Type Erasure Magic

SwiftTypeErasure.swift

// from Hector Matos speech Type Erasure Magic
//: https://realm.io/news/altconf-hector-matos-type-erasure-magic/

import UIKit

public struct Transaction {}

protocol CellReloadable: class {
    associatedtype DataType
    var selectedIndexPath: NSIndexPath? { get set }
    var loadingCellObjects: [NSIndexPath : DataType] { get set }
    
    func didFinishEditing(updated: Bool, newObject: DataType)
}

class AnyCellReloadable<ErasedDataType>: CellReloadable {
    var selectedIndexPath: NSIndexPath?
    var loadingCellObjects: [NSIndexPath : ErasedDataType] {
        get { return _getLoadingCellObjects() }
        set { _setLoadingCellObjects(newValue) }
    }
    
    private let _didFinishEditing: (Bool, ErasedDataType) -> Void
    private let _getLoadingCellObjects: (Void) -> [NSIndexPath : ErasedDataType]
    private let _setLoadingCellObjects: ([NSIndexPath : ErasedDataType]) -> Void
    
    init<Injected: CellReloadable>(_ reloadable: Injected)
        where Injected.DataType == ErasedDataType {
            
        selectedIndexPath = reloadable.selectedIndexPath
        
        _didFinishEditing = reloadable.didFinishEditing
        _getLoadingCellObjects = { reloadable.loadingCellObjects }
        _setLoadingCellObjects = { reloadable.loadingCellObjects = $0 }
    }
    
    func didFinishEditing(updated: Bool, newObject: ErasedDataType) {
        
    }
}

class TransactionViewController: UIViewController {
    var delegate: AnyCellReloadable<Transaction>?
    
    override func viewDidLoad() {
        
    }
    
    func buttonTapped() {
        delegate?.didFinishEditing(updated: true, newObject: Transaction())
        navigationController?.popViewController(animated: true)
    }
}