CODEWITHSUNDEEP
dartfirebasedart-async
st_clair_clarke
st_clair_clarke

Reputation: 5705

Firebase (Dart) future exception

I am trying to use firebase package to see if I can use it with my app | https://pub.dartlang.org/packages/firebase

I am trying to covert this code in Javascript

var upvotesRef = new Firebase('https://docs-examples.firebaseio.com/android/saving-data/fireblog/posts/-JRHTHaIs-jNPLXOQivY/upvotes');
  upvotesRef.transaction(function (current_value) {
  return (current_value || 0) + 1;
});

taken from https://www.firebase.com/docs/web/guide/saving-data.html#section-transactions

My attempt in dart is:

int getMrn() {
  var testRef = firebaseClient.child('mrn');
       return testRef.transaction((curVal) {
         return curVal == null ? 1 : curVal + 1;
       }).then((result) {

         var snapshot = result.snapshot;
         print( snapshot.val());

       });
}

The correct curVal is returned and the mrn is written correctly to the database. Also the correct value is printed.

However, I get the following error and I am uncertain why:

Exception: Uncaught Error: type '_Future' is not a subtype of type 'int' of 'function result'.
Stack Trace:
#0      getMrn (package:epimss_db/firebase_db.dart:26:15)
#1      onMenuItemSelected (package:epimss_lab/components/chempath/nephrology/electrolytes_rqst_form.dart:116:17)
#2      Function.apply (dart:core-patch/function_patch.dart:28)
#3      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#4      invoke (package:smoke/smoke.dart:43:41)
#5      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#6      BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#7      _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#8      _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#9      BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)
#10     BlinkEventTarget.dispatchEvent_Callback_1 (dart:_blink:7764)
#11     BlinkEventTarget.dispatchEvent_Callback_1_ (dart:_blink:7765)
#12     EventTarget.dispatchEvent (dart:html:14741)
#13     onMainActionClicked (package:html_components/menu/split_button.dart:34:23)
#14     Function.apply (dart:core-patch/function_patch.dart:28)
#15     GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#16     invoke (package:smoke/smoke.dart:43:41)
#17     HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#18     BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#19     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#20     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#21     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

1

Thanks

Upvotes: 1

Views: 197

Answers (1)

G&#252;nter Z&#246;chbauer
G&#252;nter Z&#246;chbauer

Reputation: 657338

Future<int> getMrn() { // <== 3
  var testRef = firebaseClient.child('mrn');
       return testRef.transaction((curVal) { // <== 2)
         return curVal == null ? 1 : curVal + 1;
       }).then((result) {
         var snapshot = result.snapshot;
         print( snapshot.val());
         return result; // <== 1)
       });
}

1) The value returned from the last chained then() is returned at 2) 3) Because this is async code a Future<int> is returned instead of int.

You call this code like

getMrn().then((result) {
  // doSomething with the code here
});

or with await

void someFunc() async { // <== async necessary in order to be able to use await
  var result = await getMrn();
}

Upvotes: 2

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