How to add variable in existing class in swift?

问题

I know that in swift we can use Extensions to add new methods to existing classes.

But what about if i want to add a variable?

extension UIViewController {

    var myVar = "xyz"
}

It gives like :

Extensions must not contain stored properties

回答1:

We can't add the stored properties to extensions directly but we can have the computed variables.

Extensions in Swift can:

Add computed instance properties and computed type properties
Define instance methods and type methods
Provide new initializers
Define subscripts
Define and use new nested types
Make an existing type conform to a protocol

For more please visit

https://docs.swift.org/swift-book/LanguageGuide/Extensions.html

回答2:

You can only add computed properties to extensions as follows...

extension UIViewController {
   var someProperty = "xyz" : String {
      return "xyz"
   }
}

If you wish to use it the way you are defining it, you might need to subclass your UIViewController

class YourCustomViewController: UIViewController {
    var someProperty: String = "xyz"
}

回答3:

you can only use computed variables: for example we have the type Int in swift, and we want it extend in a way that swift generates a random number from 0 to our number :

extension Int
{
    var arc4random : Int{
        if self > 0
        {return Int(arc4random_uniform(UInt32(UInt(self))))}

         else if self < 0
        {return -Int(arc4random_uniform(UInt32(UInt(abs(self)))))}

        else
        {return 0}
    }
}

and usage :

myArray.count.arc4random

here my array.count is an Int , and arc4random is the computed variable we have defined in our extension, u cant store a value in it

回答4:

You can try ( This is a readOnly computed property )

extension UIViewController {

    var someProperty : String {

        return "xyz"
    }
}

来源：https://stackoverflow.com/questions/52972231/how-to-add-variable-in-existing-class-in-swift

标签

ios

swift

extension-methods