Description

Objection is a lightweight dependency injection framework for Objective-C for MacOS X and iOS. For those of you that have used Guice, Objection will feel familiar. Objection was built to stay out of your way and alleviate the need to maintain a large XML container or manually construct objects.

Features

• "Annotation" Based Dependency Injection
• Seamless support for integrating custom and external dependencies
  • Custom Object Providers
  • Meta Class Bindings
  • Protocol Bindings
  • Instance Bindings
• Lazily instantiates dependencies
• Eager Singletons
• Initializer Support
  • Default and custom arguments

Using Objection

For questions, visit the mailing list

Basic Usage

A class can be registered with objection using the macros objection_register (optional) or objection_register_singleton. The objection_requires macro can be used to declare what dependencies objection should provide to all instances it creates of that class. objection_requires can be used safely with inheritance.

Example

@class Engine, Brakes;

@interface Car : NSObject
{
  Engine *engine;
  Brakes *brakes;
  BOOL awake;  
}

// Will be filled in by objection
@property(nonatomic, strong) Engine *engine;
// Will be filled in by objection
@property(nonatomic, strong) Brakes *brakes;
@property(nonatomic) BOOL awake;

@implementation Car
objection_requires(@"engine", @"brakes")
@synthesize engine, brakes, awake;
@end

Defining dependencies with selectors

You can alternatively use selectors to define dependencies. The compiler will generate a warning if a given selector is not visible or cannot be found.

Example

@implementation Car
objection_requires_sel(@selector(engine), @selector(brakes))
@synthesize engine, brakes, awake;
@end

Fetching Objects from Objection

An object can be fetched from objection by creating an injector and then asking for an instance of a particular class or protocol. An injector manages its own object context. Which means that a singleton is per injector and is not necessarily a true singleton.

- (void)someMethod {
  JSObjectionInjector *injector = [JSObjection createInjector];
  id car = [injector getObject:[Car class]];
}

A default injector can be registered with Objection which can be used throughout your application or library.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
  JSObjectionInjector *injector = [JSObjection createInjector];
  [JSObjection setDefaultInjector:injector];
}

- (void)viewDidLoad {
  id myModel = [[JSObjection defaultInjector] getObject:[MyModel class]];
}

Injecting dependencies

There may be instances where an object is allocated outside of the injector's life cycle. If the object's class declared its dependencies using objection_requires an injector can satisfy its dependencies via the injectDependencies: method.

@implementation JSTableModel
objection_requires(@"RESTClient")
- (void)awakeFromNib {
  [[JSObjection defaultInjector] injectDependencies:self];
}
@end

Subscripting

Objection has support for the subscripting operator to retrieve objects from the injection context.

- (void)someMethod {
  JSObjectionInjector *injector = [JSObjection createInjector];
  id car = injector[[Car class]];
}

Awaking from Objection

If an object is interested in knowing when it has been fully instantiated by objection it can implement the method awakeFromObjection.

Example

@implementation Car
//...
objection_register_singleton(Car)
  - (void)awakeFromObjection {
    awake = YES;
  }
@end  

Object Factory

A class can get objects from the injector context through an object factory.

Example

@interface RequestDispatcher
@property(nonatomic, strong) JSObjectFactory *objectFactory
@end

@implementation RequestDispatcher
- (void)dispatch:(NSDictionary *)params
{
  Request *request = [self.objectFactory getObject:[Request class]];
  request.params = params;
  [request send];
}
@end

Modules

A module is a set of bindings which contributes additional configuration information to the injector. It is especially useful for integrating external depencies and binding protocols to classes or instances.

Instance and Protocol Bindings

• Bind a protocol or class to a specific instance of that type
• Bind a class that is registered with Objection to a protocol

Example

@interface MyAppModule : JSObjectionModule {
  
}
@end

@implementation MyAppModule
- (void)configure {
  [self bind:[UIApplication sharedApplication] toClass:[UIApplication class]];
  [self bind:[UIApplication sharedApplication].delegate toProtocol:@protocol(UIApplicationDelegate)];
  [self bindClass:[MyAPIService class] toProtocol:@protocol(APIService)];
}

@end
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
  JSObjectionInjector *injector = [JSObjection createInjector:[[MyAppModule alloc] init]];
  [JSObjection setDefaultInjector:injector];
}

Meta Class Bindings

There are times when a dependency -- usually external -- is implemented using only class methods. Objection can explicitly support binding to the meta class instance through a protocol. This avoids having to unnecessarily create a wrapper class that passes through to the class methods. The catch, of course, is that it requires a protocol definition so that Objection knows how to bind the meta class to objects in the injector context.

