Observe a property from an ObjC class in Swift

I am using a third party library for which I have an ObjC header file. In this header file there is a property that I would like to observe from my Swift code. My question is now: can I somehow extend the ObjC class without having the .m file such that I can observe a property whenever it is changed in Swift? I thought of using KVO but then I would need to change the implementation of the ObjC class?

Thanks for your help

Assuming your Objective-C class is key-value observing compliant, you can use addObserver(_:forKeyPath:options:context:). Here's an example:

// Person.h
#import <Foundation/Foundation.h>

@interface Person : NSObject

@property NSString * name;
@property int age;

- (id) initWithName:(NSString *) name
                age:(int) age;

@end

// Person.m
#import "Person.h"

@implementation Person

- (id) initWithName:(NSString *) name
                age:(int) age
{
    if (self = [super init]) {
        self.name = name;
        self.age = age;
    }

    return self;
}

@end

And over in Swift:

extension Person {
    override public func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if let keyPath = keyPath,
            let change = change,
            let oldValue = change[NSKeyValueChangeOldKey],
            let newValue = change[NSKeyValueChangeNewKey] {

            print("'\(keyPath)' has changed from \(oldValue) to \(newValue)")
        }
    }
}

let p = Person(name: "John", age: 42)

// Start observing changes
// In this case, the object will observe itself
p.addObserver(p, forKeyPath: "name", options: [.New, .Old], context: nil)
p.addObserver(p, forKeyPath: "age", options: [.New, .Old], context: nil)

p.name = "Jack"
p.age = 50

// You must remove all observers before releasing the object
p.removeObserver(p, forKeyPath: "name")
p.removeObserver(p, forKeyPath: "age")