Welcome page display

1. add to SDWebImage library , establish WelcomeViewController.swift Inherit UIViewController

import UIKit
import SDWebImage

class WelcomeViewController: UIViewController {
    // Set the interface , The hierarchy of views 
    override func loadView() {
        // Directly use the background image as the root view , Don't worry about the scaling of the image 
        view = backImageView
        setupUI()
    }
    
    // Subsequent processing after the view is loaded , Usually used to set data 
    override func viewDidLoad() {
        super.viewDidLoad()
        // Load user avatar asynchronously 
        iconView.sd_setImage(with:UserAccountViewModel.sharedUserAccount.avatarUrl, placeholderImage: UIImage(named: "avatar_default_big"))
    }
    
    // The view is already displayed , You can usually animate / Keyboard processing 
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        //1. Change constraints  ->  Change position 
        //updateConstraints  Update already set constraints 
        //multiplier  Properties are read-only properties , After creating , No modification allowed 
        /**
          Develop with automatic layout , There is a principle 
         - All controls that use constraints to set positions , Don't set  'frame'
          reason :   The automatic layout system will be based on the set constraints , Automatically calculate the value of the control  frame
          stay  layoutSubviews  Function  frme
          If   Active modification  frame, Can cause   Automatic layout system calculation error 
         - working principle ： When a motion cycle starts , Automatic layout system , Will collect “ All constraints change ”
         - Before the end of the rerun cycle , call  layoutSubViews  Functions are set uniformly  frame
         - If you want some constraints to be updated in advance ！ Use  ‘layoutIfNeeded’  Function to make the automatic layout system , Update the constraint changes currently collected in advance 
         */
        iconView.snp.updateConstraints { make in
            // make.bottom.equalTo(view.snp.bottom).multipliedBy(0.3)
            make.bottom.equalTo(view.snp.bottom).offset(-view.bounds.height + 200)
            //make.bottom.equalTo(view.snp.bottom).offset(-view.bounds.height * 0.7)
        }
        
        //2. Animation 
        welcomeLable.alpha = 0
        UIView.animate(withDuration: 1.2, delay: 0, usingSpringWithDamping: 0.8, initialSpringVelocity: 10, options: []) {
            // Modify all ‘ Animatable ’ attribute 
            // Automatic layout animation 
            self.view.layoutIfNeeded()
        } completion: {(state: Bool) in
            UIView.animate(withDuration: 0.7) {
                self.welcomeLable.alpha = 1
            } completion: {(state: Bool) in
                // It's not recommended 
                //UIApplication.shared.keyWindow?.rootViewController = MainViewController()
                NotificationCenter.default.post(name: NSNotification.Name(SwitchRootViewControllerNotification), object: nil)
            }
        }
    }
    
    //MARK: -  Lazy load control 
    // background image 
    private lazy var backImageView:UIImageView = UIImageView(imageName: "ad_background")
    // Head portrait 
    private lazy var iconView: UIImageView = {
        let iv = UIImageView(imageName: "avatar_default_big")
        // Setting fillet 
        iv.layer.cornerRadius = 45
        iv.layer.masksToBounds = true
        return iv
    }()
    // Welcome label 
    private lazy var welcomeLable: UILabel = UILabel(title: " welcome back ", fontSize: 18)
}

2. Extension class settings UI

//MARK: -  Set the interface 
extension WelcomeViewController{
    private func setupUI(){
        //1. add controls 
        view.addSubview(iconView)
        view.addSubview(welcomeLable)
        //2. Automatic layout 
        iconView.snp.makeConstraints { make in
            make.centerX.equalTo(view.snp.centerX)
            //make.bottom.equalTo(view.snp.bottom).multipliedBy(0.7)
            make.bottom.equalTo(view.snp.bottom).offset(-200)
            make.width.height.equalTo(90)
        }
        welcomeLable.snp.makeConstraints { make in
            make.centerX.equalTo(iconView.snp.centerX)
            make.top.equalTo(iconView.snp.bottom).offset(16)
        }
    }
}