Unowned behaves similarly to weak, but both are not the same. Hence it can be nil and treated as on Optional. As the reference count might be zero when nothing holds a strong reference to this object, it might be de-allocated. You must be wondering about the ? (question mark) after self. This square bracket is called a capture list. Just after the opening of curly braces at the declaration of m圜losure, inside the square bracket, weak self () is written, which means self is captured as weak, followed by in keyword. In the below example, the self-captured as weak. As the retain count is not increased using weak, the object can be de-allocated by ARC. It doesn’t increase the retain count or reference count. Hence it’s reference count is increased by 1 and won’t get de-allocated causing a memory leak.Ī weak reference is just a pointer that points to an object.
In the above example, callFunction() referred strongly. As long as an object whose reference count or retain count is greater than 0, that object won’t get de-allocated by ARC. Capturing variable means that the referred object will not be non-allocated from memory because the count of the reference or the count of an object can be increased by 1. Let’s discuss the strong reference first. To avoid reference cycle, we need to be captured variables as weak or unowned. In this example Test class has a strong reference to m圜losure and m圜losure closure captures self strongly by calling callFunction(). The below example shows how a strong retain cycle is created using a closure. Since closures in Swift are a reference type, a retain cycle can create, especially when self-captured within it. Swift handles all of the memory management of capturing for you. It is known as closing over those constants and variables. Closures in Swift are similar to blocks in C and Objective-C and lambdas in other programming languages.Ī closure can capture and store references to any constants and variables from the context in which they defined. ClosuresĬlosures are self-contained blocks of functionality that can be passed around and used in your code. In most cases, Swift memory management works automatically. It uses the concept called Automatic Reference Counting (or simply ARC) which allows it to manage the memory. Since a weak reference may be nil, the variable captured becomes an optional.Swift is a modern, high-level programming language which automatically manages much of the memory of the iOS applications, which means it allocates and de-allocates memory on its own. This behaviors prevents the reference from becoming part of a strong reference cycle. We can break this strong reference cycle using self in the capture list with either a weak or an unowned reference:Ī weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. In this way, we are creating a strong reference cycle, since closure and self are keeping a strong reference of each other.
Then, in setupClosure, closure keeps an additional strong reference of self to use its obj property.
In this example, handler keeps a strong reference of its properties closure and obj. The capturing is transparent to us, we can use these variables without any efforts like in the following example:Ĭlass M圜lass let handler = Handler () handler. We are able to use these variables capturing them inside the closure. When we use a closure, most of the time, we need to use variables from the surrounding context-like class properties or local variables. Happy Reading! Default Variable Capturing You don’t have to be a Guru of Swift closures to understand this article, but I suggest you a good understanding of what’s a strong and weak reference before starting reading this article, otherwise something may be confusing since I don’t explain thoroughly how an object reference works in Swift.
In this article, we’ll see mainly two things: how a closure captures a variable and a specific scenario where the closure captures the self context.
#Swiftcapture how to#
Let’s find out how a closure captures a variable and how to prevent weird behaviours. We use closures pretty much every day, and, some times, it may be not very clear what happens and how a closure works.