Dynamic OneToMany ReactiveCocoa binding

Question

I have something similar to the following structure in my code:

// Model
@interface Child : NSObject
@property (nonatomic, assign) CGPoint position;
@end

@interface Father : NSObject
@property (nonatomic, strong) NSArray <Child *> *children; // (0..n)
@end

// ViewModel
@interface FatherViewModel : NSObject
@property (nonatomic, strong) Father *father;
@property (nonatomic, assign) CGPoint averagePositionOfChildren;
@end

During execution, can change (in this case I recreate the whole NSArray) and . 可以更改（在这种情况下，我将重新创建整个NSArray） 。

Does any elegant solution in ReactiveCocoa exist to map the dynamic number of children positions in the model to averagePositionOfChildren in FatherViewModel?

Answer 1

Yes, I can see you choose two strategies:

1. Use properties

Use a MutableProperty for children and then create a mapped property for averagePositionOfChildren .

2. Use KVO

Alternatively, you can use KVO to watch changes in children . Ie, you can create a DynamicProperty .

Note that both scenarios would force you to recreate the whole array, as you already noted.

Answer 2

If anyone is interested, here follows my own solution:

@interface FatherViewModel()
@property (nonatomic, strong) RACDisposable *averageBindingDisposable;
@end

@implementation FatherViewModel
- (instanceType) init {
self = [super init];
...
...
RACSignal *signalOfSignals = [RACObserve(self, father.children)
                         map:^RACSignal *(NSArray <Child *> *setOfChildren) {
                             NSArray <RACSignal *> *arrayOfSignal = [[[setOfChildren rac_sequence]
                                                                      map:^RACSignal *(Child *child) {
                                                                          return RACObserve(child, position);
                                                                      }]
                                                                     array];
                             return [RACSignal combineLatest:arrayOfSignal];
                         }];

[self.averageBindingDisposable dispose];
self.averageBindingDisposable = [[[signalOfSignals flatten]
                  map:^NSValue *(RACTuple *tuple) {
                      NSArray <NSValue *> *arrayOfWrappedCGPoints = tuple.allObjects;
                      CGPoint avg = CGPointZero;
                      // Compute here the average of the points in the array
                      return [NSValue valueWithPoint: avg];
                  }]
                  setKeyPath:@"averagePositionOfChildren" onObject: self];
}];

return self;
}
...
@end