MinimalBible.github.io/_posts/2014-05-23-viewholder-vs-holderview.md

layout	title	modified	tags	image	comments	share
post	ViewHolder vs. HolderView	2014-05-23 19:04:19 -0400	viewholder holderview caching listview	feature credit creditlink	true

Further semantics and practices

Well, I personally think that ListViews are some of the most complicated Views in the Android ecosystem. So, I'm going to make two blog posts talking about them! The first is the ViewHolder vs. HolderView pattern, the second will be some nasty issues on ViewHolder caching I ran into. So, without further ado:

What is ViewHolder?

One of the biggest problems in displaying a ListView in Android is just how inefficient it is. Specifically, calling findViewById() is really expensive, so we try and do everything we can to avoid it. That's why the ViewHolder pattern was invented. The basic principle is:

• The ListView gives us a View object when it requests a view from the adapter with getView()

• If that View is null, we need to inflate a new one. Then, cache it in the view with setTag()

• If that View is not null, we can use the View's getTag() method to retrieve a cached version of it (since it's been inflated prior)

So, what we actually cache in that sequence above is the ViewHolder object. Basically, it just stores a static reference to the inflated fields so we don't have to reinflate everything. Then, the actual adapter is responsible for updating the inflated View using the references in the ViewHolder.

If you have any other questions on ViewHolder, check out this page because the author did an amazing job of explaining everything.

What is HolderView?

The HolderView pattern has the same basic principle. We want to make as few calls to findViewById() as possible. There's a big difference though: The HolderView is responsible for its own presentation.

• The ViewHolder just stores a reference to the View elements that are being inflated (by the adapter). The HolderView actually inflates itself.
• The Adapter driving the ListView is responsible for the presentation of data in each element. The HolderView handles it's own presentation through a bind() or some similar method.
• The ViewHolder is a static class, and so can't be garbage collected. Each ViewHolder takes up incredibly little space, but if you're OCD, you still have unused objects you can't remove from memory. The HolderView is allowed to go out of scope and be deconstructed.

There are a few downsides too:

• The ViewHolder pattern is nearly ubiquitous on the Internet. Honestly, if I wasn't forced to use ViewHolder by Android Annotations, I would never have known it exists (I can't justify it, but I'm guessing a similar pattern is used on iOS development, feel free to let me know).
• The HolderView likely means you'll have another class file. You don't necessarily have to do this, but it's usually better that way.
• The HolderView is used in place of the view that the ListView is actually trying to inflate. That is, the ViewHolder just caches references to elements inside the View. The HolderView actually replaces the View (which means typecasting).

By any means, the HolderView pattern kind of acts like an MVC layer for your views. You can check out my original implementation over here.

So the biggest benefit of using a HolderView pattern is that you can move the presentation logic of a View to its own class, and thus do some pretty cool stuff (for example, Dagger won't let you inject non-static inner classes).

So those are the two patterns! Coming up next: more than you ever wanted to know about ListView caching.