NibLoader.swift

//
//  NibLoader.swift
//  NibLoader
//
//  Created by Domas on 10/02/2017.
//  Copyright © 2016 Trafi. All rights reserved.
//

import UIKit

// MARK: - Public

public extension NibLoadable where Self: UIView {
    var nibLoader: IBNibLoader<Self> { return IBNibLoader(self) }
}

public struct IBNibLoader<NibLoadableView: NibLoadable> where NibLoadableView: UIView {
    
    public func awakeAfter(using aDecoder: NSCoder, _ superMethod: @autoclosure () -> Any?) -> Any? {
        guard nonPrivateSubviews.isEmpty else { return superMethod() }
        
        let nibView = type(of: view).fromNib()
        copyProperties(to: nibView)
        copyConstraints(to: nibView)
        
        return nibView
    }
    
    public func initWithFrame() {
        let nibView = type(of: view).fromNib()
        copyProperties(to: nibView)
        SubviewsCopier.copySubviewReferences(from: nibView, to: view)
        
        nibView.frame = view.bounds
        nibView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.addSubview(nibView)
    }
    
    public func prepareForInterfaceBuilder() {
        if nonPrivateSubviews.count == 1 {
            // Used as a reference container
            view.backgroundColor = .clear
        } else {
            // Is original .xib file
            nonPrivateSubviews.first?.removeFromSuperview()
        }
    }
    
    public func setValue(_ value: Any?, forKeyPath keyPath: String) {
        guard let subview = value as? UIView else { return }
        SubviewsCopier.store(subview: subview, forKeyPath: keyPath, of: view)
    }
    
    
    // MARK: - Private
    
    private let view: NibLoadableView
    fileprivate init(_ view: NibLoadableView) {
        self.view = view
    }
    
    private var nonPrivateSubviews: [UIView] {
        return view.subviews.filter { !String(describing: type(of: $0)).hasPrefix("_") }
    }
    
    private func copyProperties(to nibView: UIView) {
        nibView.frame = view.frame
        nibView.autoresizingMask = view.autoresizingMask
        nibView.translatesAutoresizingMaskIntoConstraints = view.translatesAutoresizingMaskIntoConstraints
        nibView.clipsToBounds = view.clipsToBounds
        nibView.alpha = view.alpha
        nibView.isHidden = view.isHidden
    }
    
    private func copyConstraints(to nibView: UIView) {
        nibView.addConstraints(
            view.constraints.map {
                NSLayoutConstraint(
                    item: $0.firstItem === view ? nibView : $0.firstItem as Any,
                    attribute: $0.firstAttribute,
                    relatedBy: $0.relation,
                    toItem: $0.secondItem === view ? nibView : $0.secondItem,
                    attribute: $0.secondAttribute,
                    multiplier: $0.multiplier,
                    constant: $0.constant)
            }
        )
    }
}


fileprivate struct SubviewsCopier {
    
    static var viewKeyPathsForSubviews = [UIView: [String: UIView]]()
    
    static func store(subview: UIView, forKeyPath keyPath: String, of view: UIView) {
        if viewKeyPathsForSubviews[view] == nil {
            viewKeyPathsForSubviews[view] = [keyPath: subview]
        } else {
            viewKeyPathsForSubviews[view]?[keyPath] = subview
        }
    }
    
    static func copySubviewReferences(from view: UIView, to otherView: UIView) {
        viewKeyPathsForSubviews[view]?.forEach { otherView.setValue($0.value, forKeyPath: $0.key) }
        viewKeyPathsForSubviews[view] = nil
    }
}