Swift Q & A


What is a closure capture list in Swift?

In Swift, a closure capture list is a mechanism that allows you to control how values from the surrounding context are captured and retained by a closure. Closures in Swift capture and store references to values and objects from their surrounding context, which can lead to issues like retain cycles (memory leaks) if not managed properly. The capture list provides a way to specify which variables and constants a closure should capture, and how they should be captured, to avoid such issues.


A capture list is defined within square brackets (`[]`) immediately before the closure’s parameter list and body. It consists of one or more capture specifiers, each specifying a variable or constant to capture and an optional capture mode:


Strong Capture (`[variable]`): By default, closures capture variables and constants strongly, meaning they keep these references alive for as long as the closure itself exists. This is the most common capture mode.


Weak Capture (`[weak variable]`): You can use `weak` to capture a variable or constant weakly, which means the closure holds a weak reference to it. If the referenced object is deallocated, the captured reference becomes `nil`.


Unowned Capture (`[unowned variable]`): Similar to weak, `unowned` captures a variable or constant, but it assumes that the referenced object won’t be deallocated before the closure is executed. If this assumption is not met and the object is deallocated, accessing the captured reference results in a runtime crash.


Capture lists are particularly useful when dealing with closures that are stored as properties in classes or when closures might outlive the context in which they are defined. They help prevent strong reference cycles and memory leaks by allowing you to control the lifetime of captured values within the closure. It’s a crucial tool in Swift for managing memory and avoiding unexpected behavior when working with closures.


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Experienced iOS Engineer with 7+ years mastering Swift. Created fintech solutions, enhanced biopharma apps, and transformed retail experiences.