Android - How To Select Scope Functions In Kotlin?

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In this article, we will learn about how to select Scope Functions in Kotlin. The Kotlin standard library contains several functions whose sole purpose is to execute a block of code within the context of an object. When we call such a function on an object with a lambda expression provided, it forms a temporary scope. In this scope, we can access the object without its name. Such functions are called Scope Functions. This article provides the detailed descriptions of the differences between scope functions and the conventions on their usage in Kotlin with some authentic examples.

A famous quote about learning is :

” Tell me and I forget, teach me and I may remember, involve me and I learn.”

So Let’s begin.

Scope Functions in Kotlin

The definition of Scope function is

” Scoped functions are functions that execute a block of code within the context of an object.“

These functions provide a way to give temporary scope to the object under consideration where specific operations can be applied to the object within the block of code, thereby, resulting in a clean and concise code. There are five scoped functions in Kotlin: let, run, with, also and apply.

Basically, these functions do the same: execute a block of code on an object. What’s different is how this object becomes available inside the block and what is the result of the whole expression.

Example

Here’s a typical usage of a scope function:

Person("Alice", 20, "Amsterdam").let {
     println(it)
     it.moveTo("London")
     it.incrementAge()
     println(it)
 }

If we write the same without let, we’ll have to introduce a new variable and repeat its name whenever we use it.

val alice = Person("Alice", 20, "Amsterdam")
 println(alice)
 alice.moveTo("London")
 alice.incrementAge()
 println(alice)

The scope functions do not introduce any new technical capabilities, but they can make our code more concise and readable.

Due to the similar nature of scope functions, choosing the right one for our case can be a bit tricky. The choice mainly depends on our intent and the consistency of use in our project.

Common Difference between Scope Functions

Because the scope functions are all quite similar in nature, it’s important to understand the differences between them. There are two main differences between each scope function:

The way to refer to the context object
The return value.

Context object – this or it

Inside the lambda of a scope function, the context object is available by a short reference instead of its actual name. Each scope function uses one of two ways to access the context object: as a lambda receiver (this) or as a lambda argument (it). Both provide the same capabilities, so we’ll describe the pros and cons of each for different cases and provide recommendations on their use.

fun main() {
     val str = "Hello"
     // this
     str.run {
         println("The receiver string length: $length")
         //println("The receiver string length: ${this.length}") // does the same
     }
     // it 
     str.let {     
        println("The receiver string's length is ${it.length}")           }
}

this

run, with, and apply refer to the context object as a lambda receiver – by keyword this. Hence, in their lambdas, the object is available as it would be in ordinary class functions. In most cases, we can omit this when accessing the members of the receiver object, making the code shorter. On the other hand, if this is omitted, it can be hard to distinguish between the receiver members and external objects or functions. So, having the context object as a receiver (this) is recommended for lambdas that mainly operate on the object members: call its functions or assign properties.

val adam = Person("Adam").apply { 
     age = 20                       // same as this.age = 20 or adam.age = 20
     city = "London"
 }
 println(adam)

it

In turn, let and also have the context object as a lambda argument. If the argument name is not specified, the object is accessed by the implicit default name it. it is shorter than this and expressions with it are usually easier for reading. However, when calling the object functions or properties we don’t have the object available implicitly like this. Hence, having the context object as it is better when the object is mostly used as an argument in function calls. it is also better if we use multiple variables in the code block.

fun getRandomInt(): Int {
     return Random.nextInt(100).also {
         writeToLog("getRandomInt() generated value $it")
     }
 }
 val i = getRandomInt()

Additionally, when we pass the context object as an argument, we can provide a custom name for the context object inside the scope.

fun getRandomInt(): Int {
     return Random.nextInt(100).also { value ->
         writeToLog("getRandomInt() generated value $value")
     }
 }
 val i = getRandomInt()

Return value

The scope functions differ by the result they return:

apply and also return the context object.
let, run, and with return the lambda result.

These two options let we choose the proper function depending on what we do next in our code.

Context object

The return value of apply and also is the context object itself. Hence, they can be included into call chains as side steps: we can continue chaining function calls on the same object after them.

val numberList = mutableListOf()
 numberList.also { println("Populating the list") }
     .apply {
         add(2.71)
         add(3.14)
         add(1.0)
     }
     .also { println("Sorting the list") }
     .sort()

They also can be used in return statements of functions returning the context object.

fun getRandomInt(): Int {
     return Random.nextInt(100).also {
         writeToLog("getRandomInt() generated value $it")
     }
 }
 val i = getRandomInt()

Lambda result

let, run, and with return the lambda result. So, we can use them when assigning the result to a variable, chaining operations on the result, and so on.

val numbers = mutableListOf("one", "two", "three")
 val countEndsWithE = numbers.run { 
     add("four")
     add("five")
     count { it.endsWith("e") }
 }
 println("There are $countEndsWithE elements that end with e.")

