Question

#import "ApiService.h"

@implementation ApiService
static ApiService *sharedInstance = nil;

+ (ApiService *)sharedInstance
{
    if (sharedInstance == nil)
    {
        sharedInstance =  [[self alloc]init];
    }

    return sharedInstance;
}

- (id)init
{
    if (self = [super init])
    {
    }
    return self;
}
@end

When I call +sharedInstance what does self refer to? How am I allowed to call init from a Class method?

Answer 1

self is the class.

+ (id)create {
  return [[self alloc] init];
}

Is the same as:

+ (id)create {
  return [[SomeClass alloc] init];
}

Or in your example:

+ (ApiService *)sharedInstance
{
    if (sharedInstance == nil)
    {
        sharedInstance =  [[ApiService alloc]init];
    }

    return sharedInstance;
}

This allows you to call class methods on self from class methods. And it allows you to call them on the child class when you have inheritance, as class methods are inherited too.