Receivers

Receivers play a vital role in Kotlin, enabling extension functions and member functions. The Analysis API provides abstractions for different kinds of receivers, allowing you to analyze them in your code.

Types of Receivers

There are two primary types of receivers in Kotlin:

Dispatch receiver: Represents the instance of a class on which a member function is called. It's implicitly passed to the member function and accessible using the this keyword.
Extension receiver: Represents the object that an extension function extends. It's explicitly declared in the function signature and accessible using the this keyword within the function body.

In addition, there are context receivers, which are deprecated and will be eventually replaced with context parameters.

Dispatch Receiver

A dispatch receiver is associated with member functions, which are functions defined within a class. When a member function is called on an object, the object itself becomes the dispatch receiver.

Example:

class MyClass {
    fun memberFunction() {
        // `this` refers to the dispatch receiver (instance of MyClass)
        println(this)
    }
}

val myObject = MyClass()
myObject.memberFunction() // `myObject` is the dispatch receiver

Extension Receiver

An extension receiver is associated with extension functions, which are functions that extend the functionality of a class without modifying the class itself. The extension receiver is the object being extended.

Example:

fun String.extensionFunction() {
    // `this` refers to the extension receiver (instance of String)
    println(this.length)
}

val myString = "Hello"
myString.extensionFunction() // `myString` is the extension receiver

Key Differences

While both dispatch and extension receivers are accessible using the this keyword within the function body, there are a few differences.

Declaration. Dispatch receivers are implicit and don't appear in the function signature. Extension receivers are explicitly declared in the function signature.
Purpose. Dispatch receivers are used for member functions that operate on the state of an object. Extension receivers are used to add new functions to existing classes.

Analysis API Representation

The Analysis API represents receivers using the KaReceiverValue sealed class. The specific subclasses are:

KaExplicitReceiverValue: Represents an explicit receiver expression, e.g., foo in foo.toString();
KaImplicitReceiverValue: Represents an implicit receiver, such as in toString() (without an explicit receiver).

Declarations with both receivers

Member extension functions can have both dispatch and extension receivers. For example:

class MyClass {
    fun String.extensionFunction() {
        // `this` refers to the String receiver (extension receiver)
        // `this@MyClass` refers to the MyClass instance (dispatch receiver)
        println(this.length + this@MyClass.hashCode())
    }
}

In the Analysis API, you can access both receivers using the corresponding properties of KtCallableMemberCall:

val callInfo = ...
when (callInfo) {
    is KtSuccessCallInfo -> {
        val call = callInfo.call
        if (call is KtSimpleFunctionCall) {
            val dispatchReceiver = call.partiallyAppliedSymbol.dispatchReceiver
            val extensionReceiver = call.partiallyAppliedSymbol.extensionReceiver
            // Use dispatchReceiver and extensionReceiver
        }
    }
}

Last modified: 01 August 2024