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

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