Create or increment a value with Angular2/AngularFire2

问题

I'm using Angular 2 and AngularFire 2 to interact with Firebase. In Firebase I have a tags collection. I'd like to either create or increment the number for a tag. The code I'm using looks something like this:

let tagName = "angular";
let tagObs = this.af.database.object(`/tags/${tagName}`);
tagObs.subscribe(function(snapshot) {
    let newValue = (snapshot.$value) ? (snapshot.$value + 1) : 1;
    this.tagObs.set(newValue);
}.bind({ tagObs: tagObs ));

It's not clear to me why, but this doesn't work. It creates an infinite loop that just keeps incrementing the tag value.

Using AngularFire 2, how should I go about either creating or incrementing a value for a node (a "tag" in this case)?

Update after @Fiddle's comment

Here is the same code with a "fat arrow" function. The same problem exists... an infinite loop.

let tagName = "angular";
let tagObs = this.af.database.object(`/tags/${tagName}`);
tagObs.subscribe((snapshot) => {
    let newValue = (snapshot.$value) ? (snapshot.$value + 1) : 1;
    tagObs.set(newValue);
});

Update # 2: the code that worked

Just for the sake of clarity, this is the actual code I ended up using:

let tagObs = this.af.database.object(`/tags/${tagName}`);
tagObs.transaction(function(currentCount) {
  return currentCount + 1;
});

回答1:

Infinite loop

You got an infinite loop because the subscribe method is called every time the tagObsreference receives a new value, and the subscribe function changes the value of tabObs with the set method.

Firebase Transactions

Firebase provides a transaction method for this situation. This is really helpful because:

transaction() is used to modify the existing value to a new value, ensuring there are no conflicts with other clients writing to the same location at the same time.

tagObs.$ref.transaction(tagValue => {
  return tagValue ? tagValue + 1 : 1;
});

It's important to note that this is a method from the Firebase API (not Angularfire2), but you can still access those methods by calling $ref on your provided tagObs which looks like a FirebaseObjectObservable.

来源：https://stackoverflow.com/questions/40795605/create-or-increment-a-value-with-angular2-angularfire2