How can I create a instance like objective-c do?

Usually, I create a class's instance like this:

The Parent class:

@interface baseClass  
+ (instancetype)task;  
@end  
@implement baseClass  
+ (instancetype)task {  
    return [[[self class] alloc] init];  
}  
@end

and then in children class:

@interface childClass : baseClass  
@end  
@implement childClass  
@end

Finally, I can create a instance using:

childClass *pChild = [childClass task];

How could I implement this feature by using the Swift programming language?

In another word, how could I implement the [[[self class] alloc] init] in swift way?

Solution

I feel a bit insecure about you tagged your question as a singleton but basically the code you presented has nothing to do with singletons at all, so I tried to mimic your original classes and show a few options here.

these are written in Swift 2.2.

case #1

that works flawlessly initially:

class BaseClass {

    class func task() -> BaseClass {
        return BaseClass()
    }

    // ...

}

class ChildClass : BaseClass {

    override class func task() -> ChildClass {
        return ChildClass()
    }

    // ...

}

then you would be able to get instances like this:

let bTask: BaseClass = BaseClass.task()
let cTask: ChildClass = ChildClass.task()

case #2

you can also do something like this, if such thing looks more reasonable for you:

class BaseClass {
    
    class var task: AnyObject {
        get {
            return self.init()
        }
    }
    
    required init() {
        
    }
    
    // ...

}

class ChildClass : BaseClass {
    
    // ...

}

then you can instantiate your classes like:

let bTask: BaseClass = BaseClass.task as! BaseClass
let cTask: ChildClass = ChildClass.task as! ChildClass

case #3

here is an other option for you too, if you are not happy with any of the ideas above:

class BaseClass {
    
    class func task() -> Self {
        return self.init()
    }
    
    required init() {
        
    }
    
    // ...

}

class ChildClass : BaseClass {
    
    // ...

}

the instantiation is similar to the first case:

let bTask: BaseClass = BaseClass.task()
let cTask: ChildClass = ChildClass.task()

NOTE: you may need to refine the chosen concept for your final code if you want to deal with real singletons, these examples are not perfectly worked out as I mentioned at the beginning but they show you a few options you can have and you can use as template.