Open classes.md

Open classes

Can inherit from open class, use its protected properties, override open, but not override final methods. Overriding final methods will give an NSException, which will crash the app.

Explanations

Let's describe Kotlin's open class:

open class OpenClassWithConstructorParams(
    val param1: String,
    val param2: Boolean
) {
    protected val someField: String get() = "14"

    fun finalFunctionInClass() {
        println("Final method in open class")
    }

    open fun functionCanBeOverridden() {
        println("Open method in open class")
    }

}

On the Swift side, we can inherit from this class, use its protected properties, override open, but not override final methods (this will give an exception at runtime on construction):

class OpenClassExample : OpenClassWithConstructorParams {
    override func finalFunctionInClass() {
        print("Final function was overridden")
    }
    
    override func functionCanBeOverridden() {
        print("Open function was overridden")
    }
    
}

func example() {
    let example = OpenClassExample(param1: "123", param2: true)
    let _ = example.someField
    example.finalFunctionInClass()
    example.functionCanBeOverridden()
}

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