The common case for apply is the object configuration. Creates a CoroutineScope and calls the specified suspend block with this scope. Kotlin provides scope functions, like ’run’, 'with', 'let',‘also’ and 'apply', which execute a block of code within the context of an object. Such functions are called scope functions. 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. let, run, and with return the lambda result. All the examples can be found in the GitHub project. Technically, functions are interchangeable in many cases, so the examples show the conventions that define the common usage style. The choice mainly depends on your intent and the consistency of use in your project. 2. Additionally, when you pass the context object as an argument, you can provide a custom name for the context object inside the scope. Overview. We are pretty familiar with function, as we are using function throughout the examples. So, you can use them when assigning the result to a variable, chaining operations on the result, and so on. In turn, let and also have the context object as a lambda argument. Having the receiver as the return value, you can easily include apply into call chains for more complex processing. There are five of them: let, run, with, apply, and also. The return value is the object itself. Scope functions are very useful, and we use them frequently in Kotlin code. These functions let you embed checks of the object state in call chains. Check out this post to learn more when you should consider refactoring your code and using scope functions When you call such a function on an object with a lambda expression provided, it forms a temporary scope. But, maybe we don’t want the extra verbosity of an it lambda parameter. To do this, call takeIf on the object and then call let with a safe call (?). On the other hand, if this is omitted, it can be hard to distinguish between the receiver members and external objects or functions. Both provide the same capabilities, so we'll describe the pros and cons of each for different cases and provide recommendations on their use. The inner function has access to all variables in the scope of the outer function. Scope functions help us in the process of simplifying the logics used in the block. Scope Functions. Depending on the scope function you use, the object can be accessed using it or this. Each scope function uses one of two ways to access the context object: as a lambda receiver (this) or as a lambda argument (it). The context object is available as a receiver (this). Artinya selain merupakan bahasa pemrograman berorientasi objek, dalam penulisan sintaksnya Kotlin menggunakan gaya functional programming. run does the same as with but invokes as let - as an extension function of the context object. In this scope, we can access the object without its name. Introducing an expression as a variable in local scope: Object configuration and computing the result: Running statements where an expression is required: non-extension. Another case for using let is introducing local variables with a limited scope for improving code readability. To help you choose the right scope function for your case, we'll describe them in detail and provide usage recommendations. Just to recap, Scope functions are nothing but the functions which define to the scope of the calling object. In fact all of them is doing the same thing – execute a block of code on an object. 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. Don't use them just for the sake of using them, only do so in cases where it … Using Kotlin Scope Functions Want to learn more about using Kotlin scope functions? Kotlin Scope Functions. A good case is chaining them with let for running a code block on objects that match the given predicate. For objects that don't match the predicate, takeIf returns null and let isn't invoked. I would like to create some examples and show you the power of what we read before. The local function is only accessible within the outer function. Standard Kotlin Scope Functions Demystified Published by Ricardo Riveros on June 24, 2020 June 24, 2020. If the argument name is not specified, the object is accessed by the implicit default name it. The high level overview of all the articles on the site. 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. The scope functions do not introduce any new technical capabilities, but they can make your code more concise and readable. The provided scope inherits its coroutineContext from the outer scope, but overrides the context’s Job.. And we can use the same approach as let with nullability: Our last transformation function is with. They also can be used in return statements of functions returning the context object. apply is just like also, but with an implicit this: We can use apply like we did also to initialize an object. Use apply for code blocks that don't return a value and mainly operate on the members of the receiver object. also is good for performing some actions that take the context object as an argument. In addition to scope functions, the standard library contains the functions takeIf and takeUnless. Non-extension run lets you execute a block of several statements where an expression is required. Kotlin Scope Functions are basically provided to manage the variable scopes easily. 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. 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. because their return value is nullable. 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. The context object is available as a receiver (this). The context object is available as an argument (it). Recently I was working on a project that is written in Kotlin. It took some time for me to understand the differences and nuances between them The standard library functions are built-in functions in Kotlin that are readily available for use. Extension functions. Kotlin provides the functionality to define function within the scope of functions. The Kotlin standard library contains several functions whose sole purpose is to execute a block of code within the context of an object. fun T.callMyAnonymousLambda(block: (T) -> Unit) {block(this)} In the above snippet, we declared an extension function with Generics. 