Objective-C Key-Value Observing made easier with blocks
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Block Key-Value Observing.xcodeproj


Block Observing 2.2


Key-Value Observing made easier with blocks.

This is an extension to Key-Value Observation mechanism and allows you to use blocks as observation handlers. Block Observing can be used and mixed with classic KVO without any problems.

You should be familiar with the concepts of Key-Value Observing and Key-Value Coding.

Library and example app in this project are for iOS, but you can use in OS X project too by importing the source files directly.


  • ARC
  • iOS 5 and higher (because of __weak)


  1. Drag the project into your project (as a child or sibling).
  2. Add Block Observing to Target Dependencies in Build Phases.
  3. Add libBlockObserving.a to Link Binary With Libraries in Build Phases.
  4. Add -ObjC and -all_load as Other Linker Flags in Build Settings.
  5. Make sure you have Header Search Paths in Build Settings set up (e.g. Libraries/**).
  6. Import MTKObserving.h to your files (usually in Prefix.pch).

Integration using CocoaPods

  1. Add pod 'Block-KVO' to your Podfile.
  2. Import <Block-KVO/MTKObserving.h> to your files (usually in Prefix.pch).


Observe using block

Any object can observe its own key-path using block handler. Caller and receiver must be the same object and the key-path must be relative to the receiver.

[self observe:@keypath(self.profile.username) withBlock:
 ^(__weak typeof(self) self, NSString *oldUsername, NSString *newUsername) {
     self.usernameLabel.text = newUsername;

Block arguments has no specific type (so they are id). You are supposed to specifiy the type by yourself as you want. Primitive values are wrapped by NSNumber or NSValue instances

Quick macros

The above code example using provided macro:

MTKObservePropertySelf(profile.username, NSString *, {
    self.usernameLabel.text = newUsername;

Equality check

IMPORTANT: This is different from the standard KVO.

Once the value of observed property changes, but the values are equal (using -isEqual: method) the observation blocks are not invoked. For example self.title = self.title; will not trigger observation.

No retain cycles inside the blocks

All observation blocks have first argument the receive/caller with name self. It overrides method argument self, but contains the same object. The only difference is __weak attribute. In the example code above, you can use self and will not cause retain cycle.

Observe Using Selector

If you want to get out of the current scope, you can just provide selector instead of block.

[self observe:@keypath(self.profile.username) withSelector:@selector(didChangeUsernameFrom:to:)];

Observe more key-paths at once

There are methods that take an array of key-paths and one block (or selector).

One-way binding (mapping)

Map property to another property. Once the source key-path changes, destination is updated with the new value. Transform the value as you wish.

[self map:@keypath(self.profile.isLogged) to:@keypath(self.isLoggedLabel.text) transform:
 ^NSString *(NSNumber *isLogged) {
     return (isLogged.boolValue ? @"Logged In" : @"Not Logged In");

Also, there is convenience method for specifying replacement for null value.

[self map:@keypath(self.profile.username) to:@(self.usernameLabel.text) null:@"Unknown"];

Two-way binding (mapping)

Two-way binding can be achieved by using two one-way bindings. Don't worry about recursion, because observation is supressed if the values are equal. With this you can map user.name to textField.text, so it will always display the name and then map textField.text to user.name for the name to be updated once the user make changes.

Observe NSNotifications using blocks

Improved observation of notifications using blocks. NSNotificationCenter provides some support for this, but here you don't need to worry about removing those blocks or retain cycles.

MIT License, Copyright (c) 2012—2013 Martin Kiss