CODEWITHSUNDEEP
reactive-cocoa
Eimantas
Eimantas

Reputation: 49344

RAC'ing property to be set

Is it possible to observe a property of an object that is yet to be set?

I am trying to implement MVVM pattern and here's what I've got so far:

In my view controller I instantiate view model and model:

- (void)viewDidLoad
{
  [super viewDidLoad];
  Model * model = [Model initWithSomeData:[self dataFromSomewhere]];
  ViewModel * viewModel = [[ViewModel alloc] init];
  viewModel.model = model;

  RAC(self.propertyLabel, text) = [RACObserve(self.viewModel, someTransformedProperty];
}

Now in ViewModel class I have following code in initialiser

- (instancetype)init
{
  self = [super init];

  if (self) {
    RAC(self, someTransformedProperty = [RACObserve(self.model, plainProperty) map:id^(id value) {
      return [self transformProperty:value];
    }];
  }
}

Now the problem is that property label does not get set. Is there any way around this? Also - if plainProperty in model is nil I get a crash even if I provide nilValue as 3rd parameter to RAC macro.

Any help is much appreciated.

Upvotes: 0

Views: 114

Answers (1)

Ian Henry
Ian Henry

Reputation: 22403

Yeah! Just turn RACObserve(self.model, plainProperty) into RACObserve(self, model.plainProperty).

Oh but also:

ViewModel * viewModel = [[ViewModel alloc] init];
viewModel.model = model;
self.viewModel = viewModel; // <--- don't forget that
RAC(self.propertyLabel, text) = RACObserve(self.viewModel, someTransformedProperty);

As far as the crash goes, hard to say what that could be without more info. Is transformProperty: crashing on nil?

Upvotes: 2

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