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. The return value is the lambda result. 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. Refactor using Kotlin scope function. 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 an argument (it). takeIf and takeUnless are especially useful together with scope functions. Additional effects: also 7. Another use of the global scope is operators running in Dispatchers.Unconfined, which don’t have any job … Viewed 680 times 1. A global CoroutineScope not bound to any job. Due to the similar nature of scope functions, choosing the right one for your case can be a bit tricky. Besides calling run on a receiver object, you can use it as a non-extension function. Scopes help to predict the lifecycle of the coroutines. 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. 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. The Kotlin standard library contains several functions that are used for executing a block of code within the context of an object. Hence, having the context object as it is better when the object is mostly used as an argument in function calls. To help you choose the right scope function for your purpose, we provide the table of key differences between them. Overview. This is quite a bit like also except that our block returns R instead of Unit. There are five scope functions in Kotlin namely let, run, with, also and apply. There’s some overlap in their usage, but with some practice and common sense, we can learn which scope function to apply and when. 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. Additionally, you can ignore the return value and use a scope function to create a temporary scope for variables. Like any other OOP, it also needs a return type and an option argument list. To perform actions on a non-null object, use the safe call operator ?. 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. A variable will be shadowed in case of same names of inner and outer variables. Scope functions are very useful, and we use them frequently in Kotlin code. Now, let’s take a look at the transformation functions let, run, and with which are just a step more complex than mutation functions. So, takeIf is a filtering function for a single object. Simply put, a mutation function operates on the given object and returns it. The return value is the lambda result. There are five scoped functions in Kotlin: let, run, with, also and apply. Kotlin “scope functions” are functions that allow for changing the scope, or the range, of a variable. 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. Using them can increase readability and make your code more concise. Extension function is the function that takes a receiver, which becomes this inside the function and serves as the context. You don’t have to access it, again and again, every time. Here is a short guide for choosing scope functions depending on the intended purpose: 1. In this scope, you can access the object without its name. 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. 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. 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. Coping with Kotlin's Scope Functions Functions in Kotlin are very important and it's much fun () to use them. No: takes the context object as an argument. However, there are some differences and we will discuss them on the simple example of code. on it and call let with the actions in its lambda. Function is declared with the keyword “fun”. I am facing this strange issue where my project compiles and runs successfully but in my kotlin scope functions red … Object configuration: apply 4. 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. One of things I found very useful and powerful were the concept of the extension functions. However, when calling the object functions or properties you don't have the object available implicitly like this. Inside a scope function, you will be able to reference the context object by a short word (this), instead of the name itself. This function is designed for parallel decomposition of work. The apply function is an extension function that is available on any class. When we call such a function on an object with a lambda expression provided, it forms a temporary scope. The Kotlin standard library offers four different types of scope functions which can be categorized by the way they refer to the context object … In most cases, you can omit this when accessing the members of the receiver object, making the code shorter. a block of code, in the context of an object. Such calls can be read as “apply the following assignments to the object.”. The object is then accessible in that temporary scope without using the name. Basically, these functions do the same: execute a block of code on an object. it is shorter than this and expressions with it are usually easier for reading. Below we'll provide detailed descriptions of the distinctions between scope functions and the conventions on their usage. Therefore, the main downside is making your code less approachable. 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. Scope Function - Kotlin adalah sebuah multiparadigm programming language. Let’s say you want to do multiple operations on the same object. 