Sibling properties in Objective C - ios

Question

Is there any Objective C magic I can use to do the following:
Say I have a Class Foo, with two properties myObj A and otherObj B.
Now...
In the Class implementation file (.m) of myObj (A) I want to access
it's "sibling" property "otherObj B" in the "parent" Class Foo.
Is there something similar to this in objective C to help access sibling properties:
self.ParentClass.b; ?
or
self.OwningClass.b; ?
I know convention is probably to pass a refence to B during init of A, but I don't want to have to create many custom initalisers
Just wondered if there was a shortcut.
#interface Foo : NSObject
#property A
#property B
#end
#interface A : NSObject
#property B
#end
#implementation A : NSObject
self.B = self.parent.B;
#end
Thanks,

Answer 1

If you have
#interface A : NSObject
#property A
#end
#interface B : NSObject
#property B
#end
then A has no way to know B and vice versa. As you said: you have to pass a reference
IF you ever only have ONE instance of A and ONE instance of B then you COULD use a singleton pattern. But it introduces extra dependencies and makes the code less reusable. so while it might work, thats not a way I would recommend.

Answer 2

There's no built in "parent" or "owner" concept (large because the object could, theoretically, have strong references to it maintained from multiple objects). But just add your own parent property:
Thus, consider:
#interface Foo : NSObject
#property (nonatomic, strong) A *a;
#property (nonatomic, strong) B *b;
#end
#interface A : NSObject
#property (nonatomic, weak) Foo *parent;
- (id)initWithParent:(Foo *)foo;
#end
#interface B : NSObject
#property (nonatomic, weak) Foo *parent;
- (id)initWithParent:(Foo *)foo;
#end
So, Foo will own instances of A and B objects (thus the strong references), and A and B will maintain a weak reference to their parent. Note, the weak reference to the parent is critical, to avoid strong reference cycles (a.k.a. retain cycles).
The implementations of these classes might look like:
#implementation Foo
- (id)init
{
self = [super init];
if (self) {
_a = [[A alloc] initWithParent:self];
_b = [[B alloc] initWithParent:self];
}
return self;
}
#end
#implementation A
- (id)initWithParent:(Foo *)foo
{
self = [super init];
if (self) {
_parent = foo;
}
return self;
}
- (void)someRandomMethod
{
B *sibling = self.parent.b;
// now do whatever you want with your reference to the sibling object
}
#end
#implementation B
- (id)initWithParent:(Foo *)foo
{
self = [super init];
if (self) {
_parent = foo;
}
return self;
}
#end
Notice, in A I have a someRandomMethod which illustrates how it could get access to its sibling, B. You could do the same thing from B to A, too.
I know you say you don't want to have to write your own custom init methods, but that's the right way to do it. Or just call the default init method and manually set the parent property of the child objects, e.g.:
_a = [[A alloc] init];
_a.parent = self;
_b = [[B alloc] init];
_b.parent = self;