RAC key-value validation feature request

[Pitometsu]

As a programmer,
I want to have ability to use KVV (key-value validation) when do KVC via RAC
to reduce duplication
and avoid model properties key-path hard-coding each time.

---

Do something like this

    RACSignal *usernameSignal = [RACObserve(self, username) map:^RACTuple *_Nonnull(id _Nullable value) {
        NSError *error;
        return
        RACTuplePack(
        @([self.credentials validateValue:&value
                               forKeyPath:@"username"
                                    error:&error]),
                     error);
    }];

    self.usernameValidation = [usernameSignal map:^NSError *_Nullable(RACTuple *_Nonnull tuple) {
        return tuple.second;
    }];
    RAC(self.credentials, username) = [[RACObserve(self, username) zipWith:usernameSignal]
                                       filter:^BOOL(RACTuple *_Nonnull tuple) {
                                           return tuple.second;
                                       }];

In way like

    [RAC(self.credentials, username)
     validateTo:self.usernameValidation] = RACObserve(self, username);

Where

• self - view modeller
• self.credentials - model
• self.username - data to validate
• self.credentials.username - validating property
• self.usernameValidation - signal with results of validation.

[Pitometsu]

Is it possible to implement such behaviour with current codebase to avoid a lot of code duplication on each validated property?

Can't figure out it by myself because of heavy macros usage in RACSubscriptingAssignmentTrampoline.