In the 01.CustomFont project, the main learning is the installation and use of SnapKit, the automatic layout library.

1.SnapKit installation

The content of Podfile is as follows:

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '10.0'
use_frameworks!
target 'CustomFont' do
    pod 'SnapKit'
end

Locate the directory of Podfile in the terminal and execute pod install.

If SnapKit cannot be used in swfit5 environment, please refer to SnapKit error after conversion to swift 5 is available Solve.

2. Use of snapkit

Note: you must add the control to the view before adding constraints to the control. For example:

        self.view.addSubview(label)
        label.snp.makeConstraints { (make) in
            make.top.equalTo(100)
            make.centerX.equalTo(self.view)
        }

Property Description:

        . equalTo: equal to
        . lessthankorequalto: less than or equal to
        . greaterthankorequalto: greater than or equal to
        . edges:: edge  
        . size: size
        . center: center
        . inset: internal displacement correction
        . offset: external displacement correction
        . multiplicedby: multiplier correction

Basic use:

        let viewOne = UIView()
        viewOne.snp.makeConstraints { (make) in
            make.left.top.equalToSuperview()
            make.width.height.equalTo(40)
        }
        //The current view is the same as the title Center (centerX and center)
        make.center.equalTo(title)
        //The width and height of the current view is greater than or equal to the title
        make.size.greaterThanOrEqualTo(title)
        //The top, left, bottom, right of the current view is equal to the title
        make.edges.equalTo(title)
        //The distance between the current view and the title view is 20, 20, 20 and 30, respectively
        make.edges.equalTo(title).inset(UIEdgeInsetsMake(20, 20, 20, 30)) 
        //The current view is half of the title View
        make.size.equalTo(title).multipliedBy(0.5)



        //Center horizontally and vertically in the parent view
        make.center.equalTo(superview.snp.center)
        //Center horizontally and vertically in the parent view
        make.center.equalTo(superview)

Update, remove, and reset constraints:

        //Save reference to constraint
        var constraint:Constraint?
        title.snp.makeConstraints { (make) -> Void in
            self.constraint = make.width.height.equalTo(150).constraint
            make.center.equalTo(self.view)
        }
        
        //Remove constraints
        self.constraint?.deactivate()
        //1. Update modification constraints
        self.constraint?.update(offset: 60)      
        //2. View constraint update
        override func updateViewConstraints() {
              self.title.snp.updateConstraints{ (make) -> Void in
              //View width equal to screen width
                    make.width.equalTo(self.view)
               }
              super.updateViewConstraints()
          }

         //Redo constraint
          title.snp.remakeConstraints { (make) -> Void in
              make.width.height.equalTo(100)
         }

The source code of ViewController is as follows:

import UIKit
import SnapKit

var isSystemFont:Bool = true;

class ViewController: UIViewController {
    let label = UILabel()

    override func viewDidLoad() {
        super.viewDidLoad()
        label.text = "I am the king of the world"
        label.textAlignment = NSTextAlignment.center
        self.view.addSubview(label)
        label.snp.makeConstraints { (make) in
            make.top.equalTo(100)
            make.centerX.equalTo(self.view)
        }
        label.font = UIFont.systemFont(ofSize: 30)
        
        let changeBtn = UIButton(type: .custom)
        changeBtn.setTitle("Change Font Family", for: UIControl.State.normal)
        changeBtn.addTarget(self, action: #selector(changeFontFamily), for: UIControl.Event.touchUpInside)
        changeBtn.setTitleColor(UIColor.blue, for: UIControl.State.normal)
        self.view.addSubview(changeBtn)
        changeBtn.layer.borderColor = UIColor.blue.cgColor
        changeBtn.layer.borderWidth = 1
        changeBtn.layer.cornerRadius = 5
        changeBtn.snp.makeConstraints { (make) in
            make.top.equalTo(500)
            make.centerX.equalTo(self.view)
            make.width.equalTo(200)
        }
        printAllSupportedFontNames()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @objc func changeFontFamily() {
        if isSystemFont {
            label.font = UIFont(name: "Savoye LET", size: 30)
            isSystemFont = false
        }else{
            label.font = UIFont.systemFont(ofSize: 30)
            isSystemFont = true
        }
        
    }
    
    func printAllSupportedFontNames() {
        let familyNames = UIFont.familyNames
        for familyName in familyNames {
            print("++++++ \(familyName)")
            let fontNames = UIFont.fontNames(forFamilyName: familyName)
            for fontName in fontNames {
                print("----- \(fontName)")
            }
        }
    }

}

Learning source code from https://github.com/nimomeng/30-swift-projects-in-30-days , its Swift language version is 4.1.

During the learning process, I will change the code language version to Swift 5, code address: https://github.com/dong706/30-swift-projects-in-30-days/ .

Reference article: The use of Snapkit in iOS development

               The difference between center and snp.center when using SnapKit