index of any one item changes so that the list jumps.

If you really start thinking about this from the MVC perspective, you realize that ListAdapter does not provide a model that fits this need.

Instead of having absolute indexing (i.e. 1, 2, 3, 4, etc), what you really want is relative indexing, so instead of saying "Give me the item at index 42" you want to say "here's an item, give me the five items before it". Or you have something like a calendar date which is absolute; yet — unlike your device's memory — it has effectively no beginning or end, so what you really want here is a "window" into a section of that data.

A better data model for this would be a kind of double-ended queue that is partly a LRU cache. You place a limit on the number of items in the structure. Then as prior items are loaded (user is scrolling up) the items at back end are pushed off, and when subsequent items are added (user is scrolling down), items at the front are pushed off.

Also, you would have a threshold where if you got within a few items of of one edge of the structure, a "loadNext" or "loadPrevious" event would fire and invoke a callback that you set up to push more data onto the edge of the structure.

So once you've figured out that your model is completely different, you realize that even RecyclerView isn't going to help you here because it's tied to the absolute indexing model. You need some sort of custom ViewGroup subclass that recycles item views like a ListView, but can adapt to the double-ended queue. And when you search code repos for something like this, there's nothing out there.

Sounds like fun. I'll post a link when I get a project started. (Sadly, it won't be done in any timely manner to help you right now, sorry.)

Something that might help you a little sooner: look at Google's Calendar implementation and see how they did it: Google Calendar Git repo