Android Development with Kotlin

Companion object extensions

If a class has a companion object defined, then you can also define extension functions (and properties) for this companion object. To distinguish between an extension to a class and an extension to a companion object, .Companion needs to be added between the extension type and function name:

    class A { 
        companion object {} 
    } 
    fun A.Companion.foo() { print(2) } 

When it is defined, the foo method can be used as if it were defined inside the A companion object:

    A.foo() 

Note that we are calling this extension using the class type, not the class instance. To allow the creation of an extension function for a companion object, a companion object needs to be explicitly defined inside the class, even an empty one. Without ...