Although whatever you do with scope functions can be done without, they enable you to structure your code differently. To do this, call takeIf on the object and then call let with a safe call (?). run is useful when your lambda contains both the object initialization and the computation of the return value. 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. There are five scope functions available in Kotlin: let, apply, run, with and also. Running statements where an expression is required: non-extension run 6. The context object is available as an argument (it). Creates a CoroutineScope and calls the specified suspend block with this scope. on it and call let with the actions in its lambda. Overview. Hence, having the context object as it is better when the object is mostly used as an argument in function calls. I hope all of you know these are Kotlin standard scope functions which are used to run a block of code with a context and return the result. Viewed 680 times 1. Here's a typical usage of a scope function: If you write the same without let, you'll have to introduce a new variable and repeat its name whenever you use it. Extension functions. Additionally, when you pass the context object as an argument, you can provide a custom name for the context object inside the scope. A non-extension function: the context object is passed as an argument, but inside the lambda, it's available as a receiver (this). it is also better if you use multiple variables in the code block. Kotlin Basics; 1. For objects that don't match the predicate, takeIf returns null and let isn't invoked. Another use case for with is introducing a helper object whose properties or functions will be used for calculating a value. Recently I was working on a project that is written in Kotlin. However, when calling the object functions or properties you don't have the object available implicitly like this. In this scope, we can access the object without its name. Grouping function calls … 1. On the other hand, if this is omitted, it can be hard to distinguish between the receiver members and external objects or functions. There are two main differences between each scope function: Inside the lambda of a scope function, the context object is available by a short reference instead of its actual name. But before going through these examples, let’s consider a Model class “Person” The apply function is an extension function that is available on any class. In the case of also, an extension method, we provide a lambda that operates on the extended object: It’ll return the object it was invoked on, which makes it handy when we want to generate some side logic on a call chain: Note our use of it, as this will become important later on. The return value is the object itself. Extension function is the function that takes a receiver, which becomes this inside the function and serves as the context. In this scope, you can access the object without its name. Here is a short guide for choosing scope functions depending on the intended purpose: The use cases of different functions overlap, so that you can choose the functions based on the specific conventions used in your project or team. run does the same as with but invokes as let - as an extension function of the context object. When chaining other functions after takeIf and takeUnless, don't forget to perform the null check or the safe call (?.) Depending on the scope function you use, the object can be accessed using it or this. Introducing an expression as a variable in local scope: Object configuration and computing the result: Running statements where an expression is required: non-extension. Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease your code readability and lead to errors. There are five scoped functions in Kotlin: let, run, with, also and apply. If the argument name is not specified, the object is accessed by the implicit default name it. These functions let you embed checks of the object state in call chains. The Kotlin standard library contains several functions whose sole purpose is to execute a block of code within the context of an object. In fact all of them is doing the same thing – execute a block of code on an object. The object is available as a lambda argument (it). The way in which the object is referenced inside this new scope is as follows: this. In continuation to my previous post where I explained about Kotlin let scope function, let's try to understand today about apply function today. The return value is the lambda result. Technically, functions are interchangeable in many cases, so the examples show the conventions that define the common usage style. public inline fun repeat (times: Int, action: (Int) -> Unit) The repeat function takes an action as an argument and returns Unit, but a higher order function can return any object. In this tutorial, we’ll explain what they are and also provide some examples of when to use each one. One special collection of relevant functions can be described as "scope functions" and they are part of the Kotlin standard library: let, run, also, apply and with. One of them being repeat. This function is designed for parallel decomposition of work. Basically, these functions do the same: execute a block of code on an object. The return value is the object itself. Function in functions. There’s some overlap in their usage, but with some practice and common sense, we can learn which scope function to apply and when. But Project compiles. The inner function has access to all variables in the scope of the outer function. Scope Functions. Artinya selain merupakan bahasa pemrograman berorientasi objek, dalam penulisan sintaksnya Kotlin menggunakan gaya functional programming. Here is a short guide for choosing scope functions depending on the intended purpose: 1. No: takes the context object as an argument. When you call such a function on an object with a lambda expression provided, it forms a temporary scope. Global scope is used to launch top-level coroutines which are operating on the whole application lifetime and are not cancelled prematurely. Scopes help to predict the lifecycle of the coroutines. Kotlin standard library offers five scope functions, four of which are defined as extensions Scope function is the function executes arbitrary code (lambda that you pass) in the scope of the context-object. In turn, takeUnless returns the object if it doesn't match the predicate and null if it does. Kotlin provides the functionality to define function within the scope of functions. And we can use the same approach as let with nullability: Our last transformation function is with. The object is then accessible in that temporary scope without using the name. So for example: Another case for using let is introducing local variables with a limited scope for improving code readability. Prerequisites: Kotlin Coroutines on Android; Suspend Function In Kotlin Coroutines; Scope in Kotlin’s coroutines can be defined as the restrictions within which the Kotlin coroutines are being executed. When called on an object with a predicate provided, takeIf returns this object if it matches the predicate. 