Is it possible to define a placeholder signal for a signal that hasn't yet been defined in ReactiveCocoa?

Question

As an example, say I have a JSViewController with a UIButton. I'd like to expose a signal representing [UIButton rac_signalForControlEvents:UIControlEventsTouchUpInside] in the interface for JSViewController, but I'll be loading the UIButton in -viewDidLoad:.

This presents a problem: the UIButton's signal isn't available until the -viewDidLoad is executed.

@interface JSViewController : UIViewController
@property (nonatomic, strong) RACSignal *buttonPressedSignal;
@end

@implementation JSViewController

- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    // buttonPressedSignal needs to be defined here
}

- (void)viewDidLoad {
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    [self.view addSubview:button];
    self.buttonPressedSignal = [button rac_signalForControlEvents:UIControlEventTouchUpInside];
}

@end

Is there a way I can create a "placeholder" signal that forwards events from [button rac_signalForControlEvents:UIControlEventTouchUpInside] ?

Answer 1

Observe the view controller's isViewLoaded property, filter out NO values, and then -flattenMap: the YES value into a signal of the button's control events:

- (RACSignal *)buttonEvents
{
    @weakify(self);
    return [[[RACObserve(self, isViewLoaded)
            filter:^BOOL(NSNumber *isLoaded) {

                return isLoaded.boolValue;

            }]
            take:1]
            flattenMap:^id(id _) {

                @strongify(self);
                return [self.myButton rac_signalForControlEvents:UIControlEventTouchUpInside];

            }];
}

Answer 2

One solution is to create RACSubject as the placeholder signal, and make it subscribe UIButton's signal once the signal is created:

@interface JSViewController : UIViewController
@property (nonatomic, strong) RACSubject *buttonPressedSubject;
@end

@implementation JSViewController

- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        _buttonPressedSubject = [RACSubject subject];
    }

    return self;
}

- (void)viewDidLoad {
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    [self.view addSubview:button];
    [[button rac_signalForControlEvents:UIControlEventTouchUpInside] subscribe:self.buttonPressedSubject];
}

@end