Progressively loading ListViews

Again I received a mail from somebody who asked for lists with forward/backward paging option. These lists are not trivial to implement but I was more intrigued by the need why anyone would want such a clumsy UI construct. Then it turned out that the guy wanted to back the list with data from a web service. Obviously, he did not want to wait until the entire list is loaded through the network but wanted to present something to the user as soon as possible.

Such a list can be found in Android Market when the list of applications is progressively loaded from the network server. The problem is worth a simple test program to analyse because it clearly shows the power of the Android adapter concept.

Click here to download the test program.

Our test application does not actually connect to the network but simulates the slow data source. If you check the Datasource class, you will find that even though this "data source" is a simple array, the accessor method adds a 500 ms delay to every access. This simulates well the fact that loading from certain data sources takes time. The trick is in the PageAdapter class. This class adds a View to the end of the list ("Loading ..."). Android Market animates this view but the idea is the same. Whenever this View is drawn, the adapter kicks the loading thread which progressively fetches new elements from the data source, adds them to the Adapter's data set and notifies the adapter that the data set has changed. There is the usual fiddling with a Handler to make sure that notifyDataSetChanged() is invoked in the context of the UI thread. The Adapter constantly changes the value returned by the getCount() method as the items are loaded from the data source. When the whole data set is loaded, the "Loading ..." widget is not drawn anymore therefore nothing triggers the loading thread.

Here is how it looks like.



No, wait a minute, here is how it looks like.


Yes! After having blogged for more than 2 years about Android, I finally put my hands on an actual Android phone. The story is complicated but the essence is that a friend thought that I deserved that phone and gave it to me. Thanks for everyone involved!

Posted in: adapter,list