Kotlin, known for its concise syntax and powerful features, has gained immense popularity among developers. One of its notable features is the ability to declare kotlin inline properties. Kotlin inline properties combine the benefits of properties and inline functions, providing improved performance and better control over code structure. In this blog post, we’ll dive deep into Kotlin inline properties, covering their definition, benefits, and use cases, and providing detailed examples to solidify your understanding.

Understanding Inline Properties

An inline property is a property that is backed by an inline function. This means that when you access the property, the code of the getter function is directly inserted at the call site, similar to how inline functions work. This has significant implications for performance, as it eliminates the overhead of function calls.

What are Kotlin inline properties?

Inline properties are a Kotlin feature that allows you to improve the performance of your code by inlining the property accessors into the code that uses them. This means that the compiler will copy the body of the accessors into the call site, instead of calling them as separate functions.

Inline properties can be used for both read-only (val) and mutable (var) properties. However, they can only be used for properties that do not have a backing field.

When to use Kotlin inline properties?

Inline properties should be used when you want to improve the performance of your code by reducing the number of function calls. This is especially useful for properties that are accessed frequently or that are used in performance-critical code.

Inline properties should not be used when the property accessors are complex or when the property is not accessed frequently. In these cases, the performance benefits of inlining may not be worth the added complexity.

Declaring Kotlin Inline Properties

To declare an inline property in Kotlin, you’ll use the inline keyword before the property definition. Here’s the general syntax:

inline val propertyName: PropertyType
    get() = propertyValue

Let’s break down the code:

• inline: This keyword indicates that the property is inline, allowing its getter code to be inserted at the call site.
• val: Indicates that the property is read-only.
• propertyName: The name you give to your property.
• PropertyType: The data type of the property.
• propertyValue: The value that the property holds.

Few Simple Declarations of Kotlin inline properties

Here are some simple examples of how to use Kotlin inline properties:

// A read-only inline property
inline val foo: String
    get() = "Hello, softAai!"

// A mutable inline property
inline var bar: Int
    get() = TODO() // You need a getter function for a mutable property
    set(value) {
        // Do something with the value.
    }

// An inline property with a custom getter and setter
inline val baz: String
    get() = "This is a custom getter."
    set(value) {
        // Do something with the value.
    }

In the above code snippet:

• For the bar property, you need to provide a getter function since it’s a mutable property. In this case, I’ve used TODO() to indicate that you need to replace it with an actual getter implementation.
• The baz property is defined with a custom getter and setter. The getter provides a string value, and the setter is a placeholder where you can implement custom logic to handle the incoming value.

Use Cases for Kotlin Inline Properties

1. Simple Properties: Inline properties are ideal for cases where you have simple read-only properties that involve minimal computation. For instance, properties that return constant values or perform basic calculations can benefit from inlining.
2. Performance-Critical Code: In scenarios where performance is crucial, such as in high-frequency loops, using inline properties can significantly reduce function call overhead and lead to performance improvements.
3. DSLs (Domain-Specific Languages): Inline properties can be used to create more readable and expressive DSLs. The inlined properties can provide syntactic sugar that enhances the DSL’s usability.
4. Custom Accessors: Inline properties are useful when you want to customize the getter logic for a property without incurring function call overhead.

Examples of Kotlin Inline Properties

Let’s explore a few examples to solidify our understanding.

Example 1: Constant Inline Property

inline val pi: Double
    get() = 3.141592653589793

In this example, the pi property always returns the constant value of Pi.

Example 2: Performance Optimization

data class Point(val x: Int, val y: Int) {
    val magnitude: Double
        inline get() = Math.sqrt(x.toDouble() * x + y.toDouble() * y)
}

fun main() {
    val point = Point(3, 4)
    println("Magnitude of the point: ${point.magnitude}")
}

In this example, the inline property magnitude allows you to access the magnitude of the Point instance without invoking a separate function. The getter’s code is expanded and copied directly at the call site, eliminating the function call overhead.

Example 3: DSL Enhancement

class Element(val tagName: String) {
    inline val cssClass: String
        get() = "$tagName-class"
}

fun main() {
    val div = Element("div")
    println(div.cssClass) // Output: div-class
}

Here, the cssClass property enhances the readability of constructing CSS class names within an HTML DSL.

