Bad State: Future Already Completed

The Dart error Bad state: Future already completed occurs when you attempt to complete a Future more than once. Since a Future in Dart can only transition from an uncompleted state to a completed state exactly once, any subsequent attempt to complete it will throw this error.

Here are the most common reasons for this error:

1. Manually Completing a Completer Multiple Times

• If you are using a Completer and call Completer.complete or Completer.completeError more than once, you will encounter this error.

     final completer = Completer<int>();
     completer.complete(42); // Completes the Future
     completer.complete(43); // Throws: Bad state: Future already completed
  

2. Multiple Calls to an async Function or Callback

• If an async function, callback, or event handler tries to resolve the same Future multiple times, the error can occur. For example:

     Future<void> handleEvent() async {
  final completer = Completer<void>();
  someAsyncOperation().then((_) {
    completer.complete(); // Completes the Future
    completer.complete(); // Throws error
  });
     }
  

3. Stream Subscription and Repeated Calls

• Handling streams incorrectly and attempting to complete a Future for every event, without proper checks, can lead to this error.

     Stream<int> stream = Stream.fromIterable([1, 2, 3]);
     final completer = Completer<void>();
     stream.listen((event) {
  completer.complete(); // Completes on the first event
  // Subsequent events cause the error
     });
  

4. Rethrowing or Propagating Errors

• If an error handler or catchError block tries to complete a Future that has already been completed elsewhere in the code, this error can occur.

     final completer = Completer<void>();
     try {
  completer.complete(); // Completes successfully
  throw Exception('Error'); // Error occurs after completion
     } catch (e) {
  completer.completeError(e); // Throws: Bad state: Future already completed
     }
  

5. Race Conditions

• When two or more asynchronous tasks attempt to complete the same Future without proper synchronization, it can lead to this error. This is especially common when handling multiple listeners or operations that share a single Completer.

6. Incorrect Logic in Custom Future-Based APIs

• If you’re writing custom asynchronous APIs using Future or Completer, you might inadvertently complete the Future multiple times due to faulty logic in our implementation.

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How to Fix the Issue

1. Check Completion State Before Completing: Use the Completer.isCompleted property to verify if the Completer is already completed.

   if (!completer.isCompleted) {
     completer.complete();
   }
   

2. Guard Against Multiple Calls: Ensure that Future completion logic is invoked only once. For example, debounce or synchronize access to shared resources.

3. Refactor Code for Better Design: Avoid scenarios where multiple parts of the code compete to complete the same Future.

4. Use Debugging Tools: Add logging or breakpoints to track where the Future is being completed to identify duplicate completions.