How can I retry a Future in Dart/Flutter? - flutter

Question

I have a method that does some async processing and want it to retry X times. How can I achieve that in Dart/Flutter?

Answer 1

Use this function:
typedef Future<T> FutureGenerator<T>();
Future<T> retry<T>(int retries, FutureGenerator aFuture) async {
try {
return await aFuture();
} catch (e) {
if (retries > 1) {
return retry(retries - 1, aFuture);
}
rethrow;
}
}
And to use it:
main(List<String> arguments) {
retry(2, doSometing);
}
Future doSometing() async {
print("Doing something...");
await Future.delayed(Duration(milliseconds: 500));
return "Something";
}

Answer 2

I added an optional delay to Daniel Oliveira's answer:
typedef Future<T> FutureGenerator<T>();
Future<T> retry<T>(int retries, FutureGenerator aFuture, {Duration delay}) async {
try {
return await aFuture();
} catch (e) {
if (retries > 1) {
if (delay != null) {
await Future.delayed(delay);
}
return retry(retries - 1, aFuture);
}
rethrow;
}
}
You can use it as follows:
retry(2, doSometing, delay: const Duration(seconds: 1));

Answer 3

Retry from Dart Neat is a good API and, unofficially, it's from Google:
https://pub.dev/packages/retry

Answer 4

This is how I implemented it:
Future retry<T>(
{Future<T> Function() function,
int numberOfRetries = 3,
Duration delayToRetry = const Duration(milliseconds: 500),
String message = ''}) async {
int retry = numberOfRetries;
List<Exception> exceptions = [];
while (retry-- > 0) {
try {
return await function();
} catch (e) {
exceptions.add(e);
}
if (message != null) print('$message: retry - ${numberOfRetries - retry}');
await Future.delayed(delayToRetry);
}
AggregatedException exception = AggregatedException(message, exceptions);
throw exception;
}
class AggregatedException implements Exception {
final String message;
AggregatedException(this.message, this.exceptions)
: lastException = exceptions.last,
numberOfExceptions = exceptions.length;
final List<Exception> exceptions;
final Exception lastException;
final int numberOfExceptions;
String toString() {
String result = '';
exceptions.forEach((e) => result += e.toString() + '\\');
return result;
}
}
This is how I use it:
try {
await retry(
function: () async {
_connection = await BluetoothConnection.toAddress(device.address);
},
message: 'Bluetooth Connect');
} catch (e) {
_log.finest('Bluetooth init failed ${e.toString()}');
}