Factory methods in Objective-C - objective-c

Question

I make class factories like so,
#implementation Universe {
NSString *foo;
}
+ (instancetype)universeWithMeaning:(NSString *)meaning
{
return [[self alloc] initUniverseWithMeaning:meaning];
}
- (id)initUniverseWithMeaning:(NSString *)meaning
{
if (self = [super init]) {
foo = meaning;
}
return self;
}
- (void)showMeaning
{
NSLog(#"%#", foo);
}
#end
And create object like this,
Universe *universe = [Universe universeWithMeaning:#"42"];
[universe showMeaning]; // Prints 42
This works great, but the method signature of initUniverseWithMeaning: is the same as that of universeWithMeaning:, except that it's an instance method which allows it to save instance variables to the created object.
Is there a way to this without having to implement the initUniverseWithMeaning: instance method?
I know its necessary to be inside of an instance method to be able to access instance variables, so I've been experimenting with blocks. My idea was to pass a block containing instance variable assignations to the class method which would somehow execute it in the instance context.
Implementation,
#implementation Cat {
NSString *lives;
}
+ (Cat *)newCat:(void(^)(void))cat
{
cat(); // **Problem 1**
}
- (void)showLives
{
NSLog(#"%#", lives);
}
#end
Usage,
Cat *cat = [Cat newCat:^void (void) {
self.lives = 9; // **Problem 2**
}];
[cat showLives]; // I'd like this to print 9
Problem 1: How to create a Cat object and execute cat() inside it?
Problem 2: How to make self refer to the object in the block's execution environment?
Anyway, this is more of a curiosity than anything else, it's would only be practically useful to save me from writing alloc (I would just need to include a method prototype for initUniverseWithMeaning: in the .h file.)

Answer 1

For your problem 1 and 2, you can try this
#interface Cat ()
#property (strong) NSString *lives;
#end
#implementation Cat
+ (Cat *)newCat:(void(^)(Cat *me))cat
{
Cat *newcat = [[self alloc] init];
cat(newcat);
return newcat;
}
- (void)showLives
{
NSLog(#"%#", lives);
}
#end
Cat *cat = [Cat newCat:^(Cat *me) {
me.lives = 9;
}];
[cat showLives]; // print 9
but I can't see much use of it... Isn't this simpler?
Cat *cat = [Cat new];
cat.lives = 9;
[cat showLives];
For your real problem
Is there a way to this without having to implement the initUniverseWithMeaning: instance method?
+ (instancetype)universeWithMeaning:(NSString *)meaning
{
Universe *universe = [[self alloc] init];
if (universe) universe->foo = meaning;
return universe;
}

Answer 2

The first example you've posted is the correct way of creating Objective-C factory methods.
An Objective-C factory method is nothing more than a class method wrapper around an instance level init method. Generally speaking, every factory method should have a paired init method that takes the same number and type of arguments.
fooWithBar:(NSString *)bar should be paired with initWithBar:(NSString *)bar, etc.
An exception might come in when you have an init method that takes arguments, but you've create a handful of factory methods with default arguments for this method. For example:
- (instancetype)initWithString:(NSString *)string;
+ (instancetype)fooWithString:(NSString *)string {
return [[self alloc] initWithString:string];
}
+ (instancetype)fooWithBar {
return [[self alloc] initWithString:#"bar"];
}
Now, you can create the object with in the method, then modify it, and return the modified object.
For example:
+ (instancetype)fooWithString:(NSString *)string {
Foo *f = [[self alloc] init];
f.str = string;
return f;
}
But honestly, it's just better to have an initWithString: method.
Every class should have a designated initializer and every object of that class should go through the designated initializer.