interceptors.md

Interceptors

This is to do some action before or after execution of a Mutation. Structure of a Interceptor is like this:

abstract class Interceptor {
  bool beforeMutation(Mutation mutation);
  void afterMutation(Mutation mutation);
}

Functions in the class is self explanatory and both can modify the incoming mutation as well. In case of beforeMutation if you return false, execution of that mutation will be cancelled.

Example - Rate Limiter

This example blocks increment from executing more than once a second. Similar to a debouncer.

class RateLimiter extends Interceptor {
  var lastIncOn = DateTime.now();

  @override
  bool beforeMutation(Mutation<Store> mutation) {
    if (mutation is Increment) {
      final now = DateTime.now();

      // if the last call was not before one second cancel
      // this execution
      if (now.difference(lastIncOn) < Duration(seconds: 1)) {
        return false;
      }

      lastIncOn = now;
    }
    return true;
  }

  @override
  void afterMutation(Mutation<Store> mutation) {}
}