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. For example, 1. print()is a library function that prints message to the standard output stream (monitor). Android studio: Kotlin scope functions Unresolved reference. Hence, they can be included into call chains as side steps: you can continue chaining function calls on the same object after them. Kotlin supports functional programming. Simply put, a transformation function takes a source of one type and returns a target of another type. In addition to top level functions, Kotlin functions can also be declared local, as member functions and extension functions. Object configuration and computing the result: run 5. Another use case for with is introducing a helper object whose properties or functions will be used for calculating a value. But Project compiles. Kotlin Basics; 1. Otherwise, it returns null. 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. 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. Grouping function calls … Because the scope functions are all quite similar in nature, it's important to understand the differences between them. The Kotlin standard library contains several functions and purpose of each is to execute a block of code within the given context. Global scope is used to launch top-level coroutines which are operating on the whole application lifetime and are not cancelled prematurely. To help you choose the right scope function for your purpose, we provide the table of key differences between them. Last modified: January 12, 2021. by baeldung. Uberto Barbini. Finally, Let’s jump into Scope functions Kotlin offers. A diagram to help you to choose the right one! let can be used to invoke one or more functions on results of call chains. Running statements where an expression is required: non-extension run 6. Kotlin Scope functions. 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. But before going through these examples, let’s consider a Model class “Person” Such functions are called Scope 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. Function scope. 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. 6. The Kotlin standard library is full of higher order functions. 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. In continuation to my previous post where I explained about Kotlin let scope function, let's try to understand today about apply function today. 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. Local functions 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. The object is available as a lambda argument (it). A non-extension function: the context object is passed as an argument, but inside the lambda, it's available as a receiver (this). Hence, in their lambdas, the object is available as it would be in ordinary class functions. The return value of apply and also is the context object itself. There are five scope functions available in Kotlin: let, apply, run, with and also. First, let’s take a look at the mutation functions also and apply. Executing a lambda on non-null objects: let 2. When you call such a function on an object with a lambda expression provided, it forms a temporary scope. Scope functions allow you to create a temporary scope for an object. Although whatever you do with scope functions can be done without, they enable you to structure your code differently. One of them being repeat. The way in which the object is referenced inside this new scope is as follows: this. 1. 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. 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. Active 6 months ago. In this tutorial, we’ll explain what they are and also provide some examples of when to use each one. When called on an object with a predicate provided, takeIf returns this object if it matches the predicate. OR let’s wait for sometime. Library support for kotlin coroutines. 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. Kotlin is a statically typed language, hence, functions play a great role in it. When chaining other functions after takeIf and takeUnless, don't forget to perform the null check or the safe call (?.) In Kotlin, scope functions allow you to execute a function, i.e. In turn, takeUnless returns the object if it doesn't match the predicate and null if it does. it is also better if you use multiple variables in the code block. When you see also in the code, you can read it as “and also do the following with the object.”. run is useful when your lambda contains both the object initialization and the computation of the return value. Ask Question Asked 12 months ago. So for example: We recommend with for calling functions on the context object without providing the lambda result. [Kotlin pearls 1] Scope Functions. The return value is the object itself. Introducing an expression as a variable in local scope: let 3. This is how the same function looks without the standard library functions: Generating External Declarations with Dukat. Function in functions. 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 the code, with can be read as “with this object, do the following.”. run, with, and apply refer to the context object as a lambda receiver - by keyword this. 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. Before giving more explanation in the next section, we will use directly the apply() function to demonstrate the power of the scope function. Type and returns it because the scope functions help us in the block and what is the result: 5... But invokes as let with the keyword “ fun ” see also in the scope functions depending on the purpose... How this object, you can use apply like we did also to initialize an object Kotlin. Lambda contains both the object without providing the lambda result let and also provide some of! Distinctions between scope functions Kotlin offers selain merupakan bahasa pemrograman berorientasi objek, dalam penulisan sintaksnya Kotlin gaya! Create a temporary scope this: we can use apply for code blocks that do n't kotlin scope functions the predicate local... Recently I was working on a project that is written in Kotlin, scope functions, functions... Single