从AsyncTask事务访问Firebase数据库

Question

public class MyAsyncTask extends AsyncTask<String,Void,Void> {

    @Override
    protected Void doInBackground(String... params) {
        String para1 = params[0];
        String para2 = params[1];

        performCalculations(para1,para2);

        return null;
    }

    public void performCalculation(String para1, String para2) {
        DatbaseReference ref = FirebaseDatabase.getInstance().getReference().child(para1);

        // Run the transaction here
        ref.runTransaction(new Transaction.Handler() {
            @Override
            public Transaction.Result doTransaction(MutableData mutableData) {

                 TaskCompletionSource<DataSnapshot> taskCompletionSource = new TaskCompletionSource<>();

                 DatabaseReference ref2 = FirebaseDatabase.getInstance().getReference().child(param2);

                 ref2.addListenerForSingleValueEvent(new MyValueEventListener(taskCompletionSource));

                 try {
                    dataSnapshot = Tasks.await(taskCompletionSource.getTask());

                    // DOING SOME AWESOME THINGS HERE 
                    // And updating the mutable Data


                } catch (ExecutionException | InterruptedException e) {
                    Log.v(TAG, "Exception Catched");
                    e.printStackTrace();
                }


                mutableData.setValue(stats);
                return Transaction.success(mutableData);
            }

            @Override
            public void onComplete(DatabaseError databaseError, boolean b, DataSnapshot dataSnapshot) {
                if (databaseError != null) {
                    Log.d(TAG, "onComplete: Error: " + databaseError.getMessage());
                } else {
                    Log.d(TAG, "onComplete: Transaction was successful");

                }
            }

        }
    }

    private class MyValueEventListener implements ValueEventListener {

        private final TaskCompletionSource<DataSnapshot> taskSource;

        public MyValueEventListener(TaskCompletionSource<DataSnapshot> taskSource) {
            this.taskSource = taskSource;
        }

        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Log.v(TAG, "Looks like MyValueEventListener received the data");
            taskSource.setResult(dataSnapshot);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.v(TAG, "Looks like MyValueEventListener thrown an error");
            taskSource.setException(databaseError.toException());
        }

    }

}

把日志放在各处，我发现它到达了tasks.await语句。但是，ondatachanged回调从未被调用。

它卡在那一点上。没有错误。没有例外。

最大的问题是，从那时起，它停止了整个应用程序的所有回调。我的所有活动都被阻止了，因为它从不调用ondatachanged。看起来它在内部崩溃或被线程阻塞。

public class MyAsyncTask extends AsyncTask<String,Void,Void> {

    DataSnapshot mDataSnapshot;

    ValueEventListener mListener;

    DatabaseReference ref2;

   @Override
    protected Void doInBackground(String... params) {
        String para1 = params[0];
        String para2 = params[1];

        performCalculations(para1,para2);

        return null;
    }

    public void performCalculation(String para1, String para2) {

            TaskCompletionSource<DataSnapshot> taskCompletionSource = new TaskCompletionSource<>();

            ref2 = FirebaseDatabase.getInstance().getReference().child(param2);

            ref2.addListenerForSingleValueEvent(new MyValueEventListener(taskCompletionSource));

            try {
                    // Get the data first time before you proceed
                    dataSnapshot = Tasks.await(taskCompletionSource.getTask());

             } catch (ExecutionException | InterruptedException e) {
                    Log.v(TAG, "Exception Catched");
                    e.printStackTrace();
             }

             // Register the value event listener to keep the data updated
             mListener = new ValueEventListener() {
             @Override
             public void onDataChange(DataSnapshot dataSnapshot) {
                mDataSnapshot = dataSnapshot
                Log.v(TAG, "Data updated");
             }

             @Override
             public void onCancelled(DatabaseError databaseError) {

             }
         };

        ref2.addValueEventListener(mListener);


        DatbaseReference ref = FirebaseDatabase.getInstance().getReference().child(para1);

        // Run the transaction here
        ref.runTransaction(new Transaction.Handler() {
            @Override
            public Transaction.Result doTransaction(MutableData mutableData) {

                 // Perform some stuff based on mDataSanpshot



                mutableData.setValue(stats);
                return Transaction.success(mutableData);
            }

            @Override
            public void onComplete(DatabaseError databaseError, boolean b, DataSnapshot dataSnapshot) {
                if (databaseError != null) {
                    Log.d(TAG, "onComplete: Error: " + databaseError.getMessage());
                } else {
                    Log.d(TAG, "onComplete: Transaction was successful");
                    ref2.removeEventListener(mListener);
                }
            }

        }
    }

    // My value event listener here ... 

}

Answer 1

您可能会从Firebaser（如Fvp）那里得到更知情的答案。在此之前，我要说的是，我非常肯定您在runtransaction()中等待的任务正在创建死锁。您可以看到通过log语句运行回调的线程，如下所示：

Log.i(TAG, "doTransaction: " + Thread.currentThread().getName());

doTransaction()在FirebaseDatabaseWorker线程上运行。侦听器回调在主(UI)线程上运行，并且（我猜）是由从DB工作线程派发的事件驱动的。

阻止DB工作线程将导致侦听器回调永远不会激发。任务从未完成，因此task.await()从未释放。