Auto-increment a value in Firebase

问题

How do I auto increment a value stored in Firebase from an Android client?

Currently: I declare int id = 1. When I increment, I see the values 2, 3 etc. being stored. That's fine, but when I re-run the project, id is set equal to 1 again.

I want it to behave like a static variable, so I can create an id which will go from 1 to infinity without resetting.

UPDATED FAILED

I used the following to pass the Firebase reference and a string to the function incrementCounter.

if(language_chosen.equalsIgnoreCase("english"))
 {
   Firebase publRef = f.child("Language").child("English").child("Message");
   Firebase newPublRef = publRef.push();
   writeMsgActivity demo = new writeMsgActivity();
   demo.incrementCounter(newPublRef,the_msg_enter);                                 
  }

Now I try to use the passed reference and string at public void incrementCounter(Firebase publref,String my_msg) in the oncomplete method but it gives me an error.

   public void incrementCounter(Firebase publref,String my_msg) {
    publref.runTransaction(new Transaction.Handler() {
        @Override
        public Transaction.Result doTransaction(final MutableData currentData) {
            if (currentData.getValue() == null) {
                currentData.setValue(1);
            } else {
                currentData.setValue((Long) currentData.getValue() + 1);
            }

            return Transaction.success(currentData);
        }

        public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
            if (firebaseError != null) {
                System.out.println("Firebase counter increment failed.");
            } else {
                System.out.println("Firebase counter increment succeeded.");

                Map<String, Object> publ = new HashMap<String, Object>();
                publ.put("pubMsg", my_msg);
                publ.put("id",currentData);
                publref.setValue(publ);
            }
        }
    });
}

UPDATED SOLVED

                final Firebase upvoteref = new Firebase("https://shareurday.firebaseio.com/Message/"+msg_id+"/upvotes"); 

            upvoteref.runTransaction(new Transaction.Handler() {
                @Override
                public Transaction.Result doTransaction(final MutableData currentData) {
                    if (currentData.getValue() == null) {
                        currentData.setValue(1);
                    } else {
                        currentData.setValue((Long) currentData.getValue() + 1);
                    }
                    return Transaction.success(currentData);
                }

                public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
                    if (firebaseError != null) {
                        System.out.println("Firebase counter increment failed.");
                    } else {
                        System.out.println("Firebase counter increment succeeded.");
                    }
                }
            });

The variable msg_id is the random generated id from push.

回答1:

Here's an example method that increments a single counter. The key idea is that you are either creating the entry (setting it equal to 1) or mutating the existing entry. Using a transaction here ensures that if multiple clients attempt to increment the counter at the same time, all requests will eventually succeed. From the Firebase documentation (emphasis mine):

Use our transactions feature when working with complex data that could be corrupted by concurrent updates

public void incrementCounter() {
    firebase.runTransaction(new Transaction.Handler() {
        @Override
        public Transaction.Result doTransaction(final MutableData currentData) {
            if (currentData.getValue() == null) {
                currentData.setValue(1);
            } else {
                currentData.setValue((Long) currentData.getValue() + 1);
            }

            return Transaction.success(currentData);
        }

        @Override
        public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
            if (firebaseError != null) {
                Log.d("Firebase counter increment failed.");
            } else {
                Log.d("Firebase counter increment succeeded.");
            }
        }
    });
}

来源：https://stackoverflow.com/questions/28915706/auto-increment-a-value-in-firebase