Proposal: Data Handover Mechanism among UIViewControllers

Readme.md

Proposal for Data handover accross ViewControllers

Passing dependencies across controllers

What

This mechanism gives us a framework (and discipline) to pass data from one conroller to another.

How does it work

Protocols

Every controller conforms to DataReciever protocol which means that it will have a property model that can be set from the caller scope. Although, the controllers will not have to define a property again and again ‽, they still have to implement the prepare(with:) methhod from the protocol

‽ - The property is injected into the View controllers at the runtime using Associated Objects.

Sample

Please refer SampleVC.swift

xAssociatedObjects.swift

Ignore the x in the file name this was a conspiracy to keep readme.md on the top of the gist

SampleViewController.swift

//
//  HelloVC.swift
//  Research
//
//  Created by Gaurav Keshre on 23/11/17.
//  Copyright © 2017 Gaurav. All rights reserved.
//

import UIKit

class HelloVC: UIViewController, DataReceiver {
  
    
    typealias ModelType = String

    @IBOutlet private weak var lblName : UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    //MARK:- DataReceiver
    //--------------------------
    func prepare(with model: String?) {
        lblName.text = model
    }
}

UIViewController+DaHo.swift

//
//  UIViewController+DaHo.swift
//  Research
//
//  Created by Gaurav Keshre on 29/11/17.
//  Copyright © 2017 Gaurav. All rights reserved.
//

import Foundation
import UIKit

fileprivate var ModelTypeAssociationKey: UInt = 0

public protocol DataReceiver: class {
    
    associatedtype ModelType
    
    var model: ModelType? { get set }
    
    func prepare(with model: ModelType?)

}

extension DataReceiver where Self: UIViewController {
    var model: ModelType? {
        get {
            return ObjCAssociation.get(associatedObject: self, key: &ModelTypeAssociationKey)
        }
        
        set(newModel) {
            guard let model = newModel else{
                ObjCAssociation.remove(associatedObject: self, key: &ModelTypeAssociationKey)
                return
            }
            ObjCAssociation.set(associatedObject: self, value: model, key: &ModelTypeAssociationKey)
        }
    }
}

xAssociatedObjects.swift

//
//  AssociatedObjects.swift
//  Research
//
//  Created by Gaurav Keshre on 29/11/17.
//  Copyright © 2017 Gaurav. All rights reserved.
//

import Foundation
private var AssociagedObjKey: UInt = 0


class ObjCAssociation {
    private final class AssociatedObjWrapper<T> {
        let value: T
        init(_ val: T) {
            self.value = val
        }
    }
    static func set<T>(associatedObject object: Any, value: T, key: UnsafeRawPointer,
                       policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN_NONATOMIC) {
        let finalValue: Any
        if type(of: value) is AnyClass {
            finalValue = value
        }else {
            /// Only classes can be saved as associated objects
            finalValue = AssociatedObjWrapper(value)
        }
        objc_setAssociatedObject(object, key, finalValue, policy)
    }
    
    static func get<T>(associatedObject object: Any, key: UnsafeRawPointer) -> T? {
        let fetchedValue = objc_getAssociatedObject(object, key)
        if let v = fetchedValue as? T {
            return v
        }else if let v = fetchedValue as? AssociatedObjWrapper<T> {
            return v.value
        }
        return nil
    }
    
    static func remove(associatedObject object: Any, key: UnsafeRawPointer){
        objc_setAssociatedObject(object, key, nil, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }
}