Discuss how closure capturing and capture list works

closure.swift

import Foundation

// struct for printing out instance address
struct MemoryAddress<T>: CustomStringConvertible {

    let intValue: Int
    var description: String {
        let length = 2 + 2 * MemoryLayout<UnsafeRawPointer>.size
        return String(format: "%0\(length)p", intValue)
    }
    // for struct
    init(of structPointer: UnsafePointer<T>) {
        intValue = Int(bitPattern: structPointer)
    }
}

extension MemoryAddress where T: AnyObject {
    // for class
    init(of classInstance: T) {
        intValue = unsafeBitCast(classInstance, to: Int.self)
    }
}

/// struct example
var a = 1, b = 1

print("address of a: \(MemoryAddress(of: &a))")
print("address of b: \(MemoryAddress(of: &b))")
var captureList = { [a] in
    print("[In closure] address of b: \(MemoryAddress(of: &b))")
    print("[In closure] a: \(a), b: \(b)")
}

let strongRef = {
    a += 1
    b += 1
}

strongRef()

print("a: \(a), b: \(b)")
captureList()

// class example
print("============ Reference ============")
class aClass{
    var value = 1
}

var c1 = aClass()
var c2 = aClass()

var copyReference = { [c1] in // copy a reference to c1 instance
    print("[In copy clsoure] c1 address : \(MemoryAddress(of: c1))")
    print("[In copy clsoure] c2 address : \(MemoryAddress(of: c2))")
}

var weakCast = { [weak c1] in
    print("[In clsoure] c1 address : \(MemoryAddress(of: c1!))")
    print("[In clsoure] c2 address : \(MemoryAddress(of: c2))")
    c1?.value += 1
}

print(" c1 address : \(MemoryAddress(of: c1))")
print(" c2 address : \(MemoryAddress(of: c2))")
copyReference()
weakCast()

print("c1 retain count : \(CFGetRetainCount(c1))") // c1 has one more retain count than c2
print("c2 retain count : \(CFGetRetainCount(c2))")