How to use RACObserve in Swift

Sample.m

// Objective-C
@interface Document: NSObject
@property (nonatomic, copy) NSArray *notes;
@property (nonatomic, readonly) RACSignal *notesUpdatedSignal;
@end

@implementation Document
- (RACSignal *)notesUpdatedSignal
{
    // In Objective-C, just simply use RACObserve
    return RACObserve(self, notes);
}
@end

Sample.swift

// Swift
// 1. Do not forget to inherit NSObject (or add @objc attribute)
public class Document: NSObject {
    // 2. Make sure to add 'dynamic' modifier
    public dynamic var notes: [Note]= []
    // 3. observer should always be 'self'
    public var notesUpdatedSignal: RACSignal {
        return self.rac_valuesForKeyPath("notes", observer: self)
    }
}