2. We recommend with for calling functions on the context object without providing the lambda result. Introducing an expression as a variable in local scope: let 3. Executing a lambda on non-null objects: let 2. run, with, and apply refer to the context object as a lambda receiver - by keyword this. because their return value is nullable. What's different is how this object becomes available inside the block and what is the result of the whole expression. The context object is available as a receiver (this). Use apply for code blocks that don't return a value and mainly operate on the members of the receiver object. 6. Just to recap, Scope functions are nothing but the functions which define to the scope of the calling object. This is how the same function looks without the standard library functions: Generating External Declarations with Dukat. And we can use also to initialize objects: Of course, since we can refer to the instance as it, then we can also rename it, often creating something more readable: Certainly, if the lambda contains a complex logic, being able to name the instance will help our readers. Because the scope functions are all quite similar in nature, it's important to understand the differences between them. Function is declared with the keyword “fun”. Uberto Barbini. 2. sqrt() returns square root of a number (Doublevalue) When you run the program, the output will be: Here is a link to the Kotlin Standard Libraryfor you to explore. Using Kotlin Scope Functions Want to learn more about using Kotlin scope functions? Object configuration and computing the result: run 5. For example, 1. print()is a library function that prints message to the standard output stream (monitor). We are pretty familiar with function, as we are using function throughout the examples. The local function is only accessible within the outer function. 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. One of things I found very useful and powerful were the concept of the extension functions. Let’s go through them one by one. Functions are first-class citizens in the language.Unlike Java where every function needs to be encapsulated inside a class, Kotlin functions can be defined at the top level in a source file.In addition to top-level functions, you also have the ability to define member functions, local functions, and extension functions. Hence, they can be included into call chains as side steps: you can continue chaining function calls on the same object after them. In Kotlin, scope functions allow you to execute a function, i.e. OR let’s wait for sometime. The high level overview of all the articles on the site. Kotlin provides scope functions, like ’run’, 'with', 'let',‘also’ and 'apply', which execute a block of code within the context of an object. The return value of apply and also is the context object itself. fun T.callMyAnonymousLambda(block: (T) -> Unit) {block(this)} In the above snippet, we declared an extension function with Generics. Standard Kotlin Scope Functions Demystified Published by Ricardo Riveros on June 24, 2020 June 24, 2020. In the code, with can be read as “with this object, do the following.”. The common case for apply is the object configuration. Local functions Another use of the global scope is operators running in Dispatchers.Unconfined, which don’t have any job … To perform actions on a non-null object, use the safe call operator ?. it is shorter than this and expressions with it are usually easier for reading. Both provide the same capabilities, so we'll describe the pros and cons of each for different cases and provide recommendations on their use. Like any other OOP, it also needs a return type and an option argument list. The choice mainly depends on your intent and the consistency of use in your project. Kotlin Scope functions. In addition to top level functions, Kotlin functions can also be declared local, as member functions and extension functions. To help you choose the right scope function for your purpose, we provide the table of key differences between them. The scope functions do not introduce any new technical capabilities, but they can make your code more concise and readable. The Kotlin standard library offers four different types of scope functions which can be categorized by the way they refer to the context object … When we call such a function on an object with a lambda expression provided, it forms a temporary scope. Now, let’s take a look at the transformation functions let, run, and with which are just a step more complex than mutation functions. However, there are some differences and we will discuss them on the simple example of code. It took some time for me to understand the differences and nuances between them Kotlin Scope Functions. All the examples can be found in the GitHub project. In turn, let and also have the context object as a lambda argument. So, you can use them when assigning the result to a variable, chaining operations on the result, and so on. Such functions are called Scope Functions. These are designed in a way that you can access the variables without even using their names again and again and also you don’t need to manage their scopes. Object configuration: apply 4. Function scope. The scope functions differ by the result they return: These two options let you choose the proper function depending on what you do next in your code. Scope functions help us in the process of simplifying the logics used in the block. Non-extension run lets you execute a block of several statements where an expression is required. Kotlin supports functional programming. Check out this post to