AUv3 custom state

auv3_custom_state.m

// How to store and recall custom non-parameter based state in AUv3

- (NSDictionary*)customState { return nil; } // return your custom state here
- (void)setCustomState:(NSDictionary*)state { return; } // set your custom state here

- (void)setFullState:(NSDictionary<NSString *,id> *)fullState {
    NSDictionary *custom = fullState[@"custom_data"];

    if(custom) {
        NSMutableDictionary *m = [fullState mutableCopy];
        [m removeObjectForKey:@"custom_data"]; // to not disturb super implementation
        fullState = m;
    }
    
    // Reset preset when loading state. We could also save preset name & number in state and restore it here,
    // but that would be problematic when user saves user preset local to host, because then a name is given
    // in host but we can't know about it here, so the old preset name will be shown in our UI.
    [self willChangeValueForKey:@"currentPreset"];
    super.currentPreset = nil;
    [self didChangeValueForKey:@"currentPreset"];

    [super setFullState:fullState];
    
    if(custom)
        [self setCustomState:custom];
}

- (NSDictionary<NSString *,id> *)fullState {
    NSDictionary *custom = [self customState];
    if(!custom) return [super fullState];
    
    NSMutableDictionary *state = [[super fullState] mutableCopy];
    state[@"custom_data"] = [custom mutableCopy];

    return state;
}