Observer pattern implementation in Objective-C
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AIObservable.xcodeproj
AIObservable
AIObservableTests
.gitignore
AIObservable.h
AIObservable.m
LICENSE
NSInvocation+AIConstructors.h
NSInvocation+AIConstructors.m
README.md

README.md

AIObservable

The AIObservable class lets you to dispatch events to a list of observers. It's an implementation of the Observer design pattern. You can add and remove observers even within the notification callback and from multiple threads or queues.

This implementation has a few advantages over NSNotificationCenter:

  • You don't need to call removeObserver: on the observer's dealloc, AIObservable uses zeroing weak references.
  • When the observable is destroyed all observers are automatically removed.
  • Each observable object keeps its own set of observers. This makes it easier to debug.
  • I like having an observer protocol for each class that is observable. A protocol allows you to better document each event that the observable class can dispatch and what parameters it has. It also gives meaningul names to the methods. For instance collection:didAddObject:atIndex: as opposed to didAddObject:(NSNotification*)notification.

Known Issues

AIObservable is currently not (fully) compatible with Swift because Swift doesn't support invocations. I don't have any plans to fix this on this project as it would require a completly different approach. You can still use it in Swift but your classes (both observers and observables) need to inherit from NSObject.

Example

As an example suppose you have a chat room. The chat room object dispatches obseravable events when messages are added and people join and leave the room.

@class AIMessage;
@class AIUser;
@protocol AIChatRoomObserver;


@interface AIChatRoom : NSObject 

- (void)postMessage:(AIMessage*)message;
// ...

- (void)addObserver:(id<AIChatRoomObserver>)observer;
- (void)removeObserver:(id<AIChatRoomObserver>)observer;

@end


@protocol AIChatRoomObserver

@optional
- (void)chatRoom:(AIChatRoom*)room didGetNewMessage:(AIMessage*)message;
- (void)chatRoom:(AIChatRoom*)room userDidJoin:(AIUser*)user;
- (void)chatRoom:(AIChatRoom*)room userDidLeave:(AIUser*)user;

@end

As you can see everything has a clear intent and the observer methods are self-documenting. Here is the implementation:

#import <AIObservable.h>
#import <NSInvocation+AIConstructors.h>

@interface AIChatRoom ()

// We use composition but you could also use inheritance
@property (nonatomic, strong) AIObservable* observable;

@end


@implementation AIChatRoom

- (void)postMessage:(AIMessage*)message {
    // ...

    // Create an invocation that will be dispatched to every observer
    AIChatRoom* chatRoom = self;
    NSInvocation* invocation = [NSInvocation invocationWithProtocol:@protocol(AIChatRoomObserver)
                                                           selector:@selector(chatRoom:didGetNewMessage:)];
    [invocation setArgument:&chatRoom atIndex:2];
    [invocation setArgument:&message atIndex:3];
    [self.observable notifyObservers:invocation];
}

- (void)addObserver:(id<AIChatRoomObserver>)observer {
    [self.observable addObserver:observer];
}

- (void)removeObserver:(id<AIChatRoomObserver>)observer {
    [self.observable removeObserver:observer];
}

@end

Installation

Copy the source files to your project or add pod 'AIObservable' to your Podfile.