Additionally, we can ignore the return value and use a scope function to create a temporary scope for variables.

val numbers = mutableListOf("one", "two", "three")
 with(numbers) {
     val firstItem = first()
     val lastItem = last()        
     println("First item: $firstItem, last item: $lastItem")
 }

We can analyze the common scope functions difference summary as below diagram:

Five Scope Functions In Kotlin

To help we choose the right scope function for our case, we’ll describe them in detail and provide usage recommendations. Technically, functions are interchangeable in many cases, so the examples show the conventions that define the common usage style.

There are five scoped functions in Kotlin: let, run, with, also and apply.

Let’s go through them one by one.

Scope Function – let

The context object is available as an argument (it). The return value is the lambda result.

let can be used to invoke one or more functions on results of call chains. For example, the following code prints the results of two operations on a collection:

val numbers = mutableListOf("one", "two", "three", "four", "five")
 val resultList = numbers.map { it.length }.filter { it > 3 }
 println(resultList)

With let, we can rewrite it:

val numbers = mutableListOf("one", "two", "three", "four", "five")
 numbers.map { it.length }.filter { it > 3 }.let { 
     println(it)
     // and more function calls if needed
 }

If the code block contains a single function with it as an argument, we can use the method reference (::) instead of the lambda:

val numbers = mutableListOf("one", "two", "three", "four", "five")
 numbers.map { it.length }.filter { it > 3 }.let(::println)

let is often used for executing a code block only with non-null values. To perform actions on a non-null object, use the safe call operator ?. on it and call let with the actions in its lambda.

val str: String? = "Hello"   
 //processNonNullString(str)       // compilation error: str can be null
 val length = str?.let { 
     println("let() called on $it")        
     processNonNullString(it)      // OK: 'it' is not null inside '?.let { }'
     it.length
 }

Another case for using let is introducing local variables with a limited scope for improving code readability. To define a new variable for the context object, provide its name as the lambda argument so that it can be used instead of the default it.

val numbers = listOf("one", "two", "three", "four")
 val modifiedFirstItem = numbers.first().let { firstItem ->
     println("The first item of the list is '$firstItem'")
     if (firstItem.length >= 5) firstItem else "!" + firstItem + "!"
 }.toUpperCase()
 println("First item after modifications: '$modifiedFirstItem'")

Scope Function – with

A non-extension function: the context object is passed as an argument, but inside the lambda, it’s available as a receiver (this). The return value is the lambda result.

We recommend with for calling functions on the context object without providing the lambda result. In the code, with can be read as “with this object, do the following.”

val numbers = mutableListOf("one", "two", "three")
 with(numbers) {
     println("'with' is called with argument $this")
     println("It contains $size elements")
 }

Another use case for with is introducing a helper object whose properties or functions will be used for calculating a value.

val numbers = mutableListOf("one", "two", "three")
 val firstAndLast = with(numbers) {
     "The first element is ${first()}," +
     " the last element is ${last()}"
 }
 println(firstAndLast)

It is convenient when we have to call multiple different methods on the same object. Instead of repeating the variable containing this object on each line, we can use with. This function is used to change instance properties without the need to call dot operator over the reference every time.

Scope Function – run

The context object is available as a receiver (this). The return value is the lambda result.

run does the same as with but invokes as let – as an extension function of the context object.

run is useful when our lambda contains both the object initialization and the computation of the return value.

val service = MultiportService("https://example.kotlinlang.org", 80)
 val result = service.run {
     port = 8080
     query(prepareRequest() + " to port $port")
 }
 // the same code written with let() function:
 val letResult = service.let {
     it.port = 8080
     it.query(it.prepareRequest() + " to port ${it.port}")
 }

Besides calling run on a receiver object, we can use it as a non-extension function. Non-extension run lets us execute a block of several statements where an expression is required.

val hexNumberRegex = run {
     val digits = "0-9"
     val hexDigits = "A-Fa-f"
     val sign = "+-"
 Regex("[$sign]?[$digits$hexDigits]+")
 }
 for (match in hexNumberRegex.findAll("+1234 -FFFF not-a-number")) {
     println(match.value)
 }

run is actually a combination of with() and let().

Scope Function – apply

The context object is available as a receiver (this). The return value is the object itself.

Use apply for code blocks that don’t return a value and mainly operate on the members of the receiver object. The common case for apply is the object configuration. Such calls can be read as “apply the following assignments to the object.”

val adam = Person("Adam").apply {
     age = 32
     city = "London"        
 }
 println(adam)

Having the receiver as the return value, we can easily include apply into call chains for more complex processing.

Scope Function – also

The context object is available as an argument (it). The return value is the object itself.

also is good for performing some actions that take the context object as an argument. Use also for actions that need a reference rather to the object than to its properties and functions, or when we don’t want to shadow this reference from an outer scope.

When we see also in the code, we can read it as “and also do the following with the object.”

val numbers = mutableListOf("one", "two", "three")
 numbers
     .also { println("The list elements before adding new one: $it") }
     .add("four")

Scope Functions – run vs let

run is similar to let in terms of accepting any return value , but this is different in the context of the object terms. run function refers to the context of the object as “this” and not “it”. That is the reason we did not use “${this.name}” as it would be redundant here since the block of code understands that “name” is used here concerning the Person object.

