title: A ViewModel pattern for iOS apps with ReactiveCocoa
body: <p>I'm working on integrating RAC into my project with the goal of creating a ViewModel layer that will allow easy caching/prefetching from the network (plus all of the other benefits of MVVM). I'm not especially familiar with MVVM or FRP yet, and I'm trying to develop a nice, reusable pattern for iOS development. I have a couple of questions about this.</p>
  
  <p>First, this is sort of how I've added a ViewModel to one of my views, just to try it out. (I want this here to reference later).</p>
  
  <p><strong>In ViewController viewDidLoad:</strong> </p>
  
  <pre class="lang-objective-c prettyprint-override"><code>@weakify(self)
  
  //Setup signals
  RAC(self.navigationItem.title) = self.viewModel.nameSignal;
  RAC(self.specialtyLabel.text) = self.viewModel.specialtySignal;
  RAC(self.bioButton.hidden) = self.viewModel.hiddenBioSignal;
  RAC(self.bioTextView.text) = self.viewModel.bioSignal;
  
  RAC(self.profileImageView.hidden) = self.viewModel.hiddenProfileImageSignal;    
  
  [self.profileImageView rac_liftSelector:@selector(setImageWithContentsOfURL:placeholderImage:) withObjectsFromArray:@[self.viewModel.profileImageSignal, [RACTupleNil tupleNil]]];
  
  [self.viewModel.hasOfficesSignal subscribeNext:^(NSArray *offices) {
      self.callActionSheet = [[UIActionSheet alloc] initWithTitle:@"Choose Office" delegate:self cancelButtonTitle:nil destructiveButtonTitle:nil otherButtonTitles:nil];
      self.directionsActionSheet = [[UIActionSheet alloc] initWithTitle:@"Choose Office" delegate:self cancelButtonTitle:nil destructiveButtonTitle:nil otherButtonTitles:nil];
      self.callActionSheet.delegate = self;
      self.directionsActionSheet.delegate = self;
  }];
  
  [self.viewModel.officesSignal subscribeNext:^(NSArray *offices){
      @strongify(self)
      for (LMOffice *office in offices) {
          [self.callActionSheet addButtonWithTitle: office.name ? office.name : office.address1];
          [self.directionsActionSheet addButtonWithTitle: office.name ? office.name : office.address1];
  
          //add offices to maps
          CLLocationCoordinate2D coordinate = {office.latitude.doubleValue, office.longitude.doubleValue};
          MKPointAnnotation *point = [[MKPointAnnotation alloc] init];
          point.coordinate = coordinate;
          [self.mapView addAnnotation:point];
      }
  
      //zoom to include all offices
      MKMapRect zoomRect = MKMapRectNull;
      for (id &lt;MKAnnotation&gt; annotation in self.mapView.annotations)
      {
          MKMapPoint annotationPoint = MKMapPointForCoordinate(annotation.coordinate);
          MKMapRect pointRect = MKMapRectMake(annotationPoint.x, annotationPoint.y, 0.2, 0.2);
          zoomRect = MKMapRectUnion(zoomRect, pointRect);
      }
      [self.mapView setVisibleMapRect:zoomRect animated:YES];
  }];
  
  [self.viewModel.openingsSignal subscribeNext:^(NSArray *openings) {
      @strongify(self)
      if (openings &amp;&amp; openings.count &gt; 0) {
          [self.openingsTable reloadData];
      }
  }];
  </code></pre>
  
  <p><strong>ViewModel.h</strong></p>
  
  <pre class="lang-objective-c prettyprint-override"><code>@property (nonatomic, strong) LMProvider *doctor;
  @property (nonatomic, strong) RACSubject *fetchDoctorSubject;
  
  - (RACSignal *)nameSignal;
  - (RACSignal *)specialtySignal;
  - (RACSignal *)bioSignal;
  - (RACSignal *)profileImageSignal;
  - (RACSignal *)openingsSignal;
  - (RACSignal *)officesSignal;
  
  - (RACSignal *)hiddenBioSignal;
  - (RACSignal *)hiddenProfileImageSignal;
  - (RACSignal *)hasOfficesSignal;
  </code></pre>
  
