Demonstration of struct usage with threading

SafeAccess.swift

import Foundation

/// If T is a struct, then this class provides thread safety, because all reads and writes happen inside queue.
/// But, if T is very big, copying it may decrease performance.
class SafeAccess<T> {
    private let queue = DispatchQueue(label: "Safe")
    private var _value: T
    
    init(_ value: T) {
        _value = value
    }
    
    var value: T {
        get {
            var result: T!
            queue.sync {
                result = _value // entire struct will be copied
            }
            return result
        }
    }
    
    func modify(_ closure: (inout T) -> Void) {
        queue.sync {
            closure(&_value)
        }
    }
}


func example() {
    struct Foo {
        var x = 0
    }
    
    let access = SafeAccess(Foo())
    for _ in 0..<100 {
        DispatchQueue.global().async {
            let x = access.value.x
            let computedState1 = x + 1
            
            access.modify {
                let computedState2 = $0.x + 1
                $0.x = computedState2
                
                if computedState1 != computedState2  {
                    print("Potential inconsitency \(computedState1) != \(computedState2)")
                }
                
                print("Incremented to \(computedState2)")
            }
        }
    }
}