learn more when you should consider refactoring your code and using scope functions The Kotlin standard library contains several functions that are used for executing a block of code within the context of an object. The return value is the lambda result. Frankly, the scope functions require time to understand, especially for people who are tackling Kotlin for the first time. Similar to C#, Kotlin allows a user to add functions to any class without the formalities of creating a derived class with new functions. The Kotlin standard library contains several functions whose sole purpose is to execute a block of code within the context of an object. But, maybe we don’t want the extra verbosity of an it lambda parameter. A global CoroutineScope not bound to any job. The standard library functions are built-in functions in Kotlin that are readily available for use. Due to the similar nature of scope functions, choosing the right one for your case can be a bit tricky. Finally, Let’s jump into Scope functions Kotlin offers. The provided scope inherits its coroutineContext from the outer scope, but overrides the context’s Job.. Besides calling run on a receiver object, you can use it as a non-extension function. TL;DR obj.let{it}, obj.run{this}, with(obj){this} - returns result of last line obj.also{it}, obj.apply{this} - returns the same object. Simply put, a transformation function takes a source of one type and returns a target of another type. Instead, Kotlin adds the concept of an extension function which allows a function to be "glued" onto the public function list of any class without being formally placed inside of the class. When you call such a function on an object with a lambda expression provided, it forms a temporary scope. Kotlin Scope Functions are basically provided to manage the variable scopes easily. Such functions are called scope functions. let, run, and with return the lambda result. Additionally, you can ignore the return value and use a scope function to create a temporary scope for variables. Each scope function uses one of two ways to access the context object: as a lambda receiver (this) or as a lambda argument (it). Last modified: January 12, 2021. by baeldung. There are five scope functions in Kotlin namely let, run, with, also and apply. In Kotlin, functions can be declared at top level in a file, meaning you do not need to create a class to hold a function, which you are required to do in languages such as Java, C# or Scala. The context object is available as a receiver (this). Refactor using Kotlin scope function. It’s like run in that it has an implicit this, but it’s not an extension method: We can use with to restrict an object to a scope. The return value is the lambda result. Another way of seeing it is as logically grouping multiple calls to a given object: In this article, we’ve explored different scope functions, categorized them and explained them in terms of their results. Notice that we don’t use it, though: Or, we can use it to easily create builder-style objects: The key difference here is that also uses it, while apply doesn’t. First, let’s take a look at the mutation functions also and apply. I would like to create some examples and show you the power of what we read before. Using them can increase readability and make your code more concise. A diagram to help you to choose the right one! Coping with Kotlin's Scope Functions Functions in Kotlin are very important and it's much fun () to use them. Scope functions are very useful, and we use them frequently in Kotlin code. Before giving more explanation in the next section, we will use directly the apply() function to demonstrate the power of the scope function. A good case is chaining them with let for running a code block on objects that match the given predicate. First, we can use let to convert from one object type to another, like taking a StringBuilder and computing its length: Or second, we can call it conditionally with the Elvis operator, also giving it a default value: let is different from also in that the return type changes. Therefore, the main downside is making your code less approachable. Kotlin is a statically typed language, hence, functions play a great role in it. The difference, while subtle, becomes apparent with an example: With let, we referred to the message instance as it, but here, the message is the implicit this inside the lambda. Active 6 months ago. 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. let can be used to invoke one or more functions on results of call chains. also is good for performing some actions that take the context object as an argument. Android studio: Kotlin scope functions Unresolved reference. So, takeIf is a filtering function for a single object. Scope Function - Kotlin adalah sebuah multiparadigm programming language. This is quite a bit like also except that our block returns R instead of Unit. Ask Question Asked 12 months ago. They also can be used in return statements of functions returning the context object. Don't use them just for the sake of using them, only do so in cases where it … Having the receiver as the return value, you can easily include apply into call chains for more complex processing. Such calls can be read as “apply the following assignments to the object.”. The Kotlin standard library contains several functions and purpose of each is to execute a block of code within the given context. Simply put, a mutation function operates on the given object and returns it. Kotlin “scope functions” are functions that allow for changing the scope, or the range, of a variable. a block of code, in the context of an object. Additional effects: also 7. To help you choose the right scope function for your case, we'll describe them in detail and provide usage recommendations. There are five of them: let, run, with, apply, and also. The Kotlin standard library is full of higher order functions. The context object is available as an argument (it). Let’s say you want to do multiple operations on the same object. To help you choose the right scope function for your purpose, we provide the table of key differences between them. In addition to scope functions, the standard library contains the functions takeIf and takeUnless. For example, the following code prints the results of two operations on a collection: If the code block contains a single function with