Flutter/cloud-firestore "Task is already complete" Exception

Question

I'm writing an app using Flutter and i have to make a transaction using the Firestore.instance.runTransaction(Transaction tx) method. In my Transaction object (or method) i have to update some data using the document reference.

_firestore.runTransaction((Transaction x) async {
  await x.update(Aref, {'data': itemA - y});
  await x.update(Bref, {'data': itemB + y});
})

When the code is running it throw an exception (Here the console log):

E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): Failed to handle method call result E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): java.lang.IllegalStateException: Task is already complete E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at com.google.android.gms.common.internal.Preconditions.checkState(Unknown Source:8) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at com.google.android.gms.tasks.zzu.zzdr(Unknown Source:8) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at com.google.android.gms.tasks.zzu.setResult(Unknown Source:3) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at com.google.android.gms.tasks.TaskCompletionSource.setResult(Unknown Source:2) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at io.flutter.plugins.firebase.cloudfirestore.CloudFirestorePlugin$3$1.success(CloudFirestorePlugin.java:283) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at io.flutter.plugin.common.MethodChannel$IncomingResultHandler.reply(MethodChannel.java:169) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at io.flutter.view.FlutterNativeView.handlePlatformMessageResponse(FlutterNativeView.java:187) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at android.os.MessageQueue.nativePollOnce(Native Method) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at android.os.MessageQueue.next(MessageQueue.java:325) E/MethodChannel#plugins.flutter.io/cloud_firestore(32612): at android.os.Looper.loop(Looper.java:142)

Answer 1

The error happens when there are multiple call to the firestore api concurrently, you have to nest each funcion inside the "whenComplete(() {}" method of the previous function. This is the wrong code :

g.f.runTransaction((transaction) async {
      DocumentSnapshot snap =
          await transaction.get(/my code);

      await transaction.update(/my code).whenComplete(() {});
    });


g.f.runTransaction((transaction) async {
      DocumentSnapshot freshSnap =
              await transaction.get(/my code));

      await transaction.update(/my code).whenComplete(() {});
});  //here is the problem!! I'have to nest this inside "whenComplete(() {})

this is the error:

E/MethodChannel#plugins.flutter.io/cloud_firestore( 5337): Failed to handle method call result E/MethodChannel#plugins.flutter.io/cloud_firestore( 5337): java.lang.IllegalStateException: Task is already complete

this is right code

g.f.runTransaction((transaction) async {
  DocumentSnapshot snap =
      await transaction.get(/my code);

  await transaction.update(/my code).whenComplete(() {
    g.f.runTransaction((transaction) async {
      DocumentSnapshot freshSnap =
          await transaction.get(/my code);

      await transaction.update(/my code);
    }).whenComplete(() {});
  });
});