What is @escaping in Swift 4?

In Swift 1. x and Swift 2. x, closure parameter was @escaping by default, means that closure can be escape during the function body execution. if don’t want to escape closure parameters mark it as @nonescaping.

A non-escaping closure is a closure that is called within the function it is passed into. The execution of the closure will be before the function returns. An escaping closure is a closure that executes after the function it is passed into returns.

An autoclosure is a closure that’s automatically created to wrap an expression that’s being passed as an argument to a function. It doesn’t take any arguments, and when it’s called, it returns the value of the expression that’s wrapped inside of it.

A capture list is defined after the opening curly brace and before the parameter list and return type of the closure. An element of a capture list is the weak or unowned keyword followed by the reference to a class instance. In this example, we use a capture list to weakly capture self in the closure.

In Swift, we need to use weak self and unowned self to give ARC the required information between relationships in our code. Without using weak or unowned you’re basically telling ARC that a certain “strong reference” is needed and you’re preventing the reference count from going to zero.

If the last parameter to a function is a closure, Swift lets you use special syntax called trailing closure syntax. Rather than pass in your closure as a parameter, you pass it directly after the function inside braces. … Trailing closure syntax is extremely common in Swift, so it’s worth getting used to.

An escaping closure is a closure that’s called after the function it was passed to returns. In other words, it outlives the function it was passed to. A non-escaping closure is a closure that’s called within the function it was passed into, i.e. before it returns.

Non-escaping closures have a very clear lifecycle and have become the default closure type in Swift 3 because of it. … The closure cannot return or finish executing after the body of the calling function has returned.

Swift lets us specify a capture list to determine how values used inside the closure should be captured. The most common alternative to strong capturing is called weak capturing, and it changes two things: Weakly captured values aren’t kept alive by the closure, so they might be destroyed and be set to nil .

Swift’s @autoclosure attribute enables you to define an argument that automatically gets wrapped in a closure. It’s primarily used to defer execution of a (potentially expensive) expression to when it’s actually needed, rather than doing it directly when the argument is passed.

A weak reference is just a pointer to an object that doesn’t protect the object from being deallocated by ARC. While strong references increase the retain count of an object by 1, weak references do not. … In Swift, all weak references are non-constant Optionals (think var vs.

Weak References. A weak reference is a reference that doesn’t keep a strong hold on the instance it refers to, and so doesn’t stop ARC from disposing of the referenced instance. This behavior prevents the reference from becoming part of a strong reference cycle.

We can solve this in two ways. First, we can use [unowned self]: Now the closure doesn’t have a strong reference anymore. Just be careful when using [unowned self] since that, if the object has already been deallocated when the closure is called, a crash will occur.

A lazy var is a property whose initial value is not calculated until the first time it’s called. … A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.

$0 and $1 are Closure’s first and second shorthand arguments (a.k.a. Shorthand Argument Names or SAN for short). The shorthand argument names are automatically provided by Swift. The first argument can be referenced by $0 , the second argument can be referenced by $1 , the third one by $2 , and so on.

Swift guard is defined as a statement that is used to transfer program control out of a scope if one or more conditions aren’t met. What it means is that it is essentially a redirection or early exit of a statement or function to prevent crashing and incorrect data.

Extensions add new functionality to an existing class, structure, enumeration, or protocol type. This includes the ability to extend types for which you don’t have access to the original source code (known as retroactive modeling). Extensions are similar to categories in Objective-C.

Closures are useful because they let you associate data (the lexical environment) with a function that operates on that data. This has obvious parallels to object-oriented programming, where objects allow you to associate data (the object’s properties) with one or more methods.

Trailing Closures A trailing closure is written after the function call’s parentheses, even though it is still an argument to the function. When you use the trailing closure syntax, you don’t write the argument label for the closure as part of the function call.

Swift 2 introduces the defer keyword, which effectively means “here’s some what I want you to do later, no matter what.” That work can be whatever you want: a single method call closing a file, or 50 lines of code doing some other important clean up work.

A completion handler in Swift is a function that calls back when a task completes. This is why it is also called a callback function. A callback function is passed as an argument into another function. When this function completes running a task, it executes the callback function.

in other words, “guard let” is used when the code is 99% sure of not using the else conditional; in the other hand, “if let” when the code is 50 – 50(example) to use else condition. The variable bound by if let is only visible inside if let scope. The variable bound by guard let is visible afterwards.

Closures can capture and store references to any constants and variables from the context in which they are defined. This is known as closing over those constants and variables, hence the name “closures”. Swift handles all of the memory management of capturing for you.

The Basics. In Swift, structs, enums and tuples are all value types, while classes and closures are reference types. … In practice, this means that when a value type is assigned to a variable or passed as a parameter to a function, it is copied, creating a new and unique instance of its data.

Difference between Function and Closure Function is declared using func keyword whereas Closure doesn’t have func keyword. Function has always name but Closure doesn’t have. Function doesn’t have in keyword but closure has in the keyword.

In Swift self is a special property of an instance that holds the instance itself. Most of the times self appears in an initializer or method of a class, structure or enumeration. … Or accessing the property from a method brings sufficient context to omit self from self.

The first rule to avoid retain cycles is that an object must never retain its parent. This changes the previous diagram to the following: This is the easy case: an object should never retain its parent when it makes a pointer to the parent. Notice that the term “weak pointer” is used.

A memory leak in iOS is when an amount of allocated space in memory cannot be deallocated due to retain cycles. Since Swift uses Automatic Reference Counting (ARC), a retain cycle occurs when two or more objects hold strong references to each other.

S.No.SWIFTOBJECTIVE C05.Swift is static type.Objective C is dynamic type.

polymorphism is about provisioning a single interface to entities of different types. Although it is usually underestimated, it is one of the most powerful concepts in software engineering that lets you define different behaviors for different objects while you are still using a shared interface among them.

The main difference between weak and unowned is that weak is optional while unowned is non-optional. By declaring it weak you get to handle the case that it might be nil inside the closure at some point. If you try to access an unowned variable that happens to be nil, it will crash the whole program.

Multithreading can be defined as the process which facilitates the CPU to create and execute concurrent threads. Typically, a CPU performs one operation at a time.

strong is the default. An object remains “alive” as long as there is a strong pointer to it. weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.

The key difference between a strong and a weak or unowned reference is that a strong reference prevents the class instance it points to from being deallocated. … In other words, weak and unowned references cannot prevent a class instance from being deallocated.

An Optional is a type on its own, actually one of Swift 4’s new super-powered enums. It has two possible values, None and Some(T), where T is an associated value of the correct data type available in Swift 4. … Optionals are similar to using nil with pointers in Objective-C, but they work for any type, not just classes.

The retainCount is the number of ownership claims there are outstanding on the object. You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message.

Setting things to nil is the easiest way to clean up memory. This goes hand in hand with ARC. Once you have set all references of a Class to nil it will deinit and free up memory.