Create objects dynamically in Swift

Question

In Objective-C I had a category:

@implementation NSObject (Entity)
+ (instancetype)entity{
    Class cl = self.class;
    NSObject *obj = [[cl alloc] init];

    return obj;
}
@end

Let's assume there are classes A and B:

@interface A : NSObject
@end

@interface B : NSObject
@end

And then I can use static entity method to create objects like:

NSObject *o = [NSObject entity]; // type of NSObject
A *a = [A entity];               // type of A
B *b = [B entity];               // type of B

How can I write that category in Swift? I know it's possible to create an object by knowing the type like let s = NSString.self; var str = s.init(), but here I don't know the class, that should be something like self.self which has no sense and doesn't work for sure.

Thanks in advance

This question is at least similar to yours.

Adam
– Adam

2015-09-21 09:52:46 +00:00
Commented Sep 21, 2015 at 9:52 — Adam
– Adam, Commented Sep 21, 2015 at 9:52

LastMove · Accepted Answer · 2015-09-21 09:59:53Z

5

let s = NSString.self;
var str = s.init();

If you need the class, it's just:

print(s) // "NSString"

s is of Type "NSString.Type" it 's represent the Object class;

If you need the class of an object I think that you are searching "dynamicType":

let object = "toto"
let t = object.dynamicType // String.Type

var newObject = t.init("toto");

answered Sep 21, 2015 at 9:59

LastMove

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Comments

0x416e746f6e · Accepted Answer · 2015-09-21 21:47:04Z

3

As mentioned by others, the whole point is that Objective-C provides base class for all classes, while Swift does not. NSObject in obj-C provides alloc and init methods, hence one can expect that a descendant from NSObject will either implement or inherit those methods, hence you can have the shortcut-category you mentioned in your question.

In Swift there are no base classes, therefore you can not know which class (or other type) provides which initialisers. One way or another you will have to resort to specifying them for your purposes, and I think that protocols are the best way to do so.

E.g. consider this:

protocol DefaultInstantiatable {
    init()
}

extension DefaultInstantiatable {
    static func entity() -> Self {
        return Self()
    }
}

If your types adopt DefaultInstantiatable protocol then you could use clauses like:

struct Foo: DefaultInstantiatable {
    init() { }
}

class Bar: DefaultInstantiatable {
    required init() { }
}

let foo = Foo.entity()  // Creates an instance of Foo structure
let bar = Bar.entity()  // Creates an instance of Bar object

answered Sep 21, 2015 at 21:47

0x416e746f6e

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3 Comments

Chris Prince Over a year ago

I can't get return Self() to work. Xcode 7.3.1 gives a syntax error.

0x416e746f6e Over a year ago

It should work. There must be some reason why it errors in your case. Please create a separate question for that.

Chris Prince Over a year ago

Ahhhh! My bad! And I just learned something cool from you-- I hadn't seen that this was in the context of a protocol extension. I like that! I was just generically trying to return Self() in a method and that wasn't working.

Adam · Accepted Answer · 2015-09-21 11:38:00Z

2

As I mentioned in my comment, the reason is that there is no base class in Swift. You can still do it by implementing your own base class or by subclassing NSObject.

class A : NSObject {}
class B : NSObject {}

extension NSObject {
    class func entity() -> Self {
        return self.init()
    }
}

A.entity()
B.entity()

NSObject.entity()

edited Sep 21, 2015 at 11:38

answered Sep 21, 2015 at 10:00

Adam

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2 Comments

LastMove Over a year ago

Isn't "dynamicType" enough?

Adam Over a year ago

dynamicType will not allow you to create an universal category, which is what the OP asked for.

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