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CommonsWare Android Components: EndlessAdapter

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README.markdown

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

THIS PROJECT IS RETIRED

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.

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.

Installation

This Android library project is available as a JAR. If you wish to use the JAR, you will need to also add the JAR from the CWAC-Adapter project to your project

If you plan to use this as an Android library project in source form, you will also need to download the CWAC-Adapter project (and, if needed, modify this project's configuration to point to your copy of CWAC-Adapter library project). Alternatively, download the CWAC-Adapter JAR into the libs/ directory of your clone of this project and remove the dependency on the CWAC-Adapter library project.

This project is also available as an artifact for use with Gradle. To use that, add the following blocks to your build.gradle file:

repositories {
    maven {
        url "https://repo.commonsware.com.s3.amazonaws.com"
    }
}

dependencies {
    compile 'com.commonsware.cwac:endless:1.2.3'
}

Or, if you cannot use SSL, use http://repo.commonsware.com for the repository URL. This should automatically pull down the CWAC-Adapter dependency.

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 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.

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.

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.

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.

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.

The Threading

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.

The Overriding

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.

Limitations

EndlessAdapter does not work well as a child of other decorating adapters, notably the HeaderListViewAdapter that ListView creates if you use header or footer views.

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.

This project should work on API Level 4 and higher, except for any portions that may be noted otherwise in this document. Please report bugs if you find features that do not work on API Level 4 and are not noted as requiring a higher version.

Version

This is version v1.2.2 of this module. It should be backwards-compatible with v1.0.0, with only added methods. However, there is the one behavior regression (placeholder row no longer clickable) as noted above.

Demo

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.

License

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

Getting Help

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.

Bugs

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.

Other 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 relevant source code and stack traces if you are encountering crashes.

Release Notes

  • v1.2.3: migrated to Gradle, published AAR
  • v1.2.2: removed extraneous Log statement
  • v1.2.1: fix for IllegalStateException: The content of the adapter has changed bug
  • 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 AppendTask constructor protected
  • v0.9.0: added restartAppending() and buildTask(), refactored AppendTask, added new sample activity
  • v0.8.0: added setSerialized() and isSerialized()
  • 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()

Who Made This?

CommonsWare

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