Android Development with Kotlin

The observable delegate

The observable is the most useful standard library delegate for mutable properties. Every time a value is set (the setValue method is called), the lambda function from the declaration is invoked. A simple example of the observable delegate is as follows:

    var name: String by Delegates.observable("Empty"){ 
        property, oldValue, newValue -> // 1 
        println("$oldValue -> $newValue") // 2 
    } 
 
    // Usage 
    name = "Martin" // 3    Prints: Empty -> Martin 
    name = "Igor" // 3    Prints: Martin -> Igor 
    name = "Igor" // 3, 4     Prints: Igor -> Igor

1. The arguments of the lambda function are as follows:
   • property: A reference to the delegated property. Here, it is a reference to the name. This is the same as the property from setValue and getValue ...