如何在FiRESTORE中計算集合下的文檔數？我想讓收藏參考在CloudFiRESTORE上存在的計數，我嘗試使用：FirebaseFirestore db = FirebaseFirestore.getInstance();     final CollectionReference postsCollection = db.collection("Posts");     final TaskCompletionSource<Integer> source = new TaskCompletionSource<>();     new Thread(new Runnable() {         @Override         public void run() {             int fromWhereToStart = postsCollection.get().getResult().size();             source.setResult(fromWhereToStart);         }     }).start();     Task<Integer> task = source.getTask();     task.addOnCompleteListener(new OnCompleteListener<Integer>() {         @Override         public void onComplete(@NonNull Task<Integer> task) {             Log.e("Z_fromWhereToStart", "= " + task.getResult());         }     });但不幸的是，我得到了：java.lang.IllegalStateException: Task is not yet complete是否有另一種方法來獲取另一種方法的計數來修復IllegalStateException?