Paging sample with different network configurations

This CL adds a new sample application that uses Reddit API to showcase
3 different paging implementations.

1) DB + Network: This sample keeps the data in the database and also
uses a boundary callback to load more when necessary.

2) Network by Item Name: This sample does not persist results, instead
uses an ItemKeyedDataSource to page results.

3) Network by Response Keys: This sample is also memory only and uses
the before/after fields in the network response.

Change-Id: Id36e23fef4c6abe134243a41e44a85d2d65835fd

Author: yigit

Diff for: PagingWithNetworkSample/.gitignore

@@ -0,0 +1,9 @@
+*.iml
+.gradle
+/local.properties
+/.idea/workspace.xml
+/.idea/libraries
+.DS_Store
+/build
+/captures
+.externalNativeBuild

Diff for: PagingWithNetworkSample/README.md

@@ -0,0 +1,69 @@
+# Paging With Network Sample
+
+This sample demonstrates how to use the Paging library with a backend API (in this
+case [Reddit API][8]).
+
+There are 3 variations of the demo, which you can select in the `MainActivity` class.
+
+After selecting an option, it starts the `RedditActivity` which is the activity that
+shows the list of posts in a given subreddit.
+
+## Paging With Database And Network
+This sample, implemented in the [DbRedditPostRepository][1] class, demonstrates how to set up
+a Repository that will use the local database to page in data for the UI and also back-fill
+the database from the network as the user reaches to the end of the data in the database.
+
+It uses `Room` to create the `DataSource.Factory` ([dao][3]) and the Paging Boundary Callback
+API to get notified when the Paging library consumes the available local data ([boundary callback
+implementation][4])
+
+This usually provides the best user experience as the cached content is always available
+on the device and the user will still have a good experience even if the network is slow /
+unavailable.
+
+## Paging Using Item Keys
+This sample, implemented in the [InMemoryByItemRepository][2] class, demonstrates how to
+set up a Repository that will directly page in from the network and will use the `key` from
+the previous item to find the request parameters for the next page.
+
+[ItemKeyedSubredditDataSource][5]: The data source that uses the `key` in items
+(`name` in Reddit API) to find the next page. It extends from the `ItemKeyedDataSource` class
+in the Paging Library.
+
+## Paging Using Next Tokens From The Previous Query
+This sample, implemented in the [InMemoryByPageKeyRepository][6] class, demonstrates how to
+utilize the `before` and `after` keys in the response to discover the next page. (This is
+the intended use of the Reddit API but this sample still provides
+[ItemKeyedSubredditDataSource][5] to serve as an example if the backend does not provide
+before/after links)
+
+[PageKeyedSubredditDataSource][7]: The data source that uses the `after` and `before` fields
+in the API request response. It extends from the `PageKeyedDataSource` class
+in the Paging Library.
+
+
+### Libraries
+* [Android Support Library][support-lib]
+* [Android Architecture Components][arch]
+* [Retrofit][retrofit] for REST api communication
+* [Glide][glide] for image loading
+* [espresso][espresso] for UI tests
+* [mockito][mockito] for mocking in tests
+* [Retrofit Mock][retrofit-mock] for creating a fake API implementation for tests
+
+[1]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inDb/DbRedditPostRepository.kt
+[2]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inMemory/byItem/InMemoryByItemRepository.kt
+[3]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/db/RedditPostDao.kt
+[4]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inDb/SubredditBoundaryCallback.kt
+[5]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inMemory/byItem/ItemKeyedSubredditDataSource.kt
+[6]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inMemory/byPage/InMemoryByPageKeyRepository.kt
+[7]: app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/repository/inMemory/byPage/PageKeyedSubredditDataSource.kt
+[8]: https://www.reddit.com/dev/api/#listings
+[mockwebserver]: https://github.com/square/okhttp/tree/master/mockwebserver
+[support-lib]: https://developer.android.com/topic/libraries/support-library/index.html
+[arch]: https://developer.android.com/arch
+[espresso]: https://google.github.io/android-testing-support-library/docs/espresso/
+[retrofit]: http://square.github.io/retrofit
+[glide]: https://github.com/bumptech/glide
+[mockito]: http://site.mockito.org
+[retrofit-mock]: https://github.com/square/retrofit/tree/master/retrofit-mock