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#SortedList Playbook

##Blog post series A detailed exploration of SortedList through a series of posts is available below.

##What is a SortedList SortedList is a data structure to keep the items in the list sorted. Through its various callbacks it can provide information of what exactly has changed in the data structure when items are inserted/removed/updated.

With these callbacks SortedList.Callback, SortedList.BatchedCallback, SortedListAdapterCallback, SortedList becomes a perfect data structure to back any list based views. After all, every list in sorted in one way or the other.

##SortedList Data Exploration Before jumping into the example provided by support library, lets just begin by simple data based examples. Lets explore it via the Unit tests.

Test setup

We will be adding bunch of articles to a SortedList and understand its callbacks. Article.java encapsulates our data item. Every article contains a unique id, some content, an author who produced the content and a published timestamp, which is a unix epoch value in milliseconds.

  • Article.java is an Auto Value class.
  • All through the tests we will be working with Immutable instances of articles.
  • We will be using Java Faker to generate fake data for our testing.
  • We will use fluent assertions provided by AssertJ

When articles are presented in UI, generally we would want to sort them by their published time. Thats exactly what we will be exploring using JUnit 4 tests.

JUnit 4 Tests

SortedListTestSetup.java is a JUnit 4 Rule that sets up all the necessary things for testing SortedList. It does the following

  • 5 articles are created, 2 in the past, 1 on the current day and 2 in future.
  • orderedArticleList instance contains articles ordered according to their published time beginning from older article to the newer article.
  • shuffledArticlesinstance contains the same articles (different object instances, as our articles are immutable) shuffled.
  • SortedListCallbackRecorder class is a simple implementation of SortedList Callback that records all the callbacks emitted by SortedList. There is a utility method clear to erase all the existing records.
  • Test rule exposes SortedListCallbackRecorder instance as callbackRecorder
  • Finally test rule also gives sortedList instance which is an empty SortedList that is ready for testing.

SortedList Test Class

SortedListCallbackUnitTest.java is our test class. It sets up SortedListTestSetup rule as fixture to ensure every test case gives us a new fixture data. Lets go through each of the tests in details.

Test #1 - SortedList should be sorted.
  • The test method testAddShouldSortListIrrespectiveOfOrder adds all the 5 articles from fixture.shuffledArticles() into sorted list and compares the entries of SortedList with fixture.orderedArticleList()
Test #2 - Adding multiple same objects into sorted list should not have any effect
  • This test is performed by the method testAdditionOfSameObjectShouldNotChangeSortedList.
  • It adds a single article to fixture.sortedList()
  • It creates a duplicate of the same artcile previously added to SortedList and adds it again n times, where n can vary between 1 to 10.
  • It asserts that SortedList size remains same and that none of the SortedList callbacks get recorded.
Test #3 - Validating insertion callbacks
  • This test is performed by the method testInsertions
  • It iterates through shuffled articles. For each article, it figures out the position in which the item will get inserted and then asserts it with recorded callback from the fixture.
Test #4 - Validating changes callbacks
  • This test is performed by the method testChanges
  • It adds all the 5 artciles from the fixture to SortedList.
  • It then randomly picks up an article from the SortedList, duplicates the object, changes its data and then adds it to the SortedList
  • It validates that only changes callback gets recorded and asserts that changed content is reflected in the list.
Test #5 - Validating deletions.
  • This test is performed by method testDeletions
  • It invokes remove method on SortedList and asserts that only deletion callbacks is recorded.
Test #5 - Validating position moves.
  • This test is performed by method testMoves
  • It fetches an article, duplicates it and changes the published timestamp. A timestamp change is needed to test position change
  • It then validates that indeed position is moved properly with move callback recorded and additionally a change callbacks is triggered as well on the old position.
Test #6 - Validating batch updates.
  • This test is performed by method testBatchedCallbacks
  • It creates a new SortedList with batched callback that wraps fixture's callback recorder.
  • It inserts all 5 artciles from fixture's shuffled articles between SortedList's beginBatchedUpdates and endBatchedUpdates methods.
  • It validates that none of the callbacks get recorded before endBatchedUpdates.
  • It asserts that after endBatchedUpdates, there is only one single callback to insert with a count of 5.

The above tests mostly, if not completely, test all the features of SortedList. Play around with them to understand SortedList.

SortedList is perfect data strcuture to work with RecyclerView. Infact, all the callbacks methods have exact same signature as RecyclerView's notifyXYZ methods.

Simply using [SortedListAdapterCallback] (https://developer.android.com/reference/android/support/v7/widget/util/SortedListAdapterCallback.html), you should be able to connect SortedList with RecyclerView.