How to return a list from Firestore database as a result of a function in Kotlin?

This is a classic issue with asynchronous web APIs. You cannot return something now, that hasn't been loaded yet. With other words, you cannot simply return the places list as a result of a method because it will always be empty due the asynchronous behavior of the onComplete function. Depending on your connection speed and the state, it may take from a few hundred milliseconds to a few seconds before that data is available.

But not only Cloud Firestore loads data asynchronously, almost all of modern other web APIs do, since it may take some time to get the data. But let's take an quick example, by placing a few log statements in the code, to see more clearly what I'm talking about.

fun getListOfPlaces() : List<String> {
    Log.d("TAG", "Before attaching the listener!");
    val places = ArrayList<String>()
    placesRef.get().addOnCompleteListener { task ->
        if (task.isSuccessful) {
            Log.d("TAG", "Inside onComplete function!");
            for (document in task.result) {
                val name = document.data["name"].toString()
                places.add(name)
            }
        }
    }
    Log.d("TAG", "After attaching the listener!");
    return list;
}

If we run this code will, the output in your logcat will be:

Before attaching the listener!

After attaching the listener!

Inside onComplete function!

This is probably not what you expected, but it explains precisely why your places list is empty when returning it.

The initial response for most developers is to try and "fix" this asynchronous behavior, which I personally recommend against it. Here is an excelent article written by Doug Stevenson that I'll highly recommend you to read.

A quick solve for this problem would be to use the places list only inside the onComplete function:

fun readData() {
    placesRef.get().addOnCompleteListener { task ->
        if (task.isSuccessful) {
            val list = ArrayList<String>()
            for (document in task.result) {
                val name = document.data["name"].toString()
                list.add(name)
            }
            //Do what you need to do with your list
        }
    }
}

If you want to use the list outside, there is another approach. You need to create your own callback to wait for Firestore to return you the data. To achieve this, first you need to create an interface like this:

interface MyCallback {
    fun onCallback(value: List<String>)
}

Then you need to create a function that is actually getting the data from the database. This method should look like this:

fun readData(myCallback : MyCallback) {
    placesRef.get().addOnCompleteListener { task ->
        if (task.isSuccessful) {
            val list = ArrayList<String>()
            for (document in task.result) {
                val name = document.data["name"].toString()
                list.add(name)
            }
            myCallback.onCallback(list)
        }
    }
}

See, we don't have any return type anymore. In the end just simply call readData() function in your onCreate function and pass an instance of the MyCallback interface as an argument like this:

readData(object: MyCallback {
    override fun onCallback(value: List<String>) {
        Log.d("TAG", list.size.toString())
    }
})

If you are using Kotlin, please check the other answer.

Nowadays, Kotlin provides a simpler way to achieve the same result as in the case of using a callback. This answer is going to explain how to use Kotlin Coroutines. In order to make it work, we need to add the following dependency in our build.gradle file:

implementation "org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.2.1"

This library that we use is called Module kotlinx-coroutines-play-services and is used for the exact same purpose. As we already know, there is no way we can return a list of objects as a result of a method because get() returns immediately, while the callback from the Task it returns will be called sometime later. That's the reason why we should wait until the data is available.

When calling "get()" on the Task object that is returned, we can attach a listener so we can get the result of our query. What we need to do now is to convert this into something that is working with Kotlin Coroutines. For that, we need to create a suspend function that looks like this:

private suspend fun getListOfPlaces(): List<DocumentSnapshot> {
    val snapshot = placesRef.get().await()
    return snapshot.documents
}

As you can see, we have now an extension function called await() that will interrupt the Coroutine until the data from the database is available and then return it. Now we can simply call it from another suspend method like in the following lines of code:

private suspend fun getDataFromFirestore() {
    try {
        val listOfPlaces = getListOfPlaces()
    } catch (e: Exception) {
        Log.d(TAG, e.getMessage()) //Don't ignore potential errors!
    }
}

The reason for having a empty list got perfectly answered by Alex Mamo above.

I just like to present the same thing without needing to add an extra interface.

In Kotlin you could just implement it like so:

fun readData(myCallback: (List<String>) -> Unit) {
    placesRef.get().addOnCompleteListener { task ->
        if (task.isSuccessful) {
            val list = ArrayList<String>()
            for (document in task.result) {
                val name = document.data["name"].toString()
                list.add(name)
            }
            myCallback(list)
        }
    }
}

and then use it like so:

readData() {
   Log.d("TAG", it.size.toString())
})