How do I properly subscribe to changes? -Angularfire2

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Hello, I haven't used Firebase a lot. The method I'm using to get values feels messy...

I'm using this method to get the sum amount for each object.value in a ListObservable

Can anyone tell what would be the best way to subscribe to changes such that the totalAmount stays up to date?

//Returns a promise of the total sum for each object.amount under a category
getTotalAmountPromise(observable) {
  let totalAmount: number = 0;
  return new Promise(function(resolve, reject) {
    let count = 0;

    observable.$ref.ref.on('child_added', childs => {
      childs.ref.once('value', obj => {
        totalAmount += obj.val().amount;
        count++;

        let length = childs.numChildren();
        if(count == length) {
          resolve(totalAmount);
        }
      });
      setTimeout(() => {
        if (!(count == length))
          reject(Error('Data rejected. Count: ' + count + ', Childs: ' + length));

      }, 5000);
    });

  });
}

What I have tried:

I can't seem to find a good way to do this. Was hoping anyone here could come with some guidance.