iOS编程：如何将一个UIViewController设为另一个的背景视图？

问题 #

需要将一个UIImageView作为背景参考图置于另一个UIImageView后方。现有三个ViewController：MainViewController作为主控制器，secondViewController处理核心逻辑，thirdViewController承载背景图。已将secondViewController的mainImageView设为透明，但thirdViewController的背景仍无法显示，求可行的解决思路。

现有代码 #

MainViewController

import UIKit

class MainViewController: UIViewController {
    
    let canvasView = secondViewController()
    let canvasBackground = thirdViewController()
 
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBlue
        
        canvas()
    }
    
    func canvas() {
        addChild(canvasView)
        view.addSubview(canvasView.view)
        canvasView.didMove(toParent: self)
    }
}

secondViewController

class secondViewController: UIViewController {

 private let mainImageView: UIImageView = {
       var imageView = UIImageView()
       imageView.backgroundColor = .clear    
       return imageView
    }()
// some more code and logic implementation

   override func viewDidLoad() {
        super.viewDidLoad()
    }
}

thirdViewController

class thirdViewController: UIViewController {
    
    private let viewLayer: UIImageView = {
       let layerView = UIImageView()
        layerView.backgroundColor = .systemGreen
        return layerView
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(viewLayer)
        viewLayer.frame = view.bounds

    }
}

解决思路 #

1. 添加并正确层级布局背景控制器视图
   你创建了canvasBackground实例，但未将其视图添加到MainViewController的视图层级，也未做布局。需要先添加背景控制器视图，再添加前景的canvasView视图，确保背景处于下层：

   func canvas() {
       // 先处理背景控制器
       addChild(canvasBackground)
       view.addSubview(canvasBackground.view)
       canvasBackground.didMove(toParent: self)
       canvasBackground.view.frame = view.bounds
       
       // 再处理前景控制器
       addChild(canvasView)
       view.addSubview(canvasView.view)
       canvasView.didMove(toParent: self)
       canvasView.view.frame = view.bounds
   }
   

2. 设置secondViewController的根视图透明
   仅将mainImageView设为透明不够，secondViewController的默认根视图背景为不透明色，会完全遮挡下方的背景控制器。必须将根视图背景也设为透明：

   class secondViewController: UIViewController {
       // ... 现有代码
       override func viewDidLoad() {
           super.viewDidLoad()
           view.backgroundColor = .clear // 新增该行
       }
   }
   

3. 用Auto Layout替代固定frame（推荐）
   固定frame在屏幕旋转或不同尺寸设备上会出现适配问题，建议使用Auto Layout约束来设置视图位置：

   func canvas() {
       // 处理背景控制器
       addChild(canvasBackground)
       view.addSubview(canvasBackground.view)
       canvasBackground.didMove(toParent: self)
       canvasBackground.view.translatesAutoresizingMaskIntoConstraints = false
       NSLayoutConstraint.activate([
           canvasBackground.view.topAnchor.constraint(equalTo: view.topAnchor),
           canvasBackground.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
           canvasBackground.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
           canvasBackground.view.trailingAnchor.constraint(equalTo: view.trailingAnchor)
       ])
       
       // 处理前景控制器
       addChild(canvasView)
       view.addSubview(canvasView.view)
       canvasView.didMove(toParent: self)
       canvasView.view.translatesAutoresizingMaskIntoConstraints = false
       NSLayoutConstraint.activate([
           canvasView.view.topAnchor.constraint(equalTo: view.topAnchor),
           canvasView.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
           canvasView.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
           canvasView.view.trailingAnchor.constraint(equalTo: view.trailingAnchor)
       ])
       canvasView.view.backgroundColor = .clear
   }
   

内容的提问来源于stack exchange，提问作者willd