new.m

RACSignal *milestoneSignal = [[[[[RACAble(self.eventManagedObjectID)
    distinctUntilChanged]
    flattenMap:^(NSManagedObjectID *nextEventManagedObjectID) {
        @strongify(self);

        if (TCBIsEmpty(nextEventManagedObjectID)) {
            return [RACSignal error:[NSError errorWithDomain:WhateverDomain code:EmptyManagedObjectID userInfo:nil]];
        }

        NSError *objectRetrievalError;
        TBDMilestone *milestone = (TBDMilestone *)[self.controllerManagedObjectContext existingObjectWithID:nextEventManagedObjectID error:&objectRetrievalError];

        if (TCBIsEmpty(milestone)) {
            return [RACSignal error:objectRetrievalError];
        }

        return [RACSignal return:milestone];
    }]
    doError:^(NSError *error) {
        TCBLogError(error);

        [[RACScheduler mainThreadScheduler] schedule:^{
            self.startDate = [NSDate date]; 
        }];
    }]
    retry]
    replayLast];

RAC(self.label) = [[milestoneSignal
    map:^(TBDMilestone *milestone) {
        return milestone.label;
    }]
    deliverOn:[RACScheduler mainThreadScheduler]];

RAC(self.photo) = [[milestoneSignal
    map:^(TBDMilestone *milestone) {
        return [UIImage imageWithData:milestone.photo.data];
    }]
    deliverOn:[RACScheduler mainThreadScheduler]];

RAC(self.startDate) = [[milestoneSignal
    map:^(TBDMilestone *milestone) {
        return milestone.startedAt;
    }]
    deliverOn:[RACScheduler mainThreadScheduler]];

old.m

    RAC(self.milestone) = [[RACAble(self.eventManagedObjectID) distinctUntilChanged] map:^TBDMilestone *(NSManagedObjectID *nextEventManagedObjectID) {
        @strongify(self);

        if (TCBIsEmpty(nextEventManagedObjectID)) {
            return nil;
        }

        NSError *objectRetrievalError;
        TBDMilestone *milestone = (TBDMilestone *)[self.controllerManagedObjectContext existingObjectWithID:nextEventManagedObjectID error:&objectRetrievalError];

        if (TCBIsEmpty(milestone)) {
            TCBLogError(objectRetrievalError);
        }

        return milestone;
    }];
    
    [RACAbleWithStart(self.milestone) subscribeNext :^(TBDMilestone *nextMilestone) {
        @strongify(self);
        self.label = nextMilestone.label;
        self.photo = [UIImage imageWithData:nextMilestone.photo.data];
        self.startDate = TCBIsNotEmpty(nextMilestone) ? nextMilestone.startedAt :[NSDate date];
    }];