CWAC EndlessAdapter: It Just Keeps Going and Going And...

AJAX Web sites have sometimes taken up the "endless page" model, where scrolling automatically loads in new content, so you never have to click a "Next" link or anything like that.

Wouldn't it be cool to have that in an Android application? Kinda like how the Android Market does it?

EndlessAdapter is one approach to solving this problem.

It is designed to wrap around another adapter, where you have your "real" data. Hence, it follows the Decorator pattern, augmenting your current adapter with new Endless Technology(TM).

To use it, you extend EndlessAdapter to provide details about how to handle the endlessness. Specifically, you need to be able to provide a row View, independent from any of the rows in your actual adapter, that will serve as a placeholder while you, in another method, load in the actual data to your main adapter. Then, with a little help from you, it seamlessly transitions in the new data.

So, this is not truly "endless" insofar as the user does see when we load in new data. However, it should work well for Android applications backed by Web services or the like that work on "page-at-a-time" metaphors -- users get the additional data quickly and do not incur the bandwidth to download that data until and unless they scroll all the way to the bottom.

Note that this has been tested with ArrayAdapter extensively but may not work with other adapter types, particularly SimpleAdapter.

This is available as a JAR file from the downloads area of this GitHub repo. The project itself is set up as an Android library project, in case you wish to use the source code in that fashion.

Usage

To use EndlessAdapter, you need to create a subclass that will control the endlessness, specifying what View to use for the "loading" placeholder, and then updating that placeholder with an actual row once data has been loaded.

EndlessAdapter assumes there is at least one more "batch" of data to be fetched. If everything was retrieved for your ListAdapter the first time out (e.g., the Web search returned only one "page" of results), do not wrap it in EndlessAdapter, and your users will not perceive a difference.

Constructors

EndlessAdapter has two constructors. The original one takes a ListAdapter as a parameter, representing the existing adapter to be made endless. Your EndlessAdapter subclass will need to override this constructor and chain upwards. For example, the DemoAdapter inside the demo project takes an ArrayList<String> as a constructor parameter and wraps it in a ListAdapter to supply to EndlessAdapter.

The second constructor takes a Context and resource ID along with the ListAdapter. These will be used to create the placeholder (see below).

The Placeholder

Your EndlessAdapter subclass can implement getPendingView(). This method works a bit like the traditional getView(), in that it receives a ViewGroup parameter and is supposed to return a row View. The major difference is that this method needs to return a row View that can serve as a placeholder, indicating to the user that you are fetching more data in the background (see below). This View is not cached by EndlessAdapter, so if you wish to reuse it, cache it yourself.

If you use the constructor that takes a Context and resource ID along with the ListAdapter, you can skip getPendingView(), and EndlessAdapter will inflate the supplied layout resource as needed to create this placeholder.

The Loading

Your EndlessAdapter subclass also needs to implement cacheInBackground(). This method will be called from a background thread, and it needs to download more data that will eventually be added to the ListAdapter you used in the constructor. While the demo application simply sleeps for 10 seconds, a real application might make a Web service call or otherwise load in more data.

This method returns a boolean, which needs to be true if there is more data yet to be fetched, false otherwise.

Since this method is called on a background thread, you do not need to fork your own thread. However, at the same time, do not try to update the UI directly.

If you expected to be able to retrieve data, but failed (e.g., network error), that is fine. However, you should then return false, indicating that you have no more data.

The Attaching

Your EndlessAdapter subclass also needs to implement appendCachedData(), which should take the data cached by cacheInBackground() and append it to the ListAdapter you used in the constructor. While cacheInBackground() is called on a background thread, appendCachedData() is called on the main application thread.

If you had a network error in cacheInBackground(), simply do nothing in appendCachedData(). So long as you returned false from cacheInBackground(), EndlessAdapter will remove the placeholder View and will operate as a normal fixed-length list. Or, override onException() to get control on the main application thread and be passed the Exception raised by cacheInBackground(), so you can do something to let the user know what went wrong. Have onException() return true if you want to retry loading data in the background, false otherwise.

Dependencies

This project relies upon the CWAC AdapterWrapper project. A copy of compatible JARs can be found in the libs/ directory of the project, though you are welcome to try newer ones, or ones that you have patched yourself.

Version

This is version v0.7.0 of this module, meaning it is slowly becoming a respected member of the Android community.

Demo

In the demo/ sub-project you will find a sample activity that demonstrates the use of EndlessAdapter.

Note that when you build the JAR via ant jar, the sample activity is not included, nor any resources -- only the compiled classes for the actual library are put into the JAR.

License

The code in this project is licensed under the Apache Software License 2.0, per the terms of the included LICENSE file.

Questions

If you have questions regarding the use of this code, please post a question on StackOverflow tagged with commonsware and android. Be sure to indicate what CWAC module you are having issues with, and be sure to include source code and stack traces if you are encountering crashes.

Release Notes

v0.7.0: cacheInBackground() can now throw checked exceptions, new getContext() method available for subclasses
v0.6.1: merged bug fix from rgladwell/cwac-endless; added @Override annotations
v0.6.0: added pending View support via constructor
v0.5.0: added onException()
v0.4.0: eliminated need for rebindPendingView(), documented the no-data scenario
v0.3.1: fixed bug in manifest
v0.3.0: converted to Android library project, added call to notifyDataSetChanged()

leotsem / android-endless