Return when task is completed

Question

I'm facing a problem and I don't know how to solve it.

I'm using firebase firestore to retrieve me an exact Item. This works. The problem comes when I want the task to retun that Item in a custom class. Since firebase firestore task is asynchronous it returns null before it's completed.

Here's the code in DataAdapter class:

public Data infoFile(int position){

    DocumentReference fileRef = getInfoFile(position); //Method that retrieves the document.

    //Array to set later the Data Object that will retrieve
    final Data[] dataArray = {new Data()};

    fileRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>()
    {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task)
        {

            if(task.isSuccessful()){
                DocumentSnapshot documentSnapshot = task.getResult();

                if(documentSnapshot.exists()){

                    Data data = documentSnapshot.toObject(Data.class); //Get the data
                    dataArray[0] = data; //Set data
                    System.out.println("before put bundle" + dataArray[0].getName()); //This gets the data
                    //Can't return here.

                }
            }
        }
    }).addOnFailureListener(new OnFailureListener()
    {
        @Override
        public void onFailure(@NonNull Exception e)
        {

        }
    });
    System.out.println("before put bundle and almost return " + dataArray[0].getName());

    //This returns null because task isn't completed yet.  
    return dataArray[0];
}

I've read about creating and interface and then passing the Data to the method and recieve it. But I don't know how to create/understand that. Maybe I'm wrong and that doesn't work.

Question: How can I get to return Data model when task is completed?

Answer 1

The last line will return null because it will get executed before the onComplete() is finished.

onComplete() on the other hand has return type void and you can't return a value in that function

You need to create a function inside the class from where you call intoFile function

public void setData(Data data){
    //Set value and use
}

And inside onComplete()

Do this

    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task)
    {

        if(task.isSuccessful()){
            DocumentSnapshot documentSnapshot = task.getResult();

            if(documentSnapshot.exists()){

                Data data = documentSnapshot.toObject(Data.class); //Get the data
                dataArray[0] = data; //Set data
                System.out.println("before put bundle" + dataArray[0].getName()); //This gets the data
                setData(data)                }
        }
    }

Once the value is set you can perform the next operations from inside setData or you can call a function from setData which performs the next operations like this.

public void setData(Data data){
    this.data = data;
    executeNextTask(data);
}

Answer 2

Here you can not return data because you do not know when the data is ready to return it so you can just change the method you invoke this function:

Now I think you use the function:

Data d =  infoFile(position);
handleData(d);

change it to be:

infoFile(position,new OnCompleteListener<DocumentSnapshot>()
    {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task)
        {

            if(task.isSuccessful()){
                DocumentSnapshot documentSnapshot = task.getResult();

                if(documentSnapshot.exists()){

                    Data data = documentSnapshot.toObject(Data.class); //Get the data
                    dataArray[0] = data; //Set data
                    System.out.println("before put bundle" + dataArray[0].getName());   

                    //move the handling to here.
                    handleData(data);

                }
            }
        }
    }).addOnFailureListener(new OnFailureListener()
    {
        @Override
        public void onFailure(@NonNull Exception e)
        {

        }
    });

Then change the function to:

public void infoFile(int position,OnCompleteListener<DocumentSnapshot> listener){

    DocumentReference fileRef = getInfoFile(position); //Method that retrieves the document.

    //Array to set later the Data Object that will retrieve
    final Data[] dataArray = {new Data()};

    fileRef.get().addOnCompleteListener(listener);
    System.out.println("before put bundle and almost return " + dataArray[0].getName());

}