Example

@protocol ExternalUtility
  - (void)doSomething;
@end

@interface ExternalUtility
  + (void)doSomething;
@end

@implementation ExternalUtility
  + (void)doSomething {...}
@end

// Module Configuration
- (void)configure {
  [self bindMetaClass:[ExternalUtility class] toProtocol:@protocol(ExternalUtility)];    
}

@interface SomeClass
{
  ...
}
// Use 'assign' because a meta class is not subject to the normal retain/release lifecycle. 
// It will exist until the application is terminated (Class Initialization -> Application Termination)
// regardless of the number of objects in the runtime that reference it.
@property (nonatomic, assign) id<ExternalUtility> externalUtility
@end

Providers

Occasionally you'll want to manually construct an object within Objection. Providers allow you to use a custom mechanism for building objects that are bound to a type. You can create a class that conforms to the ObjectionProvider protocol or you can use a block to build the object.

Example

@interface CarProvider : NSObject <JSObjectionProvider>
@end

@implementation CarProvider
- (id)provide:(JSObjectionInjector *)context arguments:(NSArray *)arguments {
  // Manually build object
  return car;
}
@end

@implementation MyAppModule
- (void)configure {
    [self bindProvider:[[CarProvider alloc] init] toClass:[Car class]];
    [self bindBlock:^(JSObjectionInjector *context) {
      // Manually build object
      return car;          
    } toClass:[Car class]];
}
@end

Scopes

A class can be scoped as a singleton in a module. Conversely, a registered singleton can be demoted to a normal lifecycle with in the injector's context.

Example

@implementation MyAppModule
- (void)configure {
    [self bindClass:[Singleton class] inScope:JSObjectionScopeNormal];
    [self bindClass:[Car class] inScope:JSObjectionScopeSingleton];
}
@end

Eager Singletons

You can mark registered singleton classes as eager singletons. Eager singletons will be instantiated during the creation of the injector rather than being lazily instantiated.

Example

@implementation MyAppModule
- (void)configure {
  [self registerEagerSingleton:[Car class]];
}

@end

Deriving a new injector from an existing injector

A new injector can be created from an existing injector using the withModule: method. A new injector will be created containing the same bindings as the injector it was derived from. The new injector will also contain additional bindings provided by the new module.

Conversley, if withoutModuleOfType: is used the new injector will not contain the bindings of the removed module.

Example

injector = [otherInjector withModule:[[Level18Module alloc] init]] 
                          withoutModuleOfType:[Level17Module class]];
                          

Initializers

By default, Objection allocates objects with the default initializer init. If you'd like to instantiate an object with an alternate ininitializer the objection_initializer macro can be used to do so. The macro supports passing in default arguments (scalar values are not currently supported) as well.

Default Arguments Example

@implementation ViewController
objection_initializer(initWithNibName:bundle:, @"ViewController")
@end

Custom Arguments Example

@implementation ConfigurableCar
objection_requires(@"engine", @"brakes")
objection_initializer(initWithMake:model:)

@synthesize make;
@synthesize model;

- (id)initWithMake:(NSString *)make model:(NSString *)model {
  ...
}
@end

- (void)buildCar {
  ConfigurableCar *car = [self.objectFactory getObjectWithArgs:[ConfigurableCar class], @"VW", @"Passat", nil];
  NSLog(@"Make: %@ Model: %@", car.make, car.model);
}

Class Method Initializer

@implementation Truck
objection_requires(@"engine", @"brakes")
objection_initializer(truckWithMake:model:)
+ (id)truckWithMake:(NSString *) make model: (NSString *)model {
  ...
}
@end

Testing

If you're using Kiwi for testing, checkout MSSpec. It provides a convenient way inject mocks into your specs using Objection.

TODO

• Add a motivation section that speaks to why Objection was created

Installation

Static Framework and Linkable Framework

It can be downloaded here

Building Static Framework

git clone git://github.com/atomicobject/objection.git
git checkout 1.4

iOS

1. rake artifact:ios
2. cp -R build/Release-iphoneuniversal/Objection-iOS.framework ${DEST_DIR}
3. In XCode -> Project Icon -> Your Target -> Build Phases -> Link Binary With Libraries -> Add (+) -> Add Other
4. Add -ObjC and -all_load to Other Link Flags in your project

Include framework

#import <Objection-iOS/Objection.h>

MacOS X

1. rake artifact:osx
2. cp -R build/Release/Objection.framework ${DEST_DIR}
3. In XCode -> Project Icon -> Your Target -> Build Phases -> Link Binary With Libraries -> Add (+) -> Add Other

Include framework

#import <Objection/Objection.h>

CocoaPods

Edit your Pofile

edit Podfile
pod 'Objection', '1.4'

Now you can install Objection

pod install

Include framework

#import <Objection/Objection.h>

Learn more at CocoaPods.

Ruby Motion

A companion library for Objection was created called motion-objection

gem install motion-objection

Requirements

• MacOS X 10.7 +
• iOS 5.0 +

Authors

• Justin DeWind ([email protected], @dewind on Twitter)
• © 2013 Atomic Object
• More Atomic Object open source projects

Other Dependency Injection Libraries

One only has to search GitHub

Applications that use Objection

• Bubble Island
• Monster World
• Pocket Village
• SideReel
• Google Wallet