ArrayList<String> 方法返回一个从 firebase 获取数据的空数组

Question

我的返回有问题..由于下载数据需要几秒钟，当方法返回 ArrayList 时，它仍然是空的。事实上，如果我把那行代码Log.v("array", String.valueOf(partecipantsArrayList)); 首先它打印 0，然后打印填充的数组。所以我必须在return语句之前等待..有什么办法可以做到这一点？

public ArrayList<String> getPartecipantsList(){
    String email = getEmail();
    String groupTitle = getTitleBar();
    DocumentReference docRef = db.collection("users").document(email).collection("Group").document(groupTitle);

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


                        DocumentSnapshot document = task.getResult();


                        //Extracting participants ArrayList from the document
                        for(Object item : task.getResult().getData().values()) {

                            String[] values = String.valueOf(item).replace("[", "").replace("]", "").split(",");

                            for (String value : values){

                                    partecipantsArrayList.add(value);

                            }

                        }
                        partecipantsArrayList.remove(String.valueOf("["));
                        partecipantsArrayList.remove(partecipantsArrayList.size() - 1);
                        Log.v("array", String.valueOf(partecipantsArrayList));

                    }

                })

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

                }
            });


    return partecipantsArrayList;
}

Answer 1

这是异步任务，您可以像下面这样调整此代码。

将该方法的签名更改为return void

public void getPartecipantsList();

on 完成后执行下一步操作

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

                        ...............
                        partecipantsArrayList.remove(String.valueOf("["));
                        partecipantsArrayList.remove(partecipantsArrayList.size() - 1);

                        Log.v("array", String.valueOf(partecipantsArrayList));
                       // do your action by calling next method from here
                      //eg: setResultFromArray(partecipantsArrayList);
                    }

当onFailure 中的失败时执行失败操作或调用方法。

                @Override
                public void onFailure(@NonNull Exception e) {
                     //......
                }