title: Firebase how to check transaction success or fail?
body: <p>I am trying to update a firebase node in transaction, simple stuff.  Followed the doc:</p>
  
  <p><a href="https://www.firebase.com/docs/ios/guide/saving-data.html" rel="noreferrer">https://www.firebase.com/docs/ios/guide/saving-data.html</a></p>
  
  <pre><code>Firebase* upvotesRef = [[Firebase alloc] initWithUrl: @"https://docs-examples.firebaseio.com/web/saving-data/fireblog/posts/-JRHTHaIs-jNPLXOQivY/upvotes"];
  
  [upvotesRef runTransactionBlock:^FTransactionResult *(FMutableData *currentData) {
      NSNumber *value = currentData.value;
      if (currentData.value == [NSNull null]) {
        value = 0;
      }
      [currentData setValue:[NSNumber numberWithInt:(1 + [value intValue])]];
      return [FTransactionResult successWithValue:currentData];
  }];
  </code></pre>
  
  <p>The big question is:
   <strong>How do I check the outcome of this transaction(success/fail)?  I wish to make some UI changes depends on the outcome of it.</strong></p>
  
  <p>There is another method in SDK document appears to have a callback, but it did not explain which value should I check against. And it says something about the method could be run multiple times.  How do I make sure when it gives the "final" result?
  <a href="https://www.firebase.com/docs/ios-api/Classes/Firebase.html#//api/name/runTransactionBlock:andCompletionBlock" rel="noreferrer">https://www.firebase.com/docs/ios-api/Classes/Firebase.html#//api/name/runTransactionBlock:andCompletionBlock</a>:</p>
  
  <p>Sorry I am such a beginner that apple style doc really doesn't come together without some examples.</p>