一、topLayoutGuide和bottomLayoutGuide

extension UIViewController {
open var topLayoutGuide: UILayoutSupport { get }
open var bottomLayoutGuide: UILayoutSupport { get }
}
topLayoutGuide和bottomLayoutGuide是UIViewController的属性，ios7开始用于在自动布局时，帮助开发者避开顶部的状态栏，导航栏以及底部的tabbar等。

class ViewController: UIViewController {
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        print(self.topLayoutGuide.topAnchor)
        print(self.topLayoutGuide.bottomAnchor)
        print(self.bottomLayoutGuide.topAnchor)
        print(self.bottomLayoutGuide.bottomAnchor)
        
        print(self.topLayoutGuide.length)//20
        print(self.bottomLayoutGuide.length)//0
        let myview = UIView()
        self.view.addSubview(myview)
        myview.backgroundColor = .red
        myview.snp.makeConstraints { (make) in
            make.top.equalTo(self.topLayoutGuide.snp.bottom)
            make.width.equalTo(100)
            make.height.equalTo(40)
            make.centerX.equalToSuperview()
        }
    }
}

• 对于每一个UIViewController，都会有一个bottomLayoutGuide是UIViewController的属性，在自动布局时，可以通过设置控制器的bottomLayoutGuide进行约束，来避开顶部的状态栏，导航栏以及底部的tabbar等。
• iOS11以后 topLayoutGuide和bottomLayoutGuide废弃，用safeAreaLayoutGuide代替

二、 UILayoutGuide

iOS9开始，苹果新增加了一个UILayoutGuide的类。UILayoutGuide用于提供一个矩形区域可以用Auto Layout来定制一些约束特性，作为一个虚拟的view使用。

class ViewController: UIViewController {
 
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        let view  = UIView()
        self.view.addSubview(view)
        view.backgroundColor = .red
        view.snp.makeConstraints { (make) in
            make.width.equalTo(300)
            make.height.equalTo(300)
            make.centerX.equalTo(self.view)
            make.top.equalTo(self.topLayoutGuide.snp.bottom)
        }
        
        let space1 = UILayoutGuide()
        view.addLayoutGuide(space1)
        space1.snp.makeConstraints { (make) in
            make.top.equalTo(view.snp.top).offset(10)
            make.left.equalToSuperview().offset(10)
            make.right.equalToSuperview().offset(-10)
            make.height.equalTo(100)
        }
        let yelloview = UIView()
        yelloview.backgroundColor = .yellow
        view.addSubview(yelloview)
        yelloview.snp.makeConstraints { (make) in
            make.top.equalTo(space1)
            make.left.right.bottom.equalTo(space1)
        }
        print(view.layoutGuides)
        print(view.subviews)
        print(space1.owningView)
    
    }
}

import UIKit
import SnapKit
class ViewController: UIViewController {
 
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

       let container = UILayoutGuide()
        self.view.addLayoutGuide(container)
        container.snp.makeConstraints { (make) in
            make.height.equalTo(20)
            make.top.equalTo(self.topLayoutGuide.snp.bottom).offset(100)
            make.centerX.equalToSuperview()
        }
        
        let redView = UIView()
        redView.backgroundColor = .red
        self.view.addSubview(redView)
        redView.snp.makeConstraints { (make) in
            make.top.equalTo(container)
            make.left.equalTo(container)
            make.width.equalTo(30)
            make.bottom.equalTo(container)
        }
        
        let yellowView = UIView()
        yellowView.backgroundColor = .yellow
        self.view.addSubview(yellowView)
        yellowView.snp.makeConstraints { (make) in
            make.left.equalTo(redView.snp.right).offset(10)
            make.width.equalTo(65);
            make.top.bottom.equalTo(container)
            make.right.equalTo(container)
        }
    }
}

• 当我们需要一个虚拟View帮助的事情都可以交给UILayoutGuide来做。UILayoutGuide并没有真正的创建一个View，只是创建了一个矩形空间，只在进行auto layout时参与进来计算，它没有参与视图层次结构，也不会影响事件的传递，更高效、更轻量级。

三、safeAreaLayoutGuide

iOS11又引入了一个Safe Area（安全区域）的概念,苹果果建议在这个安全区域内放置UI控件。这个安全区域的范围其实就是整个屏幕隔离出状态栏，导航栏，tabar，以及iPhone X顶部刘海，底部虚拟home手势区域的范围。
从这个介绍可以看得出，所谓的Safe Area其实也就是升级版本的topLayoutGuide/bottomLayoutGuide，以前只能限制top/bottom的Layout，现在更加强大了。

extension UIView {
var safeAreaLayoutGuide: UILayoutGuide { get }
var safeAreaInsets: UIEdgeInsets { get }
}

class ViewController: UIViewController {
    var redView:UIView?
    
    override func viewDidLoad() {
        self.redView = UIView()
        self.view.addSubview(self.redView!)
        self.redView?.backgroundColor = .red
        self.redView?.snp.makeConstraints({ (make) in
            make.top.equalTo(self.topLayoutGuide.snp.bottom).offset(-30);
            make.width.equalTo(100)
            make.height.equalTo(100)
            make.left.equalTo(self.view).offset(20)
        })
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        print(self.view.safeAreaLayoutGuide)
        self.view.backgroundColor = .yellow
        print(self.view.safeAreaLayoutGuide.layoutFrame) (0.0, 44.0, 414.0, 818.0)
        print(self.redView?.safeAreaLayoutGuide.layoutFrame) (0.0, 30.0, 100.0, 70.0)

    print(self.view.safeAreaInsets) # UIEdgeInsets(top: 44.0, left: 0.0, bottom: 34.0, right: 0.0)
    print(self.redView!.safeAreaInsets) #UIEdgeInsets(top: 44.0, left: 0.0, bottom: 34.0, right: 0.0)
    }
}
注：代码是在以xr模拟器测试

• 系统会为每个UIView创建一个safeAreaLayoutGuide（安全区域）
• safeAreaInsets表示的是view的边界与自身安全区域的间距

class ViewController: UIViewController {
    var redView:UIView?
    
    override func viewDidLoad() {
        self.redView = UIView()
        self.view.addSubview(self.redView!)
        self.redView?.backgroundColor = .red
        self.redView?.snp.makeConstraints({ (make) in
            make.top.equalTo(self.view.safeAreaLayoutGuide.snp.top).offset(30);
            make.width.equalTo(100)
            make.height.equalTo(100)
            make.left.equalTo(self.view).offset(20)
        })
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        print(self.view.safeAreaLayoutGuide)
        self.view.backgroundColor = .yellow
        print(self.view.safeAreaLayoutGuide.layoutFrame) (0.0, 44.0, 414.0, 818.0)

        print(self.redView!.safeAreaLayoutGuide.layoutFrame)  (0.0, 0.0, 100.0, 100.0)
        
        print(self.view.safeAreaInsets) UIEdgeInsets(top: 44.0, left: 0.0, bottom: 34.0, right: 0.0)
        print(self.redView!.safeAreaInsets) UIEdgeInsets(top: 0, left: 0.0, bottom: 34.0, right: 0.0)
    }
}

• 在自动布局时就可以相对于view的安全区域做约束，有效的避开顶部的状态栏，导航栏以及底部的tabbar等。