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Kotlin Compact

Scoping Functions

Explore Kotlin's scoping functions such as apply, let, run, and also to write concise, readable, and well-structured code. Understand how they control variable scope and improve object initialization and method invocation.

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Introduction

On the one hand, scoping functions offer the possibility of restricting the scope of variables. On the other hand, they lend a compact notation for bundling contextually related code in a clear and legible manner.

Often, we write code that first creates a new object and shortly thereafter invokes some of its methods or sets some properties.

In Java, this might look like this:

Java
Person person = new Person();
person.setFirstName("Alex");
person.setLastName("Prufrock"); 
person.setDateOfBirth(1984,3,5); 
person.calculateAge();

Thanks to Kotlin’s properties, we can write this more elegantly:

Kotlin 1.5
val person = Person() 
person.firstName = "Alex" 
person.lastName = "Prufrock" 
person.setDateOfBirth(1984,3,5) 
person.calculateAge()

It gets even better with the first possible scoping function called apply:

Kotlin 1.5
val person = Person().apply { 
  firstName = "Alex"
  lastName = "Prufrock"
  setDateOfBirth(1984,3,5) 
  }
  print("${person.firstName} ${person.lastName}\n${person.dob.year}-${person.dob.month}-${person.dob.day}")

This is what the signature of apply looks like:

inline fun <T> T.apply(block: T.() -> Unit) : T

The function calls the passed ...