Python decorator-like design pattern for Dart/Flutter?

user_1087973 0
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I'd like to have common try/catch/finally logic in a decorator-like feature that can "wrap" a function or class method. Consider the scenario:

Class MyClass {
  void someMethodA() {
    doSomeInitialWork();
    
    try {
      doSomething();
    } catch (err) {
      throw err;
    } finally {
      doSomeCleanUpWork();
    }
  }

  void someMethodB() {
    doSomeInitialWork();
    
    try {
      doSomethingElse();
    } catch (err) {
      throw err;
    } finally {
      doSomeCleanUpWork();
    }
  }
}

So on and so forth. The unique parts of each method are just the try body. If I have a bunch of methods, some which require the same logic, is there a "nice" way to avoid redundant code?

Ideally it could be syntax like:

@wrapper
void someMethodA() {
  doSomething();
}

@wrapper
void someMethodB() {
  doSomethingElse();
}

MyClassInstance.someMethodA(); // call it like this and the wrapper takes care of everything

but I know those are annotations in Dart and not applicable here.

UPDATE

Following jamesdlin answer, I am trying to incorporate the anonymous function solution to a futures/async/await scenario:

Future<dynamic> trySomething(Future<dynamic> Function() callback) async {
  doSomeInitialWork();

  try {
    return await callback();
  } catch (err) {
    throw err;
  } finally {
    doSomeCleanUpWork();
  }
}

class MyClass {
  Future<List<String>> someMethodA() async {
    return await trySomething(() async {
      return await someApiCall();
    });
  }
}

That seems to work, but it looks kind of messy. I'm not sure if what I'm doing in the async/await example is appropriate.

dart flutter callback decorator design-patterns
1 Answers

Anonymous functions in Dart are rather common (unlike Python, where lambda is very restricted).

You therefore could make a helper function that takes the unique part as a callback.

void trySomething(void Function() body) {
  doSomeInitialWork();

  try {
    body();
  } catch (err) {
    throw err;
  } finally {
    doSomeCleanUpWork();
  }
}

void someMethodA() {
  trySomething(() {
    doSomething();
  });
}

void someMethodB() {
  trySomething(() {
    doSomethingElse();
  });
}

That's basically what test() from package:test (or testWidgets() from Flutter) do.

Update for the case described in the comment: It's not much different if the methods return Futures. For example, if you start with:

Future<List<String>> someMethodA() async {
  return await blah();
}

then you could do:

Future<R> trySomethingAsync<R>(Future<R> Function() body) async {
  doSomeInitialWork();

  try {
    return await body();
  } catch (err) {
    throw err;
  } finally {
    doSomeCleanUpWork();
  }
}

Future<List<String>> someMethodA() {
  return trySomethingAsync(() async {
    return await blah();
  });
}
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