***Scope Functions – run redundant this***

Another point here is that since the context is referred to as “this”, it cannot be renamed to a readable lambda parameter. So depending on the use case and requirement , we have to choose between the let and the run operator. The “run” operator also helps in easy null checks similar to the “let” operator.

var name: String? = "Abcd" 
private fun performRunOperation() {     
val name = Person().name?.run {         
"The name of the Person is: $this"     
}     
print(name) 
}

Scope Functions – with vs run

Let’s consider a case where a Person object can be nullable.

***Scope Functions With nullable Value***

we can see that the context of the object referred to as “this” is a nullable type of Person. And hence, to correct this, we need to change the code as:

private fun performWithOperation() {     
val person: Person? = null     
with(person) {         
  this?.name = "asdf"         
  this?.contactNumber = "1234"         
  this?.address = "wasd"         
  this?.displayInfo()     
 } 
}

So performing a null check using a “with” operator is difficult and this is where we can replace it with “run” as follows:

private fun performRunOperation() {     
 val person: Person? = null     
 person?.run {         
   name = "asdf"         
   contactNumber = "1234"         
   address = "wasd"         
   displayInfo()     
 } 
}

This looks a lot cleaner.

Scope Functions – run vs apply

So let’s see the difference between run and apply functions.

We can see that run accepts a return statement whereas apply does not accept a return statement(we can see the error thrown by the IDE in the image) and always returns the same object which it is referring to.

Scope Functions – let vs also

So let’s see the difference between let and also functions.

We can see that let accepts a return statement whereas “also” does not accept a return statement(we can see the error thrown by the IDE in the image) and always returns the same object which it is referring to.

Standard Library Scope Functions – takeIf and takeUnless

In addition to scope functions, the standard library contains the functions takeIf and takeUnless. These functions let us embed checks of the object state in call chains.

When we called on an object with a predicate provided, takeIf returns this object if it matches the predicate. Otherwise, it returns null. So, takeIf is a filtering function for a single object. In turn, takeUnless returns the object if it doesn’t match the predicate and null if it does. The object is available as a lambda argument (it).

val number = Random.nextInt(100)
val evenOrNull = number.takeIf { it % 2 == 0 }
val oddOrNull = number.takeUnless { it % 2 == 0 }
println("even: $evenOrNull, odd: $oddOrNull")

When we do chaining other functions after takeIf and takeUnless, we don’t forget to perform the null check or the safe call (?.) because their return value is nullable.

val str = "Hello"
 val caps = str.takeIf { it.isNotEmpty() }?.toUpperCase()
 //val caps = str.takeIf { it.isNotEmpty() }.toUpperCase() //compilation error
 println(caps)

takeIf and takeUnless are especially useful together with scope functions. A good case is chaining them with let for running a code block on objects that match the given predicate. To do this, call takeIf on the object and then call let with a safe call (?). For objects that don’t match the predicate, takeIf returns null and let isn’t invoked.

fun displaySubstringPosition(input: String, sub: String) {
     input.indexOf(sub).takeIf { it >= 0 }?.let {
         println("The substring $sub is found in $input.")
         println("Its start position is $it.")
     }
 }
 displaySubstringPosition("010000011", "11")
 displaySubstringPosition("010000011", "12")

This is how the same function looks without the standard library functions:

fun displaySubstringPosition(input: String, sub: String) {
     val index = input.indexOf(sub)
     if (index >= 0) {
         println("The substring $sub is found in $input.")
         println("Its start position is $index.")
     }
 }
 displaySubstringPosition("010000011", "11")
 displaySubstringPosition("010000011", "12")

The Selection Of Scope Functions

To help we choose the right scope function for our purpose, we provide the table of key differences between them.

Function	Object reference	Return value	Is extension function
`let`	`it`	Lambda result	Yes
`run`	`this`	Lambda result	Yes
`run`	–	Lambda result	No: called without the context object
`with`	`this`	Lambda result	No: takes the context object as an argument.
`apply`	`this`	Context object	Yes
`also`	`it`	Context object	Yes

Key differences between Scope Function

Here is a short guidelines for choosing scope functions depending on the intended purpose:

Executing a lambda on non-null objects: let
Introducing an expression as a variable in local scope: let
Object configuration: apply
Object configuration and computing the result: run
Running statements where an expression is required: non-extension run
Additional effects: also
Grouping function calls on an object: with

The use cases of different scope functions overlap, so that we can choose the functions based on the specific conventions used in our project or team.

That’s all about in this article.

Conclusion

In this article, we understood about how to select Scope Functions in Kotlin. Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease our code readability and lead to errors. Avoid nesting scope functions and be careful when chaining them: it’s easy to get confused about the current context object and the value of this or it. This article showed the detailed descriptions of the differences between scope functions and the conventions on their usage in Kotlin with some authentic examples.

Thanks for reading! I hope you enjoyed and learned about Scope Functions concepts in Kotlin. Reading is one thing, but the only way to master it is to do it yourself.

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