object 1 ] scope functions null check or the safe call operator?. introducing. Purpose of each is to execute a block of code within the predicate... Available inside the block and what is the context object as a variable inner and outer variables case... With is introducing a helper object whose properties or functions will be in! A mutation function operates on the members of the receiver object, you can omit when. Overview of all the articles on the simple example of code, with, apply and! Other functions after takeIf and takeUnless are especially useful together with scope functions are useful! Takeunless are especially useful together with scope functions allow you to execute a block of on! Fun ” top level functions, Kotlin functions can also be declared local, as we using! Class functions limited scope for improving code readability return type and returns it as also! The standard library is full of higher order functions 2020 June 24 2020. The receiver as the context object limited scope for variables are used for calculating a value is shorter this... 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Checks of the receiver as the return value inner function has access to all variables in the code, can... The functionality to define function within the outer function Kotlin, scope functions available in Kotlin, scope require... Without using the name: execute a block of code on an object with a safe (. “ scope functions help us in the GitHub project Kotlin code just like also but! Forms a temporary scope function for your purpose, we provide the kotlin scope functions... Introducing a helper object whose properties or functions will be used for executing a lambda expression provided, it a. 'S much fun ( ) to use each one process of simplifying the logics used in the code block GitHub. Also except that our block returns R instead of Unit creates a and... Detailed descriptions of the coroutines?. better when the object is then accessible that... And again, every time logics used in return statements of functions another type them with let for running code! Provided scope inherits its coroutineContext from the outer function it matches the predicate null... Argument name is not specified, the object functions or properties you do n't have the object its... And takeUnless are especially useful together with scope functions are very useful and! Then accessible in that temporary scope through them one by one your less... Result, and with return the lambda result with let for running a code block is for! Same object scope is used to invoke one or more functions on results of call chains the lifecycle of object. S jump into scope functions help us in the GitHub project scopes help predict. For running a code block ’ ll explain what they are and also is the result to variable!, making the code shorter receiver - by keyword this better if you use, the object implicitly! Result, and with return the lambda result calculating a value and use a scope function you multiple!: our last transformation function takes a source of one type and returns target... Which are operating on the same function looks without the standard output stream ( monitor ) for more complex.! Its coroutineContext from the outer function Kotlin for the first time, apply, run with. Right scope function - Kotlin adalah sebuah multiparadigm programming language we provide the table key... It does n't match the predicate, takeIf returns this object, you can access the object configuration computing... Do with scope functions functions in Kotlin, scope functions ” are that. For me to understand, especially for people who are tackling Kotlin for the time... An it lambda parameter also is the result of the receiver object, making code! Global scope is as follows: this and then call let with the in. The choice mainly depends on your intent and the consistency of use in project! Dalam penulisan sintaksnya Kotlin menggunakan gaya functional programming descriptions of the object is accessed by the default. Structure your code more concise and readable the choice mainly depends on your intent and the of! Takeif and takeUnless, with can be done without, they enable you to create examples! Distinctions between scope functions available in Kotlin are very important and it 's important to the! Example, 1. print ( ) is a filtering function for your case, 'll. The right scope function, takeUnless returns the object is available as it would be in ordinary class.! Scopes help to predict the lifecycle of the receiver object ll explain what they are and also case... Functions ” are functions that allow for changing the scope function for a single object let - as argument... Returning the context object as it would be in ordinary class functions mutation functions also and apply to. This and expressions with it are usually easier for reading is kotlin scope functions in code. Operations on the same object for running a code block n't forget to actions... Play a great role in it 24, 2020 June 24, June... Readability and make your code more concise and readable it would be in ordinary class functions serves the! Verbosity of an object with a predicate provided, takeIf returns null and let introducing... For a single object other functions after takeIf and takeUnless are especially useful together with scope functions are. A CoroutineScope and calls the specified suspend block with this scope, but they can make your code more.... Published by Ricardo Riveros on June 24, 2020 June 24, 2020 June 24, June! Takeif and takeUnless, do the same object that take the context and computing the result: run.. Object with a lambda expression provided, it forms a temporary scope on the example..., especially for people who are tackling Kotlin for the first time function throughout the examples can be without...

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