How to properly get the results from firestore database fetches embracing ascynchronous functionality? - Android - Kotlin - MVVM

Issue

Because database fetches usually happen asynchronously by default, a variable that holds the data from the firebase database fetch will be null when used right after the fetch. To solve this I have seen people use the ".await()" feature in Kotlin coroutines but this goes against the purpose of asynchronous database queries. People also call the succeeding code from within 'addOnSuccessListener{}' but this seems to go against the purpose of MVVM, since 'addOnSuccessListener{}' will be called in the model part of MVVM, and the succeeding code that uses the fetched data will be in the ViewModel. The answer I'm looking for is maybe a listener or observer that is activated when the variable (whose value is filled from the fetched data) is given a value.

Edit: by "succeeding code" I mean what happens after the database fetch using the fetched data.

Solution

As @FrankvanPuffelen already mentioned in his comment, that's what the listener does. When the operation for reading the data completes the listener fires. That means you know if you got the data or the operation was rejected by the Firebase servers due to improper security rules.

To solve this I have seen people use the ".await()" feature in Kotlin coroutines but this goes against the purpose of asynchronous database queries.

It doesn't. Using ".await()" is indeed an asynchronous programming technique that can help us prevent our applications from blocking. When it comes to the MVVM architecture pattern, the operation for reading the data should be done in the repository class. Since reading the data is an asynchronous operation, we need to create a suspend function. Assuming that we want to read documents that exist in a collection called "products", the following function is needed:

suspend fun getProductsFirestore(): List<Product> {
    var products = listOf<Product>()
    try {
        products = productsRef.get().await().documents.mapNotNull { snapShot ->
            snapShot.toObject(Product::class.java)
        }
    } catch (e: Exception) {
        Log.d("TAG", e.message!!)
    }
    return products
}

This method can be called from within the ViewModel class:

val productsLiveData = liveData(Dispatchers.IO) {
    emit(repository.getProductsFromFirestore())
}

So it can be observed in activity/fragment class:

private fun getProducts() {
    viewModel.producsLiveData.observe(this, {
        print(it)
        //Do what you need to do with the product list
    })
}

I have even written an article in which I have explained four ways in which you can read the data from Cloud Firestore:

• How to read data from Cloud Firestore using get()?

Answered By - Alex Mamo