HW5: Trying out Firebase Firestore cloud storage

This homework is based on the base project we used in Lab 5 . It's recommended to complete the lab before proceeding. The goal is to get to know some basics of Firestore and switch out the Room local database (done in the lab) with a Firestore DB.

Setting up Firebase (1 pt)

We will need to create a new Firebase project and use Firestore database for it. This requires a Google account.

Show steps for creating the Firebase project online

Verifying your App can use Firestore (show steps)

    // Get instance of database
    val db = FirebaseFirestore.getInstance()

        val cities = db.collection("cities")

        val dataTallinn = hashMapOf(
            "name" to "Tallinn",
            "population" to 434562,
            "area_km2" to 159
        )
        cities.document("TLL").set(dataTallinn)
            .addOnSuccessListener { Log.d(TAG, "Document successfully written!") }
            .addOnFailureListener { e -> Log.w(TAG, "Error writing document", e) }


        val dataTartu = hashMapOf(
            "name" to "Tartu",
            "population" to 93865,
            "area_km2" to 39
        )
        cities.document("TRT").set(dataTartu)

After running your app with the above steps, the data should become visible in the web dashboard of Firestore. This part was just to test you have set up Firestore correctly, you should delete it for the final HW solution.

Task Requirements

Now, study the Firestore Getting Started Guide and other Docs and try to update the project so that the following functionality works and is using Firestore:

1. Listing all recipes in the DB in MainActivity (2 pts)
   • Should use a query that queries for all recipes (all Firestore documents in the recipes collection), but sorts them by recipe title!
2. Creation of new Recipes ( 1pt)
   • Insert 1 new document based on the user-entered values, should use a query similar to the 'cities' example above
3. Display of Recipe Details view ( 1 pts)
   • Should work based on a query that selects/filters by ID! (get a document by its ID)
     • Note that Firestore document ID-s are Strings, not Ints

Tips

Some pointers

• How to query/read data and log it?
• How to sort results?
• What are documents and collections?
• Lecture 5 slides PDF has some basic examples of Firestore usage at the end.

Tip: Updating the UI / Activity / Adapter after getting Firebase query results

The Firebase library makes requests to the DB asynchronously and you get the results in callback methods (e.g. via .addOnSuccessListener { result -> /** do something with the query result */ } .

The UI should be notified that new data is available from inside the callback, not right after the code block defining the request. In the lab, we set Room not to make requests asynchronously. This means that code like this:

    // MainActivity
    override fun onResume() {
        super.onResume()

        model.refresh() // uses DB to update model.recipeArray
        recipesAdapter.data = model.recipeArray
        recipesAdapter.notifyDataSetChanged()
    }

will not work well out of the box for Firestore, since model.refresh() with firebase will do most of its work asynchronously in the background, meaning that the next lines of code (which update the adapter) will execute before the background DB query has finished. As a result, you may not immediately see the latest data in your RecyclerView with this approach.

To overcome this, here are some options:

• If you are unsure, go ahead and make all Firestore queries in your Activities and ignore using ViewModel completely for this homework.
• A nicer approach would be to define your own callback / interface
  • Recall how we defined and used a MessageHandler interface in Lab4. There, it allowed the BroadcastReceiver to trigger MainActivity's adapter.
  • In this homework, you could introduce a similar interface as an extra argument to the refresh() method, for instance. Then the ViewModel, after pulling data from the DB, can trigger some logic in MainActivity through the interface, just like MessageHandler in Lab4.

In week 6, we will learn more about how to manage background work.

Tip: Data Type returned by Firestore

Firestore returns documents as (Hash)Maps, so a straight-forward way is to manually construct RecipeEntities based on the various values (title, author, etc) contained in the HashMap.

Alternatively you could replace RecipeEntity entirely by HashMap. So, instead of having Array<RecipeEntity> in viewmodel, you might have Array<HashMap> instead.

Another option is to use tell Firebase which Custom data class (e.g. RecipeEntitiy) to translate the querie responses to, for more info see here: https://firebase.google.com/docs/firestore/query-data/get-data#custom_objects

• You should remove all previous Room-related classes and code (Dao, LocalRecipeDb). You can re-use the RecipeEntity data class, but remove the room @-annotations.

Other Troubleshooting

If you get an error about no of method references exceeding 65,536 methods after adding the Google services libraries, you probably have to enable multidex:

https://developer.android.com/studio/build/multidex#mdex-gradle