Returning value after Firestore collection.get() has finished the operation

Question

I am running code similar to the problem in this question except, I am fetching arrays from each document and adding their elements to a global ArrayList. After that, outside of the collection.get(), I perform an operation on that array and return. However, If I Log.d the global ArrayList size from within the onSuccess listener, it gives the correct answer but if I do so outside of the collection.get() it always gives 0. My problem is that I cannot return from within the onSuccess because this will not return for my whole function and so I need to do it after the collection.get() where the ArrayList has not been populate, presumably since the get() operation returns a Task<> which is an asynchronous operation and so it doesn't wait for it to complete. Here is my code:

public static boolean Validator(final String sometext) {
    FirebaseFirestore db = FirebaseFirestore.getInstance();

    final ArrayList<String> mArrayList = new ArrayList<>();

    db.collection("my_collection").get()
        .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                if (queryDocumentSnapshots.isEmpty()) {
                    Log.d(TAG, "Validator onSuccess: LIST EMPTY");
                    return;
                } else {

                    // Convert the Query Snapshot to a list of Document Snapshots, in this
                    // case, one document for each my_collection
                    List<DocumentSnapshot> my_collections = queryDocumentSnapshots.getDocuments();


                    for (int i = 0; i < my_collections.size(); i++) {
                        ArrayList<String> mArrayitems = (ArrayList<String>) my_collections.get(i).get("array");

                        for (int j = 0; j < mArrayitems.size(); j++) {
                            Log.d(TAG, "Adding array: " + mArrayitems.get(j).toString());
                            mArrayList.add(mArrayitems.get(j).toString());
                        }
                    }
                    Log.d(TAG, "Validator onSuccess: array list built");
                }
            }
        })
        .addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                Log.d(TAG, "Validator onFailure: " + e);
            }
        });

  // check if the given sometext matches a array in the list
  Log.d(TAG, "array list size: " + mArrayList.size());
  if (mArrayList.size() > 0) {
    for (int i = 0; i < mArrayList.size(); i++) {
        if (sometext.endsWith(mArrayList.get(i))) {
            return true
        }
      }
   }
   return false;
 }

EDIT:
I'm trying to do this with a callback which looks like:

// constructor
public Validator(String sometext) {
    this.valid = false;
    this.sometext = sometext
}

public interface myCallback{
    void onCallback(boolean valid);
}

public boolean validate() {

  readData(new MyCallback() {
        @Override
        public void onCallback(boolean valid) {
            setValid(valid);
        }
    });
  return this.valid;
 }

private void readData(final MyCallback myCallback) {
    FirebaseFirestore db = FirebaseFirestore.getInstance();

    db.collection("my_collection").get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                    if (queryDocumentSnapshots.isEmpty()) {
                        Log.d(TAG, "Validator onSuccess: LIST EMPTY");
                    } else {

                        List<DocumentSnapshot> my_collections = queryDocumentSnapshots.getDocuments();

                        ArrayList<String> mArrayList = new ArrayList<>();

                        for (int i = 0; i < my_collections.size(); i++) {
                            ArrayList<String> mArrayItems = (ArrayList<String>) my_collections.get(i).get("array");
                            for (int j = 0; j < mArrayItems.size(); j++) {
                                Log.d(TAG, "Adding array: " + mArrayItems.get(j).toString());
                                mArrayList.add(mArrayItems.get(j).toString());
                            }
                        }

                        boolean result = false;

                        Log.d(TAG, "array list size: " + mArrayList.size());
                        if (mArrayList.size() > 0) {
                            for (int i = 0; i < mArrayList.size(); i++) {
                                if (sometext.endsWith(mArrayList.get(i))) {
                                    result = true;
                                    Log.d(TAG, "MATCH FOUND");
                                    break;
                                }
                            }
                        }

                        myCallback.onCallback(result);

                    }
                }
            })
            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    Log.d(TAG, "Validator onFailure: " + e);
                }
            });
}

The problem I'm having is that in validate() I want to return the correct valid value after the callback is finished because it currently returns the incorrect value before the callback has finished.

It seems like you know what you need to do - only use the result from the get from within the callback, or kick off other work from the callback that uses the result. — Doug Stevenson, May 08 '18 at 16:54
@DougStevenson My problem is I can't return from within the callback because that just returns for onSuccess, not for Validator. And if I use the variable outside of the callback, I get the wrong value. — Paradox, May 08 '18 at 16:56
Possible duplicate of [Firestore - object with inner Object](https://stackoverflow.com/questions/48499310/firestore-object-with-inner-object) — Alex Mamo, May 08 '18 at 16:57
Please check the duplicate to see why is this happening and how you can solve this using a custom callback. — Alex Mamo, May 08 '18 at 16:57
You shouldn't be returning values from callbacks. Just call another method and pass the result into it from the callback. — Doug Stevenson, May 08 '18 at 16:59
@AlexMamo I've updated my question after trying to use the callback but It's still returning false. I've updated my question to show you what I mean. — Paradox, May 10 '18 at 00:59
That's not correct. Please follow the exact stepts from that post but instead of using `UserAccountSettings` use your list. It will be helpful for you if I'll write you some code? — Alex Mamo, May 10 '18 at 09:24
@AlexMamo I'm not sure where I made the mistake. Should I set the callback for the queryDocumentSnapshots instead of the mArrayList? If you don't mind, would you be able to write some code? — Paradox, May 10 '18 at 13:52
Before writing some, please try to change your code. You cannot use `return valid;` outside the callback. It will always be `false`. You need to use it only inside, right? — Alex Mamo, May 10 '18 at 14:00
@AlexMamo I can't return valid from within the onCallback method though since it's void. I need Validator() to return a boolean value so I don't know how else to do it? — Paradox, May 10 '18 at 14:17
I see you added as an argument an `ArrayList mArrayList` and not a boolean. If you need a boolean, change to a boolean or use two arguments. — Alex Mamo, May 10 '18 at 14:24
@AlexMamo Edited question to show more of my code and use boolean. But I'm still having the issue of validate() returning the wrong value of valid. I need to return the valid value after onCallback has completed. — Paradox, May 10 '18 at 14:40
Inside this method call `setValid(valid);`, the `valid` is looks good and I'm sure that it works but you cannot use `return this.valid;`. Is the same thing as in the first place. — Alex Mamo, May 10 '18 at 14:55
@AlexMamo I tried making validate() void and have it only do the setValid(valid) within the onCallback, then from the calling class, it does a validate() then an isValid() but it still gave the same problem of a false valid. — Paradox, May 10 '18 at 15:03

score 0 · Accepted Answer · answered May 10 '18 at 16:13

I was able to solve my issue by first using @AlexMamo's solution here and then listen for the valid variable being changed by using this solution here.

public class ValidatorVariable {
    private boolean valid;
    private ChangeListener listener;

    public boolean isValid(){
        return valid;
    }

    public void setValid(boolean valid){
        this.valid = valid;
        if (listener != null) listener.onChange();
    }

    public ChangeListener getListener(){
        return listener;
    }

    public void setListener(ChangeListener listener){
        this.listener = listener;
    }

    public interface ChangeListener {
        void onChange();
    }
}

which is used with:

ValidatorVariable vv = new ValidatorVariable();
        vv.setListener(new ValidatorVariable.ChangeListener() {
            @Override
            public void onChange() {
                if (vv.isValid()){
                    // do stuf
                }
            }
        });

Returning value after Firestore collection.get() has finished the operation

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