errflow 0.4.0

A package for making it somewhat easier to comprehend the flow of errors and handle them.

errflow

A Dart/Flutter package for making it somewhat easier to comprehend the flow of errors and handle them.

---

apologies for the breaking changes in v0.1.4.
Please see the Changelog for the list of the changes.
The package should be safer now in exchange for the inconvenience caused by them.

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Motivation

I made this package because I found it hard to handle exceptions:

• An app stops on an exception if it is not caught.
• It is sometimes unclear if an exception has already been caught somewhere.
• It is not preferable to catch an exception in a different layer that should be agnostic about a specific exception (e.g. a DB error / a network error).
• However, it is difficult to return an error value (instead of the exception itself to avoid the above issue) together with the result of some processing from a method to its caller located in another layer.

Solutions:

• Result in package:async
• package:errflow (this package)

These look very different, but roughly speaking, they are similar in that they provide a way to pass a result and/or an error from a method to the caller; the former holds either of the values, and the latter enables the caller to evaluate both values.

A big difference is that this package also provides handlers and a logger to enable errors to be handled more easily in a unified manner.

Isn't it inconvenient to have to pass a notifier?

It is probably possible to remove the hassle to have to pass over an object of ErrNotifier, but I choose not to do so because method signatures with a parameter of type ErrNotifier helps you spot which methods require error handling.

Usage

Initialisation and clean-up

Instantiate ErrFlow, with the default value representing that there is no error. The value is used as the initial value in the notifier of each scope().

When the ErrFlow object is no longer needed, call dispose() to ensure that the resources held in the object will not be used any more.

enum CustomError {
  none,
  foo,
  bar,
}

...

final errFlow = ErrFlow<CustomError>(CustomError.none);

...

errFlow.dispose();

If you prefer using Exception and its subtypes instead of a custom error type, specify Exception as the error type, and pass null or none to the constructor to use null as the default value.

final errFlow = ErrFlow<Exception>();

Setting/logging an error

1. Call set() on an ErrNotifier object when some exception happens.
   • The object is passed from scope(), which is described later in this document.
2. The listener is notified of the error and stores it as the last error (lastError) so that it can be checked later inside the function executed by scope().
3. The listener also calls the logger to log a set of information about the exception if it is provided via set() or log().

Future<bool> yourMethod(ErrNotifier notifier) async {
  try {
    return await errorProneProcess();
  } catch(e, s) {
    // This updates the last error value and also triggers logging.
    notifier.set(CustomError.foo, e, s, 'additional info');
  }

  // You can use hasError to check if some error was set.
  if (notifier.hasError) {
    ...
  }

  return false;
}

You can also use only the first argument of set() to not trigger the logger:

notifier.set(CustomError.foo);

or use log() for only logging:

notifier.log(e, s, 'additional info');

Handling errors

scope() executes a function, and handles errors occurring inside there when the function finishes according to the conditions specified by errorIf and criticalIf. Use both or either of them to set the conditions of whether to treat the function result as a non-critical/critical error. The condition of criticalIf is evaluated prior to that of errorIf.

If either of the conditions is met, the relevant handler, onError or onCriticalError, is called. Do some error handling in these handlers, like showing different messages depending on the severity of the error.

final result = await errFlow.scope<bool>(
  (notifier) => yourMethod(notifier),
  errorIf: (result, error) => error == CustomError.foo,
  criticalIf: (result, error) => error == CustomError.bar,
  onError: (result, error) => _onError(result, error),
  onCriticalError: (result, error) => _onCriticalError(result, error),
);

The handler functions receive the function result and the error value, which means you can combine them to customise the conditions for your preference.

e.g. To trigger the onError handler if any error was set:

errorIf: (result, error) => error != errFlow.defaultError

e.g. To trigger the onError handler when the process fails for reasons other than a connection error:

errorIf: (result, error) => !result && error != CustomError.connection

Ignoring errors

If a method, in which set() can be used, is called from some different places in your code, you may want to show an error message at some of them but not at the others. It is possible with the use of loggingScope and ignorableScope, allowing you to only log errors without handling them, or ignore them completely.

loggingScope()

notifier passed from loggingScope is an object of LoggingErrNotifier. Calling set() on that object only updates the value of lastError and triggers the logger (and added listener functions), without triggering the error handlers.

final result = await errFlow.loggingScope<bool>(
  (LoggingErrNotifier notifier) => yourMethod(notifier),
);

bool yourMethod(ErrNotifier notifier) {
  try {
    return ...;
  } catch(e, s) {
    notifier.set(CustomError.foo, e, s);  // Only updates lastError and logs the error.
    return false;
  }
}

ignorableScope()

notifier passed from ignorableScope is an object of IgnorableErrNotifier. Calling set() and log() on that object does not trigger the error handlers nor the logger. set() only updates the value of lastError.

final result = await errFlow.ignorableScope<bool>(
  (IgnorableErrNotifier notifier) => yourMethod(notifier),
);

bool yourMethod(ErrNotifier notifier) {
  try {
    return ...;
  } catch(e, s) {
    notifier.set(CustomError.foo, e, s);  // Only updates lastError.
    return false;
  }
}

Default error handlers

You may want to consistently use a particular handler for non-critical errors, and the same or another one for critical errors. In such a case, errorHandler and criticalErrorHandler will come in handy. You can specify in advance how errors should be handled, and omit onError and onCriticalError in scope().

void _errorHandler<T>(T result, CustomError? error) {
  switch (error) {
    case CustomError.foo:
      // Handle the foo error (e.g. showing the error details)
      break;
    default:
      // Handle other errors
      break;
  }
}

...

errFlow
  ..errorHandler = _errorhandler
  ..criticalErrorHandler = _errorHandler;

final result = await errFlow.scope<bool>(
  (notifier) => yourMethod(notifier),
  errorIf: (result, error) => !result,
);

Logger

To use the default logger, which simply prints information to the console, call useDefaultLogger() before the first logging.

errFlow.useDefaultLogger();

If it lacks functionality you need, set your own logger.

// The return type can be Future or non-Future.
// Note: Even if a Future is returned, set() and log() won't await it.
void _logger(Object e, StackTrace? s, {Object? reason}) {
  // Logging operations
}

...

errFlow.logger = _logger;

In flutter, the recordError() method of the firebase_crashlytics package can be assigned to the logger as is.

import 'package:firebase_crashlytics/firebase_crashlytics.dart';

...

errFlow.logger = FirebaseCrashlytics.recordError;

Make sure to set the default or a custom logger, otherwise an assertion error will occur in the debug mode.

Adding/removing a listener

This is usually unnecessary, but you can add a custom listener for your special needs.

void _listener({CustomError? error, Object? exception, StackTrace? stack, Object? context}) {
  // Some processing
}

...

errFlow.addListener(_listener);

...

errFlow.removeListener(_listener);