Rules for Kotlin Inline Properties

1. Inline Modifier

To declare an inline property, use the inline modifier before the property definition. This indicates that the property getter’s code will be inserted directly at the call site.

inline val propertyName: PropertyType
    get() = propertyValue

Example:

inline val pi: Double
    get() = 3.141592653589793

2. Read-Only Properties

Inline properties are read-only; they don’t support custom setters(as don’t have backing fields). You can only define the getter logic. It might feel confusing but don’t worry, you will get a clearer idea as we proceed. So, bear with me.

Example:

data class Point(val x: Int, val y: Int)

inline val Point.magnitude: Double
    get() = Math.sqrt(x.toDouble() * x + y.toDouble() * y)

3. Limited Logic in Getters

Keep the logic inside the inline property getter minimal and straightforward. Avoid complex computations or excessive branching.

Example:

inline val half: Int
    get() = 100 / 2

4. No Property Initialization

You can’t directly initialize the inline property’s value within the property declaration.

Example:

// Not allowed
inline val invalid: Int = 42   

// We will get this compilation error: Inline property cannot have backing field

5. Interaction with Inline Functions

When an inline property is accessed within an inline function, the property’s code is also inlined. This can create a hierarchy of inlining that affects performance and code size.

Example:

inline val greeting: String
    get() = "Hello"

inline fun printGreeting() {
    println(greeting) // The code of 'greeting' property will be inlined here
}

By marking both the property and the function as inline, the property’s getter code is directly placed into the function’s call site. This can optimize performance by avoiding the function call overhead. However, it might lead to larger compiled code if the same property’s getter logic is used in multiple locations.

6. Parameterization with Higher-Order Functions

Inline properties can’t take parameters directly. You can use higher-order functions or lambdas for parameterized behavior.

Example:

inline val greeting: (String) -> String
    get() = { name -> "Hello, $name!" }

fun main() {
    val greetFunction = greeting // Assign the lambda to a variable
    val message = greetFunction("softAai") // Call the lambda with a name
    println(message)   // o/p : Hello, softAai!
}

Inline Modifier and Inline Properties

The inline modifier can be used on accessors of properties that don’t have backing fields. You can annotate individual property accessors. That means we can mark entire property or individual accessors (getter and setter) as inline:

Inline Getter

var ageProperty: Int
    inline get() {
        ...
    }
    set(value) {
        ...
    }

Inline Setter

var ageProperty: Int
    get() {
        ...
    }
    inline set(value) {
        ...
    }

Inline Entire Property

You can also annotate an entire property, which marks both of its accessors as inline:

inline val ageProperty: Int
    get() {
       ...
    }
    set(value) {
        ...
    }

Remember, when you use inline accessors in Kotlin, whether for getters or setters, their code behaves like regular inline functions at the call site. This means that the code inside the accessor is inserted directly where the property is accessed or modified, similar to how inline functions work.

Kotlin Inline Properties and Backing Fields

In Kotlin, properties are usually associated with a backing field — a hidden field that stores the actual value of the property. This backing field is automatically generated by the compiler for properties with custom getters or setters. However, inline properties differ in this aspect.

What Does “No Backing Field” Mean?

When you declare an inline property, the property’s getter code is directly inserted at the call site where the property is accessed. This means that there’s no separate backing field holding the value of the property. Instead, the getter logic is inlined into the code that accesses the property, eliminating the need for a distinct memory location to store the property value.

Implications of No Backing Field

1. Memory Efficiency: Since inline properties don’t require a backing field, they can be more memory-efficient compared to regular properties with backing fields. This can be especially beneficial when dealing with large data structures or frequent property accesses.
2. Direct Calculation: The absence of a backing field means that any calculations performed within the inline property’s getter are done directly at the call site. This can lead to improved performance by avoiding unnecessary memory accesses.

Example: Understanding No Backing Field

Consider the following example of a regular property with a backing field and an inline property:

class Rectangle(val width: Int, val height: Int) {
    // Regular property with backing field
    val area: Int
        get() = width * height
    
    // Inline property without backing field
    inline val perimeter: Int
        get() = 2 * (width + height)
}

fun main() {
    val rectangle = Rectangle(5, 10)
    println("Area: ${rectangle.area}")   // o/p : 50
    println("Perimeter: ${rectangle.perimeter}")  // o/p : 30
}

In this example, the area property has a backing field that stores the result of the area calculation. On the other hand, the perimeter inline property doesn’t have a backing field; its getter code is directly inserted wherever it’s accessed.

When to Use Kotlin Inline Properties without Backing Fields

Inline properties without backing fields are suitable for cases where you want to perform direct calculations or return simple values without the need for separate memory storage. They are particularly useful when the logic within the getter is straightforward and lightweight.

