为什么我会收到有关变量初始化的错误？

Question

The error is Variable 'uid2' must be initialized and I've put a comment where it's saying this. My goal is to query a uid from one collection and then insert it into another collection.

fun addFriend(username: String) { 

        var uid2: String

        var docRef = firebaseFirestore.collection(collUsers).whereEqualTo("username", username)
            .get()
            .addOnSuccessListener { documents ->
                for (document in documents) {
                    Log.d(tag, "${document.id} => ${document.data}")
                    uid2 = document.data.getValue("uid").toString()
                    Log.d(tag, "uid_2: $uid2")
                }
            }
            .addOnFailureListener { exception ->
                Log.w(tag, "Error getting documents: ", exception)
            }

        var collRelationships2: CollectionReference = firebaseFirestore.collection(collRelationships)
        var relationshipsMap: HashMap<String, Any> = hashMapOf(
                                        "uid_1" to firebaseAuth.uid.toString()
                                        , "uid_2" to uid2 //Error is here
                                        , "stat" to 1
                                        , "createDate" to FieldValue.serverTimestamp()
                                        , "modifiedDate" to FieldValue.serverTimestamp()
                                    )

        collRelationships2.add(relationshipsMap)

    }

Answer 1

get() is asynchronous, which means that another task will be executed before even retrieving the data from firestore. In your code, the creation of the hash map is happening before retrieving the data from the database, therefore you get the error that uid2 is not initialized. You can do the following:

        var docRef = firebaseFirestore.collection(collUsers).whereEqualTo("username", username)
            .get()
            .addOnSuccessListener { documents ->
                for (document in documents) {
                    Log.d(tag, "${document.id} => ${document.data}")
                    uid2 = document.data.getValue("uid").toString()
                    Log.d(tag, "uid_2: $uid2")

                var collRelationships2: CollectionReference = firebaseFirestore.collection(collRelationships)
                var relationshipsMap: HashMap<String, Any> = hashMapOf(
                                        "uid_1" to firebaseAuth.uid.toString()
                                        , "uid_2" to uid2 //Error is here
                                        , "stat" to 1
                                        , "createDate" to FieldValue.serverTimestamp()
                                        , "modifiedDate" to FieldValue.serverTimestamp()
                                    )

        collRelationships2.add(relationshipsMap)
                }
            }
            .addOnFailureListener { exception ->
                Log.w(tag, "Error getting documents: ", exception)
            }

Another way is to use await() which will make the code very simple.

Answer 2

The thing to be aware of here is that Firestore queries are asynchronous, and return immediately before the query is complete. The success and error callbacks are invoked some time later on the main thread with the results of the query.

Your code is attempting to use uid2 before it will eventually be given a value in the future callback. If you want to continue work with the results of the query, the code to work with those results must be within the callback itself, not outside the callback.

(The exception is if you rewrite your code to use coroutines, but that's outside the scope of this question.)