Reactive Cocoa - Convert certain signal values into error or completed - objective-c

Question

Perhaps I'm still struggling on the reactive learning curve but I am having a hard time figuring out how to bridge a non reactive class with the rest of my reactive code. I am using a category to extend the non-reactive class.
The property is just an Enum representing the current state of a network action, states like New, Submitted, Processing and Completed. Right now I have written the following method in my category:
#implementation JRequestBase (RACExtensions)
- (RACSignal*) rac_RequestStateSignal
{
return RACAble(self, state);
}
#end
However, when state transitions from Processing -> Completed or from any state to Errored I want this signal to send Completed or Error instead of Next Value. How can I accomplish this in a category? I want to do something like:
#implementation JRequestBase (RACExtensions)
- (RACSignal*) rac_RequestStateSignal
{
return [RACAble(self, state) map:^(NSNumber *state){
if ([state intValue] == iRequestStateComplete)
{
# SEND COMPLETE
}
else if ([state intValue] == iRequestStateErrored)
{
# SEND ERROR
}
else
{
return state;
}
}];
}
#end
edit: I took a look at the GHAPIDemo and have come up with the following:
- (RACSignal*) rac_RequestSignal
{
RACSubject *subject = [[RACReplaySubject alloc] init];
[[RACAble(self, state) subscribeNext:^(NSNumber* s){
if ( [s intValue] == JRequestStateCompleted)
{
[subject sendNext:self];
[subject sendCompleted];
}
else if ([s intValue] == JRequestStateErrored)
{
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
// .. Set up dict with necessary values.
NSError *error = [NSError errorWithDomain:#"blah" code:1 userInfo:dict];
[subject sendError:error];
}
}];
return subject;
}
I'm not 100% sure this is the right way but it seems to be working.

Answer 1

Whenever you want to map values → signal events, instead of values → values, you should use -flattenMap: to return a signal corresponding to each input value. Then, as the "flatten" in the name implies, they'll be combined into one resulting signal.
However, this case is a little different, because you want to terminate the signal as soon as you get the Complete value. We'll use -takeUntilBlock: to represent that part.
The resulting code looks something like this:
- (RACSignal*) rac_RequestStateSignal
{
return [[RACObserve(self, state)
takeUntilBlock:^ BOOL (NSNumber *state){
return [state intValue] == iRequestStateComplete;
}]
flattenMap:^(NSNumber *state){
if ([state intValue] == iRequestStateErrored)
{
// Create a meaningful NSError here if you can.
return [RACSignal error:nil];
}
else
{
return [RACSignal return:state];
}
}];
}
(I used RACObserve because ReactiveCocoa 2.0 is now the only supported version, but you can use RACAble until you're ready to upgrade.)
As a general rule, you should avoid using subjects when possible, since they make code more stateful and reduce laziness.