  <p><strong>ViewModel.m</strong></p>
  
  <pre class="lang-objective-c prettyprint-override"><code>- (id)init {
      self = [super init];
      if (self) {
          _fetchDoctorSubject = [RACSubject subject];
  
          //fetch doctor details when signalled
          @weakify(self)
          [self.fetchDoctorSubject subscribeNext:^(id shouldFetch) {
              @strongify(self)
              if ([shouldFetch boolValue]) {
                  [self.doctor fetchWithCompletion:^(NSError *error){
                      if (error) {
                          //TODO: display error message
                          NSLog(@"Error fetching single doctor info: %@", error);
                      }
                  }];
              }
          }];
      }
      return self;
  }
  
  - (RACSignal *)nameSignal {
      return [RACAbleWithStart(self.doctor.displayName) distinctUntilChanged];
  }
  
  - (RACSignal *)specialtySignal {
      return [RACAbleWithStart(self.doctor.primarySpecialty.name) distinctUntilChanged];
  }
  
  - (RACSignal *)bioSignal {
      return [RACAbleWithStart(self.doctor.bio) distinctUntilChanged];
  }
  
  - (RACSignal *)profileImageSignal {
      return [[[RACAbleWithStart(self.doctor.profilePhotoURL) distinctUntilChanged]
              map:^id(NSURL *url){
                  if (url &amp;&amp; ![url.absoluteString hasPrefix:@"https:"]) {
                      url = [NSURL URLWithString:[NSString stringWithFormat:@"https:%@", url.absoluteString]];
                  }
                  return url;
              }]
              filter:^BOOL(NSURL *url){
                  return (url != nil &amp;&amp; ![url.absoluteString isEqualToString:@""]);
              }];
  }
  
  - (RACSignal *)openingsSignal {
      return [RACAbleWithStart(self.doctor.openings) distinctUntilChanged];
  }
  
  - (RACSignal *)officesSignal {
      return [RACAbleWithStart(self.doctor.offices) distinctUntilChanged];
  }
  
  - (RACSignal *)hiddenBioSignal {
      return [[self bioSignal] map:^id(NSString *bioString) {
          return @(bioString == nil || [bioString isEqualToString:@""]);
      }];
  }
  
  - (RACSignal *)hiddenProfileImageSignal {
      return [[self profileImageSignal] map:^id(NSURL *url) {
          return @(url == nil || [url.absoluteString isEqualToString:@""]);
      }];
  }
  
  - (RACSignal *)hasOfficesSignal {
      return [[self officesSignal] map:^id(NSArray *array) {
          return @(array.count &gt; 0);
      }];
  }
  </code></pre>
  
  <p>Am I right in the way I'm using signals? Specifically, does it make sense to have <code>bioSignal</code> to update the data as well as a <code>hiddenBioSignal</code> to directly bind to the hidden property of a textView?</p>
  
  <p>My <em>primary</em> question comes with moving concerns that would have been handled by delegates into the ViewModel (hopefully). Delegates are so common in iOS world that I'd like to figure out the best, or even just a moderately workable, solution to this.</p>
  
  <p>For a UITableView, for example, we need to provide both a delegate and a dataSource. Should I have a property on my controller <code>NSUInteger numberOfRowsInTable</code> and bind it to a signal on the ViewModel? And I'm really unclear on how to use RAC to provide my TableView with cells in <code>tableView: cellForRowAtIndexPath:</code>. Do I just need to do these the "traditional" way or is it possible to have some sort of signal provider for the cells? Or maybe it's best to leave it how it is, because a ViewModel shouldn't really be concerned with building the views, just modifying the source of the views?</p>
  
  <p>Further, is there a better approach than my use of a subject (fetchDoctorSubject)?</p>
  
  <p>Any other comments would be appreciated as well. The goal of this work is to make a prefetching/caching ViewModel layer that can be signalled whenever needed to load data in the background, and thus reduce wait times on the device. If anything reusable comes out of this (other than a pattern) it will of course be open source.</p>
  
  <p>Edit: And another question: It looks like according to the documentation, I should be using properties for all of the signals in my ViewModel instead of methods? I think I should configure them in init? Or should I leave it as-is so that getters return new signals?</p>
  
  <p>Should I have an <code>active</code> property as in the ViewModel example in ReactiveCocoa's github account? </p>