Key-value observing is a particularly useful technique for communicating between layers in a Model-View-Controller application. KVOController builds on Cocoa's time-tested key-value observing implementation. It offers a simple, modern API, that is also thread safe. Benefits include:

• Notification using blocks, custom actions, or NSKeyValueObserving callback.
• No exceptions on observer removal.
• Implicit observer removal on controller dealloc.
• Thread-safety with special guards against observer resurrection – rdar://15985376.

For more information on KVO, see Apple's Introduction to Key-Value Observing.

Example apps for iOS and OS X are included with the project. Here is one simple usage pattern:

// create KVO controller with observer
FBKVOController *KVOController = [FBKVOController controllerWithObserver:self];
self.KVOController = KVOController;

// observe clock date property
[self.KVOController observe:clock keyPath:@"date" options:NSKeyValueObservingOptionInitial|NSKeyValueObservingOptionNew block:^(ClockView *clockView, Clock *clock, NSDictionary *change) {

  // update clock view with new value
  clockView.date = change[NSKeyValueChangeNewKey];
}];

While simple, the above example is complete. A clock view creates a KVO controller to observe the clock date property. A block callback is used to handle initial and change notification. Unobservation happens implicitly on controller deallocation, since a strong reference to the KVOController is kept.

Note: the observer specified must support weak references. The zeroing weak reference guards against notification of a deallocated observer instance.

For an even easier usage, just #import <KVOController/NSObject+FBKVOController.h> for an automatic KVOController property on all objects.

[self.KVOController observe:clock keyPath:@"date" options:NSKeyValueObservingOptionInitial|NSKeyValueObservingOptionNew action:@selector(updateClockWithDateChange:)];

KVOController works great in Swift but there are few requirements:

• Your observer should subclass NSObject.
• Properties that you observe must be marked as dynamic.

Check the following example:

class TasksListViewModel: NSObject {

  dynamic var tasksList: [TaskList] = []
}

/// In ViewController.swift

import KVOController

kvoController.observe(viewModel,
                      keyPath: "listsDidChange",
                      options: [.new, .initial]) { (viewController, viewModel, change) in
    
  self.taskListsTableView.reloadData()
}

KVOController takes advantage of recent Objective-C runtime advances, including ARC and weak collections. It requires:

• iOS 6 or later.
• OS X 10.7 or later.

To install using CocoaPods, add the following to your project Podfile:

pod 'KVOController'

To install using Carthage, add the following to your project Cartfile:

github "facebook/KVOController"

Alternatively, drag and drop FBKVOController.h and FBKVOController.m into your Xcode project, agreeing to copy files if needed. For iOS applications, you can choose to link against the static library target of the KVOController project.

Having installed using CocoaPods or Carthage, add the following to import in Objective-C:

#import <KVOController/KVOController.h>

The unit tests included use CocoaPods for managing dependencies. Install CocoaPods if you haven't already done so. Then, at the command line, navigate to the root KVOController directory and type:

pod install

This will install and add test dependencies on OCHamcrest and OCMockito. Re-open the Xcode KVOController workspace and Test, ⌘U.

KVOController is released under a BSD License. See LICENSE file for details.