Creating a custom UIView from a XIB with IBDesinable.

XibView-IBDesignable.swift

//
//  XibView-IBDesignable.swift
//
//  Created by MiharuNaruse on 2020/06/14.
//  Copyright © 2020 m_rn. All rights reserved.
//

/**
 https://medium.com/zenchef-tech-and-product/how-to-visualize-reusable-xibs-in-storyboards-using-ibdesignable-c0488c7f525d

 If you use `Bundle.main.loadNibNamed("XibView", owner: self, options: nil)` , an error will occur like below...
 ~~~
 error: IB Designables: Failed to render and update auto layout status for UseXibViewFirstViewController (ZaM-GP-gg5): The agent threw an exception.
 ~~~

 The main bundle represents the bundle directory that contains the currently executing code.

 If your app interacts directly with plug-ins, frameworks, or other bundled content, you can use other methods of this class to create appropriate bundle objects.
 ~~~
 // Get the app's main bundle
 let mainBundle = Bundle.main

 // Get the bundle containing the specified private class.
 let myBundle = Bundle(for: NSClassFromString("MyPrivateClass")!)
 ~~~
 */

import UIKit

@IBDesignable
final class XibView: UIView {
    var contentView: UIView!

    // コードから初期化
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }

    // IBから初期化
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }

    func commonInit() {
        contentView = loadViewFromNib()

        addSubview(contentView)
        contentView.frame = bounds
        contentView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    }

    /// nibファイルからビューをロードする。
    ///
    /// `Bundle.main.loadNibNamed("ContactView", owner: self, options: nil)` だと、 `@IBDesignable` が機能しない。
    /// - Returns: nibファイルから読み込んだビュー
    func loadViewFromNib() -> UIView? {
        let bundle = Bundle(for: type(of: self))
        let nib = UINib(nibName: "XibView", bundle: bundle)
        return nib.instantiate(withOwner: self, options: nil).first as? UIView
    }
}