However, remember that inline properties are read-only and can’t have custom setters. We cannot set values to inline properties in the same way we do with regular properties. However, we can use custom setters for performing additional operations other than simple value assignment to it. Therefore, they’re most appropriate for scenarios where the value is determined by a simple calculation or constant.

Restriction: No Backing Field with inline Accessors

In Kotlin, when you use the inline modifier on property accessors (getter or setter), it’s important to note that this modifier is only allowed for accessors that don’t have a backing field associated with them. This means that properties with inline accessors cannot have a separate storage location (backing field) to hold their values.

Reason for the Restriction

The restriction on using inline with properties that have backing fields is in place to prevent potential issues with infinite loops and unexpected behavior. The inlining process could lead to situations where the inlined accessor is calling itself, creating a loop. By disallowing inline on properties with backing fields, Kotlin ensures that this kind of situation doesn’t occur.

Hypothetical Example

Consider the following hypothetical scenario, which would result in an infinite loop if the restriction wasn’t in place:

class InfiniteLoopExample {
    private var _value: Int = 0

    inline var value: Int
        get() = value   // This could lead to an infinite loop
        set(v) {
            _value = v
        }
}

In this example, if the inline modifier were allowed on the getter, an infinite loop would occur since the inline getter is calling itself.

To fix the code and prevent the infinite loop, you should reference the backing property _value in the getter and also make it public, as shown below:

class InfiniteLoopExample {
    var _value: Int = 0   // // Change visibility to public

    inline var value: Int
        get() = _value   // Use the backing property here
        set(v) {
            _value = v
        }
}

fun main() {
    val example = InfiniteLoopExample()
    example.value = 42
    println(example.value)
}

Note: By changing the visibility to ‘public,’ you introduce a security risk as it exposes the internal details of your class. This approach is not recommended; even though it violates the ‘no backing field’ rule, I only made these changes for the sake of understanding. Instead, it’s better to follow the rules and guidelines for inline properties.

Real Life Example

inline var votingAge: Int
    get() {
        return 18    // Minimum voting age in India
    }
    set(value) {
        if (value < 18) {
            val waitingValue = 18 - value
            println("Setting: Still waiting $waitingValue years to voting age")
        } else {
            println("Setting: No more waiting years to voting age")
        }
    }

fun main() {
    votingAge = 4
    val votableAge = votingAge
    println("The votable age in India is $votableAge")
}

When you run the code, the following output will be produced:

Setting: Still waiting 14 years to voting age
The votable age in India is 18

In India, the minimum voting age is 18 years old. This means that a person must be at least 18 years old in order to vote in an election. The inline property here stores the minimum voting age in India, and it can be used to check if a person is old enough to vote.

In the code, the value of the votingAge property is set to 4. However, the setter checks if the value is less than 18. Since it is, the setter prints a message saying that the person is still waiting to reach the voting age. The value of the votingAge property is not changed.

This code snippet can be used to implement a real-world application that checks if a person is old enough to vote. For example, it could be used to validate the age of a voter before they are allowed to cast their vote.

Benefits of Kotlin Inline Properties

There are several benefits to using Kotlin inline properties:

1. Performance Optimization: Inline properties eliminate the overhead of function calls, resulting in improved performance by reducing the runtime costs associated with accessing properties.
2. Control over Inlining: Inline properties give you explicit control over which properties should be inlined, allowing you to fine-tune performance optimizations for specific parts of your codebase.
3. Cleaner Syntax: Inline properties can lead to cleaner and more concise code by reducing the need for explicit getter methods.
4. Reduced Object Creation: In some cases, inline properties can help avoid unnecessary object creation, as the getter code is inserted directly into the calling code.

• Smaller code size: Inline properties can reduce the size of your compiled code by eliminating the need to create separate functions for the property accessors.
• Easier debugging: Inline properties can make it easier to debug your code by making it easier to see where the property accessors are being called.

Drawbacks of using Kotlin inline properties

There are a few drawbacks to using Kotlin inline properties:

• Increased complexity: Inline properties can make your code more complex, especially if the property accessors are complex.
• Reduced flexibility: Inline properties can reduce the flexibility of your code, because you cannot override or extend the property accessors.

Conclusion

Kotlin’s inline properties provide a powerful mechanism for optimizing code performance and enhancing code structure. By using inline properties, you gain the benefits of both properties and inline functions, leading to more readable and performant code. Understanding when and how to use inline properties can elevate your Kotlin programming skills and contribute to the efficiency of your projects.