Delegation in Kotlin

Delegation is a powerful concept that's widely used in Kotlin. It's a software design pattern that allows functionality to be transparently delegated from one object to another. In layman's terms, it's a way of saying, "I can't handle this task myself, but I know someone who can." Kotlin provides some nice features that make it easy to use delegation in your programs.

Understanding Delegation

Before jumping into the Kotlin specifics, let's first understand what delegation is in a general sense. Delegation is a method of passing 'work' off to another class or method. It's a way to achieve code reuse and code organization.

Let's consider an example. Imagine you have a Printer class which has a method print(). Now, you want another class HighResPrinter that should also have a print() method. Instead of rewriting the code for print(), you can delegate this functionality to the Printer class.

How Delegation Works in Kotlin

Kotlin has a built-in delegation pattern, using the keyword by. The syntax is simple and concise.

Let's continue our printer example. Here's how you might write it in Kotlin:

interface Printer {
    fun print()
}

class PrinterImpl : Printer {
    override fun print() = println("Printing...")
}

class HighResPrinter(printer: Printer) : Printer by printer

In this example, HighResPrinter is delegating the print() functionality to the PrinterImpl class.

The Benefits of Delegation in Kotlin

There are several benefits to using delegation in Kotlin:

1. Code Reusability: You can reuse code easily without worrying about the complexities of inheritance.
2. Enhanced Readability: Delegation makes your code more readable and easier to understand.
3. Separation of Concerns: Delegation allows you to separate the different concerns of your code, leading to cleaner and more maintainable code.

Property Delegation

In addition to class delegation, Kotlin also supports property delegation. This means that you can delegate the getter and setter of a property to another class.

Kotlin provides some standard delegates to be used with properties:

• Lazy: The value gets computed only upon first access.
• Observable: Observes changes to the property and executes some logic every time it changes.
• Vetoable: Similar to Observable, but we can prevent a new value from being assigned.
• NotNull: Throws an exception if you try to assign a null value.
• Map: Allows you to store properties in a map rather than a separate field for each property.

Here's an example of how to use the lazy delegate:

val lazyValue: String by lazy {
    println("Computed!")
    "Hello"
}

fun main() {
    println(lazyValue) // "Computed!" and "Hello" will be printed.
    println(lazyValue) // Only "Hello" will be printed.
}

In this case, the lazyValue is computed only once when it's first accessed. The next time it's accessed, the cached value is returned.

Conclusion

Delegation is a powerful feature in Kotlin that allows you to reuse code, increase readability, and separate concerns. By mastering this concept, you can write more robust and maintainable applications. So go ahead, start delegating!