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

Question

I'm building an app for a friend and I use Firestore. What I want is to display a list of favorite places but for some reason, the list is always empty.

I cannot get the data from Firestore. This is my code:

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

If I try to print, let's say the size of the list in onCreate function, the size is always 0.

Log.d("TAG", getListOfPlaces().size().toString()); // Is 0 !!!

I can confirm Firebase is successfully installed. What am I missing?

Answer 1

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.