Xcode8已经发布，带了Swift3的预览版本，以后都是默认采用Swift3的语法。

这个例子主要是演示iOS中如何用纯代码实现自动布局，先看看效果。

ios-auto-layout.png

还是先创建程序入口文件main.swift:

import UIKit

let argc = Process.argc
let argv = UnsafeMutablePointer<UnsafeMutablePointer<CChar>>(Process.unsafeArgv)

UIApplicationMain(argc, argv, NSStringFromClass(MainApp), NSStringFromClass(MainAppDelegate))

创建UI程序入口App.swift，增加了一个导航栏：

import UIKit

class MainApp: UIApplication {
    override func sendEvent(_ event: UIEvent) {
        super.sendEvent(event)
    }
}

class MainAppDelegate: UIResponder, UIApplicationDelegate {
    
    var window: UIWindow?
    
    func application(_ app: UIApplication, didFinishLaunchingWithOptions opt: [NSObject: AnyObject]?) -> Bool {
        
        self.window = UIWindow(frame: UIScreen.main().bounds)

        self.window!.rootViewController = UINavigationController(rootViewController:MainViewController())
        self.window!.backgroundColor = UIColor.white()
        self.window!.makeKeyAndVisible()
        
        return true
    }
}

然后创建视图控制器ViewController.swift, 在这里实现自动布局：

import UIKit

class MainViewController: UIViewController {

    var label0_: UILabel!
    var label1_: UILabel!
    var label2_: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.title = "主视图"

        self.label0_ = {
            let label = UILabel()
            label.textAlignment = .center
            label.text = "Hello, AutoLayout!"
            return label
        }()

        self.label1_ = {
            let label = UILabel()
            label.textColor = UIColor.red()
            label.textAlignment = .center
            label.text = "=== 你好, AutoLayout! ==="
            return label
        }()
        
        self.label2_ = {
            let label = UILabel()
            label.textColor = UIColor.blue()
            label.textAlignment = .center
            label.text = "=== 底部 ==="
            return label
        }()

        self.view.addSubview(self.label0_)
        self.view.addSubview(self.label1_)
        self.view.addSubview(self.label2_)

        self.view.setupAutoLayout {
            return (
                layouts: [
                    ("H:|-(20)-[label0]-20-|",[]),
                    ("H:|-(20)-[label1]-20-|",[]),
                    ("H:|-(20)-[label2]-20-|",[]),

                    ("V:|[topGuide]-(0)-[label0(20)]-20-[label1(20)]-(>=0)-[label2]-[bottomGuide]|",[])
                ],
                viewsMap: [
                    "topGuide": self.topLayoutGuide,
                    "bottomGuide": self.bottomLayoutGuide,
                    
                    "label0": self.label0_,
                    "label1": self.label1_,
                    "label2": self.label2_
                ]
            )
        }
    }
}

其中self.view.setupAutoLayout是针对UIView类型作的扩展:

extension UIView {
    func setupAutoLayout(closure: () -> (layouts: [(String,NSLayoutFormatOptions)], viewsMap: [String:AnyObject]) ) {
        let (viewsLayouts, viewsMap) = closure()
        
        // 采用自动布局
        for view in viewsMap.values {
            if let v = view as? UIView {
                v.translatesAutoresizingMaskIntoConstraints = false
            }
        }
        
        // 添加自动布局规则
        for layout in viewsLayouts {
            self.addConstraints(
                NSLayoutConstraint.constraints(
                    withVisualFormat: layout.0,
                    options: layout.1, metrics: nil,
                    views: viewsMap
                )
            )
        }
    }
}

完成。