How does Firebase completion callbacks work in Kotlin

Question

I have successfully read data from Firebase and displayed it in an imageView asynchronously using a completion callback. Being new to programming,

Answer 1

To solve this, you need to create your own callback to wait for Firebase to return you the data. To achieve this, first you need to create an interface like this:

interface FirebaseCallback {
    fun onCallback(list: MutableList<RecipeTemplate>)
}

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

fun readFirebaseData(firebaseCallback: FirebaseCallback) {
    ref.addListenerForSingleValueEvent(object : ValueEventListener {
        override fun onDataChange(dataSnapshot: DataSnapshot) {
            val list = ArrayList<RecipeTemplate>()
            for (ds in dataSnapshot.getChildren()) {
                val recipeTemplate = ds.getValue(RecipeTemplate::class.java!!)
                list.add(recipeTemplate)
            }
            firebaseCallback.onCallback(list)
        }

        override fun onCancelled(databaseError: DatabaseError) {}
    })
}

In the end just simply call readData() function and pass an instance of the FirebaseCallback interface as an argument wherever you need it like this:

readFirebaseData(object : FirebaseCallback {
    override fun onCallback(list: MutableList<RecipeTemplate>) {
        //Do what you need to do with your list
    }
})

This is the only way in which you can use that value outside onDataChange() function.