title: RACObserve(), RAC() - how to set a BOOL value based on an NSString
body: <p>Being a ReactiveCocoa newbie, I'm hoping for some advice with this:</p>
  
  <p>I'm trying to create a dynamic form that contains multiple Field objects parsed from an XML file.  Each Field can have muliple validation rules that will run against the Field's <code>NSString *value</code> param.</p>
  
  <p>For the RAC part of the question- </p>
  
  <p>inside each Field object, I want to bind <code>BOOL completed</code> to a signal that checks the Field's <code>*value</code> param against an array of rules.  So far I've gotten here with my thinking:</p>
  
  <pre><code>@implementation Field
  
  self = [super init];
  if (self) {
  
      RAC(self, completed) = [RACObserve(self, value) filter:^BOOL(NSString *fieldValue) {
          NSLog(@"%s::self.completed = %d\n", sel_getName(_cmd), self.completed);  // trying to watch the values here, with no luck 
          NSLog(@"%s::fieldValue = %@\n", sel_getName(_cmd), fieldValue);  // same here, I'd like to be able to view the `*value` here but so far no luck
  
          return [self validateCurrentValue]; // currently this method just checks value.length &gt; 5 
      }];
  }
  return self;
  </code></pre>
  
  <p>The <code>*value</code> param has already been bound to my view model (successfully) and it gets updated each time a textfield changes.  </p>
  
  <p>What I'm looking for is a basic example or best-practice, the code above crashes when run so I know I'm missing something fundamental. </p>
  
  <p>Thanks all</p>