Question

Can I use a @protocol for interfacing between classes? My main goal is to do some dependency injection like in Java (with interfaces and implements).

I’ve got the following classes: SignUpServiceImpl (which has an interface called SignUpService) and ServiceHelperImpl (interface is ServiceHelper).

I don’t want to hard wire both implementations together so I use a @protocol in ServiceHelper which is implemented by ServiceHelperImpl. Then SignUpServiceImpl is initialized with ServiceHelper like this:

- (id)initWithHelper:(ServiceHelper *)myServiceHelper

Is what I’m trying to accomplish possible? It looks so much easier in Java….

Answer 1

An objc protocol is very similar to a Java interface.

The blocking point for you may be how you expect things are actually tied together — or protocol syntax.

Declare a protocol:

@protocol ServiceHelperProtocol
- (void)help;
@end

Use it in a class:

@interface SomeClass : NSObject
- (id)initWithServiceHelper:(id<ServiceHelperProtocol>)inServiceHelper;
@end

@implementation SomeClass

- (id)initWithServiceHelper:(id<ServiceHelperProtocol>)inServiceHelper
{
  self = [super init];
  if (nil != self) {
    [inServiceHelper help];
  }
  return self;
}

@end

MONHelper adopts the protocol:

@interface MONHelper : NSObject < ServiceHelperProtocol > 
...
@end

@implementation MONHelper
- (void)help { NSLog(@"helping..."); }
@end

In use:

MONHelper * helper = [MONHelper new];
SomeClass * someClass = [[SomeClass alloc] initWithServiceHelper:helper];
...