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. It also will only work with a ListView or
possibly other one-View-at-a-time AdapterView implementations.
This is available as a JAR file. The project itself is set up as an Android library project, in case you wish to use the source code in that fashion.
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.
EndlessAdapter has four 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 remaining pair are the same as the first, but take a boolean
value to indicate whether or not there should be more data to add.
Normally, you would not use these, but instead would skip the
EndlessAdapter altogether if you have no data to append. However,
there may be cases where that is inconvenient, and simply keeping
the EndlessAdapter is easier, but there is no point in bothering
actually having "endless" behavior.
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.
This placeholder row, when visible, will be disabled for list
item clicks, as of v1.2 of this library. This is a regression, but
presently an unavoidable one, as clicking on that row while the background
data fetch was going on could raise an IllegalStateException. Hence, few
(if any) developers were likely relying upon clicks on the placeholder row,
so this regression hopefully will harm few people.
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.
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.
If you returned false from onException() and whatever circumstances
caused the exception should now be resolved (e.g., you now have Internet
access where before you did not), call restartAppending(), and the
normal "endless" behavior will resume on the next scroll-to-the-bottom.
By default, EndlessAdapter will use AsyncTask with the classic
thread pool. If you would prefer your EndlessAdapter use the
serialized pool on API Level 13+ projects, call setSerialized(true).
And if that paragraph was clear as mud,
here is a blog post covering the changes to AsyncTask
that pertain to the serialized pool.
If you wish to extend what is done in this AsyncTask, create your
own subclass of the static EndlessAdapter.AppendTask, implement what you need
(chaining to the superclass to inherit existing behavior), and
override buildTask() in your EndlessAdapter subclass to create
an instance of your own custom task class.
If you would prefer that EndlessAdapter not run its own AsyncTask,
then call setRunInBackground(false). In this mode, your cacheInBackground()
method will be called on the main application thread. It is up to you
to arrange to do the work on your own background thread, then call onDataReady()
when you want the adapter to update to reflect the newly added data. Note
that appendCachedData() will not be used in this scenario.
In addition to the methods mentioned above that you could override in a
custom subclass of EndlessAdapter, note that if you elect to override
isEnabled(), you should either return false or the value of
EndlessAdapter's own isEnabled() via a chain to super. EndlessAdapter
needs to mark the pending row placeholder as disabled. EndlessAdapter
already then forwards isEnabled() for all other rows to your wrapped
adapter, so it is simpler for you to just override isEnabled() there
rather than by overriding EndlessAdapter's implementation.
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.
This is version v1.2 of this module. It should be backwards-compatible with v1.0, with only added methods. However, there is the one behavior regression (placeholder row no longer clickable) as noted above.
In the demo/ sub-project you will find
three sample activities that demonstrate the use of EndlessAdapter.
Included in this is EndlessAdapterFragmentDemo, which shows how
to use EndlessAdapter in a retained fragment. Note that while
the demo/ sample requires API Level 11 (as EndlessAdapterFragmentDemo
uses native fragments and the native action bar), EndlessAdapter
should work back to API Level 3.
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.
The code in this project is licensed under the Apache Software License 2.0, per the terms of the included LICENSE file.
Do not ask for help via Twitter.
And, for those of you who skipped over that sentence: do not ask for help on Twitter. Anyone who thinks that developer support can be handled in 140-character chunks should not be attempting to use a CWAC component.
Now, that being said, the rest of your help will be for bugs or questions.
If you are experiencing some sort of problem using this component, where you are fairly certain the component
itself is at fault, please create a project that can reproduce the problem. Then, post the source code to
that project somewhere (e.g., a public GitHub repo). Next, file an
issue, pointing to your project and providing
instructions on how to reproduce the problem. Note that if you can reproduce the error with the project's
own demo/ sub-project, just provide the steps to reproduce the problem.
Do not file an issue if you cannot reproduce the problem, or with only partial source code that may or may not be related to the problem.
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 relevant source code
and stack traces if you are encountering crashes.
- v1.2: made pending view disabled for list item clicks, made fewer mods in BG thread
- v1.1: merged two pull requests, adding new constructors and
stopAppending() - v1.0.0: made this the official 1.0 release
- v0.10.0: added support for
setRunInBackground()(patch courtesy of brk3), cleaned up demos a bit - v0.9.1: made
AppendTaskconstructorprotected - v0.9.0: added
restartAppending()andbuildTask(), refactoredAppendTask, added new sample activity - v0.8.0: added
setSerialized()andisSerialized() - v0.7.0:
cacheInBackground()can now throw checked exceptions, newgetContext()method available for subclasses - v0.6.1: merged bug fix from rgladwell/cwac-endless; added @Override annotations
- v0.6.0: added pending
Viewsupport 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()
