A tract on modular and compositional design for Objective-C projects

1. modularity à la taliban
with compositional events
Jon Sterling
jonmsterling.com
@jonsterling

2. modularity à la taliban
with compositional events
Jon Sterling
jonmsterling.com
@jonsterling

3. reality-based design
with compositional events
Jon Sterling
jonmsterling.com
@jonsterling

4. mvvm is not enough

5. example
view controller:!
RAC(viewModel, message) = textField.rac_textSignal;
submitButton.action = viewModel.submitAction;
view model:!
_submitAction = [RACDynamicSignalGenerator
generatorWithBlock:^(id input) {
@strongify(self);
return sendRequest(self.message);
}].action;

6. differentiate state from
input

7. the old way
@interface MessagesViewModel : NSObject
@property (copy) NSString *message;
/// submitAction : RACAction () _
@property (readonly) RACAction *submitAction;
@end

8. the new way
@interface MessagesViewModel : NSObject
/// submitAction : RACAction NSString _
@property (readonly) RACAction *submitAction;
@end

9. example
view controller:!
submitButton.action = [[textField.rac_textSignal take:1].action
postCompose:viewModel.submitAction].action;
view model:!
_submitAction = [RACDynamicSignalGenerator
generatorWithBlock:^(NSString *message) {
return sendRequest(message);
}].action;

10. where did the state go?
@interface MessagesViewModel : NSObject
/// submitAction : RACAction NSString _
@property (readonly) RACAction *submitAction;
/// statesSignal : RACSignal MessagesState
@property (readonly) RACSignal *statesSignal;
@end

11. the state
@interface MessagesState : NSObject
/// messages : NSArray NSString
@property (readonly) NSArray *messages;
+ (instancetype)emptyState;
- (instancetype)stateByAddingMessage:(NSString
*)msg;
@end

12. example
view model:!
_submitAction = [RACDynamicSignalGenerator
generatorWithBlock:^(NSString *message) {
return [sendRequest(self.message) then:^{
return [RACSignal return:^(MessagesState
*state) {
return [state
stateByAddingMessage:message];
}];
}];
}].action;

13. example
view model:!
_submitAction = …;
RACSubject *statesSubject = [RACSubject subject];
[submitAction.results
scanWithStart:[MessagesState emptyState]
reduce:^(MessageState *state, MessageState
*(^transform)(MessageState *)) {
return transform(state);
}] subscribe:statesSubject];
_statesSignal = statesSubject;

14. example
view controller:!
RAC(self, messagesView.items) =
[self.viewModel.statesSignal map:^(MessagesState
*state) {
return state.messages;
}];

15. separate signature
from structure

16. • The view controller does not need to know how to
create a view model
• The view controller does not need to know how to
create a model state

17. signature vs. structure
public signature (exposed to view controller):!
@protocol MessagesState <NSObject>
/// messages : NSArray NSString
@property (readonly) NSArray *messages;
@end
structure private to view model:!
@interface MessagesState : NSObject <MessagesState>
+ (instancetype)emptyState;
- (instancetype)stateByAddingMessage:(NSString *)msg;
@end

18. signature vs. structure
public signature (exposed to view controller):!
@protocol MessagesViewModel <NSObject>
@property (readonly) RACAction *submitAction;
@property (readonly) RACSignal *statesSignal;
@end
structure private to creator of view model:!
@interface MessagesViewModel : NSObject <
MessagesViewModel>
- (id)initWithRecipient:(Recipient *)recipient;
@end

19. signature vs. structure
view controller!
@interface MessagesViewController : NSObject
- (id)initWithViewModel:
(id<MessagesViewModel>)viewModel
@end

20. DEMO