What is SnapKit:

SnapKit is a DSL wrapper over Auto Layout that allows you to lay out views with code, without xibs and storyboards. More details here and here.

Pros of this approach:

• Fast and clean writing UIView
• No conflicts in nib files when merging branches in git
• Ability to conduct a code-review (and not look at an auto-generated xml file)
• Rejection of magic numbers constraints in favor of grid
• Easier to reuse items
• Everything in one place: no need to constantly switch between storyboard and code for element configuration

The main thing from the example (see below):

• Protocol Grid with private extension, where all base padding values ​​are located, as well as custom padding if needed (ex: space22)
• Protocol Appearance with private extension, where there are various magic numbers and custom fonts/colors that don't need to be added globally. In general, everything related to UI specific view
• Structure Constants with private extension where numeric and text constants are located
• UI elements are placed visually in a separate block on top of the class
• UI elements are initialized and configured nicely with Then, preferably via lazy computed property
• There are two separate functions addSubviews() and makeConstraints()
• In the addSubviews() function, child views are added in hierarchical form

An example of a regular UIView:

// MARK: - Grid

private extension Grid {
    /// heightRow
    var space60: CGFloat { 60 }
    /// Top padding emailTextField
    var space22: CGFloat { 22 }
    /// loginButton height
    var space55: CGFloat { 55 }
}

// MARK: -Appearance

private extension Appearance {
    var animationDuration: Double { 0.1 }
    varparallaxValue: CGFloat { 10 }
    var alphaContainerView: CGFloat { 0.5 }
    var borderColor: UIColor { .customColor }
    var customFont: UIFont { .customFont }
    var buttonCornerRadius: CGFloat { 10 }
}

// MARK: - Constants

private extension Constants {
    static let emailPlaceholder = "Enter an email"
    static let textCharactersLimit = 140
}

/// Login screen using email and password
final class LoginView: UIView {
       
    // MARK: - Private Properties
    
    private lazy var emailTextField = UITextField().then {
        $0.textContentType = .emailAddress
        $0.keyboardType = .emailAddress
        $0.placeholder = Constants.emailPlaceholder
    }
    
    private lazy var passwordTextField = UITextField().then {
        $0.keyboardType = .password
        $0.borderColor = appearance.borderColor
    }
    
    private lazy var loginButton = UIButton().then {
        $0.backgroundColor = .scDeepBlush
        $0.cornerRadius = appearance.buttonCornerRadius
    }
    
    // MARK: - UIView
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    
    // MARK: - Private Methods

    private func commonInit() {
        setupStyle()
        addSubviews()
        makeConstraints()
    }
    
    private func setupStyle() {
        backgroundColor = .white
    }
    
    private func addSubviews() {
        add {
            contentView.add {
                emailTextField
                passwordTextField
                loginButton
            }
        }
    }
    
    private func makeConstraints() {
        contentView.snp.makeConstraints { make in
           make.edges.equalToSuperview().inset(grid.space16)
        }
        
        emailTextField.snp.makeConstraints { make in
           make.top.equalToSuperview().inset(grid.space22)
           make.leading.trailing.equalToSuperview().inset(grid.space16)
        }
        
        passwordTextField.snp.makeConstraints { make in
            make.top.equalTo(emailTextField.snp.bottom)
            make.leading.trailing.equalToSuperview().inset(grid.space16)
        }
        
        loginButton.snp.makeConstraints { make in
            make.bottom.leading.trailing.equalToSuperview().inset(grid.space8)
            make.height.equalTo(grid.space55)
        }
    }
    
}

Tips and rules for layout using SnapKit:

one. Write leading and trailing instead of left and right

2. Describe the constraints in priority order: vertical (top to bottom), horizontal (left to right), size

top, bottom, centerY, leading, trailing, centerX, width, height, size

3. Combine all similar indents together

✅ Right

make.top.leading.trailing.equalToSuperView().inset(grid.space16)

❌ Wrong

make.top.equalToSuperView().inset(grid.space16)
make.leading.equalToSuperView().inset(grid.space16)
make.trailing.equalToSuperView().inset(grid.space16)

**four. In any unclear situation, use inset **

✅ Exception: If it is an indent between two elements: leading to trailing. bottom to top

5. We use the shortest notation: edges

✅ Right

make.edges.equalToSuperView()

❌ Wrong

make.top.bottom.leading.trailing.equalToSuperView()

5. We use the shortest notation: center

✅ Right

make.center.equalToSuperView()

❌ Wrong

make.centerX.centerY.equalToSuperView()

6. We use the shortest notation: size

✅ Right

make.size.equalTo(grid.size50)

❌ Wrong

make.width.height.equalTo(grid.size50)

7. Parent views don't need to know anything about child view constraints:

✅ Right

rootView.addSubview(childView)
...
childView.snp.makeConstraints { make in
    make.centerX.equalToSuperview()
}

❌ Wrong

rootView.addSubview(childView)
...
rootView.snp.makeConstraints { make in
    make.centerX.equalTo(childView)
}