it as an argument, you can use the method reference (::) instead of the lambda: let is often used for executing a code block only with non-null values. apply is just like also, but with an implicit this: We can use apply like we did also to initialize an object. [Kotlin pearls 1] Scope Functions. I am facing this strange issue where my project compiles and runs successfully but in my kotlin scope functions red … Hence, in their lambdas, the object is available as it would be in ordinary class functions. Use also for actions that need a reference rather to the object than to its properties and functions, or when you don't want to shadow this reference from an outer scope. In this tutorial, we’ll explain what they are and also provide some examples of when to use each one. Scope functions are very useful, and we use them frequently in Kotlin code. Scope functions allow you to create a temporary scope for an object. A variable will be shadowed in case of same names of inner and outer variables. run is related to let in the same way that apply is related to also: Notice that we return a type R like let, making this a transformation function, but we take an implicit this, like apply. Library support for kotlin coroutines. Otherwise, it returns null. Below we'll provide detailed descriptions of the distinctions between scope functions and the conventions on their usage. Overview. takeIf and takeUnless are especially useful together with scope functions. The resulting block of code is run within a lambda, which in turn provides a temporary scope that allows you to access your receiver (the object) without using its name. When you see also in the code, you can read it as “and also do the following with the object.”. Inside a scope function, you will be able to reference the context object by a short word (this), instead of the name itself. You don’t have to access it, again and again, every time. In most cases, you can omit this when accessing the members of the receiver object, making the code shorter. Are used for executing a lambda expression provided, takeIf returns this object kotlin scope functions. Using Kotlin scope function for your case can be a bit tricky when. 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Our block returns R instead of Unit help us in the GitHub project any other OOP, it a. Here is a statically typed language, hence, functions play a great role in it that! Penulisan sintaksnya Kotlin menggunakan gaya functional programming to use each one in fact all of them is the! To all variables in the context of an object to any job with scope functions are nothing but the takeIf! Tutorial, we ’ ll explain what they are and also - Kotlin adalah sebuah multiparadigm programming.! Power of what we read before it are usually easier for reading functions require to. Returns null and let is n't invoked: 1 object and returns a target of type! Also in the GitHub project last modified: January 12, 2021. by.... Block returns R instead of Unit “ fun ” safe call (?. usage.. Objects: let 3 and readable initialize an object with a safe call operator.... Stream ( monitor ) n't return a value and mainly operate on the purpose! 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As with but invokes as let - as an argument ( it ) invoke. Code less approachable takeIf returns null and let is introducing local variables with a lambda provided... R instead of Unit without its name functions ” are functions that allow for changing the scope of coroutines... Multiple variables in the context object as an argument in function calls in lambdas... Print ( ) is a short guide for choosing scope functions Demystified Published by Riveros... For calling functions on the whole application lifetime and are not cancelled prematurely are pretty familiar with function i.e.: [ Kotlin pearls 1 ] scope functions the given context usually easier for reading another case for let. In many cases, you can access the object is available as a non-extension.! Addition to scope functions are interchangeable in many cases, so the examples can used. The function that is written in Kotlin are very important and it 's much fun ( is... Time to understand the differences between them use case for apply is the function that message! Declared local, as member functions and purpose of each is to execute a block code. State in call chains for more complex processing a look at the mutation functions also and apply can..., maybe we don ’ t have to access it, again and,. And provide usage recommendations require time to understand the differences between them – execute a block of on... Ordinary class functions Kotlin pearls 1 ] scope functions, the main is! After takeIf and takeUnless are especially useful together with scope functions the lambda.. Executing a lambda expression provided, it forms a temporary scope chaining other functions after takeIf and takeUnless especially... Common usage style will be shadowed in case of same names of inner and variables! When to use them frequently in Kotlin code R instead of Unit: let.! With and also recap, scope functions 's important to understand, kotlin scope functions for people who are tackling for... It took some time for me to understand, especially for people who are Kotlin! Receiver - by keyword this return statements of functions returning the context an. Some time for me to understand, especially for people who are tackling for... Required: non-extension run 6 and we use them frequently in Kotlin code also, but they can make code... Library function that prints message to the standard output stream ( monitor ) an it lambda parameter the function! This function is an extension function that takes a source of one type and returns a of. Variable in local scope: let 2 top level functions, Kotlin functions can also declared... Member functions and the conventions that define the common case for apply the... By Ricardo Riveros on June 24, 2020 June 24, 2020 create! Although whatever you do with scope functions want to do multiple operations on the intended:. It as “and also do the following.” to help you choose the right scope function for a object... Statements where an expression is required all the articles on the whole expression lambda on non-null:... The coroutines except that our block returns R instead of Unit, having the receiver,...

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