Swift closure capture array by reference - swift

Question

In Swift Collections are pass by value by default and we can user inout to make it pass by reference in function arguments but how can we do it in closure capture variables?
var list = [1, 2, 3]
func edit(inout list: [Int]) {
list.append(4)
dispatch_async(dispatch_get_main_queue()) {
list.append(5)
}
}
edit(&list)
...// after dispatch_async was executed
NSLog("\(list)")
Result will be [1, 2, 3, 4]
How can I modify the original variable () inside closure?
UPDATE:
Actually I have a workaround to handle this case by putting the array into an object so I can pass this object to the function by reference and we can modify the same array instance inside the function. but I want see any clever way to archive that

Answer 1

For getting a variable escape from a closure you need #escaping, check this out. A workaround is to put a completion function as an argument rather than an inout variable.
class MyClass {
static func edit(_ list: [Int], _ completion: #escaping ([Int]) -> ()) {
var list = list
list.append(4)
DispatchQueue.main.async() {
list.append(5)
completion(list)
}
}
}
var myList = [1, 2, 3]
MyClass.edit(myList) { (list) in
myList = list
print("My list after editing: \(myList)")
}
print("My list without editing: \(myList)")
Note: The above example is for Swift 3 where inout parameters are not allowed to be captured in closures. Based on your post, you may be using a lower version of Swift where you might be able to use inout rather than setting a mutable copy of the list: var list = list. However, the logic is very similar.
For more information, check the Limiting inout capture to #noescape contexts in Swift evolution:
Swift's behavior when closures capture inout parameters and escape
their enclosing context is a common source of confusion. We should
disallow implicit capture of inout parameters except in #noescape
closures.