mutable-view.swift

final class Storage {
    
    // not empty for testing
    var keys: [String] = ["0"]
    var values: [Int] = [0] 
}

public struct Document {
    
    var _storageReference: Storage
    
    public init() {
        
        self._storageReference = Storage()
    }
    
    public init(_ values: DocumentValues) {
        
        self._storageReference = values._storageReference
    }
    
    public var values: DocumentValues {
        
        get { return DocumentValues(self) }
        
        set {
            print("mutable view setter")
            self = Document(newValue)
        }
    }
}

public struct DocumentValues : MutableCollection {
    
    let _storageReference: Storage
    
    init(_ document: Document) {
        
        self._storageReference = document._storageReference
    }
    
    public var startIndex: Int {
        
        return self._storageReference.keys.startIndex
    }
    
    public var endIndex: Int {
        
        return self._storageReference.keys.endIndex
    }
    
    public func index(after i: Int) -> Int {
        
        return self._storageReference.keys.index(after: i)
    }
    
    public subscript(position: Int) -> Int {
        
        get { return _storageReference.values[position] }
        
        set {
            print("subscript setter")
            self._storageReference.values[position] = newValue
        }
    }
}


// test

var document = Document()

let copy = document 

document.values[0] = 1 // prints only (subscript setter)

/*

Problem:

- The view should clone the reference on the parrent and itself.
- The view should mutate in-place the value.
- Copy should remain with old values.

*/