Swift iOS：KVO

KVO是Key Value Observer的缩写，能够用于监视一个对象的属性值变化，而后执行一个代码块（块、函数、闭包等）。Facebook开源了一个KVO框架，KVOController。

这个案例使用KVOController，用于App主题改变通知。经过KVO监视主题的值，当设置主题变化时，通知全部的UI类的主题处理器。代码以下：

import UIKit
import KVOController
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        self.window = UIWindow();
        self.window?.frame=UIScreen.main.bounds;
        self.window?.makeKeyAndVisible();
        self.window?.rootViewController = Page()
        return true
    }
}
class Page: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let button   = UIButton(type: .system)
        button.frame = CGRect(x: 120, y: 150, width: 220, height: 50)
        button.contentHorizontalAlignment = .left
        button.setTitle("Peace",for: .normal)
        button.addTarget(self, action: #selector(buttonAction(_:)), for: .touchUpInside)
        view.addSubview(button)
        let button1   = UIButton(type: .system)
        button1.frame = CGRect(x: 120, y: 200, width: 220, height: 50)
        button1.contentHorizontalAlignment = .left
        button1.setTitle("Blood",for: .normal)
        button1.addTarget(self, action: #selector(buttonAction1(_:)), for: .touchUpInside)
        view.addSubview(button1)
        self.themeChangedHandler = {[weak self] (style) -> Void in
            if "Peace" == style{
                self!.view.backgroundColor = .blue
            }
            else{
                self!.view.backgroundColor = .red
            }
        }
    }
    func buttonAction(_ sender:UIButton!){
        ThemeColor.shared.style = "Peace"
    }
    func buttonAction1(_ sender:UIButton!){
        ThemeColor.shared.style = "Blood"
    }
}
class ThemeColor :NSObject  {
    dynamic var style:String
    static let shared = ThemeColor()
    fileprivate override init(){
        style = "Blood"// or Peace
        super.init()
    }
}
extension NSObject {
    fileprivate struct AssociatedKeys {
        static var themeChanged = "themeChanged"
    }
    public typealias ThemeChangedClosure = @convention(block) (_ style:String) -> Void
    var themeChangedHandler:ThemeChangedClosure? {
        get {
            let closureObject: AnyObject? = objc_getAssociatedObject(self, &AssociatedKeys.themeChanged) as AnyObject?
            guard closureObject != nil else{
                return nil
            }
            let closure = unsafeBitCast(closureObject, to: ThemeChangedClosure.self)
            return closure
        }
        set{
            guard let value = newValue else{
                return
            }
            let dealObject: AnyObject = unsafeBitCast(value, to: AnyObject.self)
            objc_setAssociatedObject(self, &AssociatedKeys.themeChanged,dealObject,objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            self.kvoController.observe(ThemeColor.shared, keyPath: "style", options: [.initial,.new] , block: {[weak self] (nav, color, change) -> Void in
                self?.themeChangedHandler?(ThemeColor.shared.style)
                }
            )
        }
    }
}复制代码

代码运行后，能够看到两个按钮，点击不一样的按钮会切换主题。

Pod文件为：

pod 'KVOController', '~> 1.2.0'复制代码

代码的效果就是，当修改：

ThemeColor.shared.style = "Peace"复制代码

就会引起当前对象的块对象：themeChangedHandler的调用。由于使用了KVO监视了ThemeColor.shared.style，当值变化的时候，就会引起themeChangedHandler所指向的闭包的执行。闭包在set的时候被使用objc_setAssociatedObject存储于指定键（themeChanged）的值内。当运行get的时候，会使用objc_getAssociatedObject函数把它再取出来。

函数对：objc_getAssociatedObject和objc_setAssociatedObject看起来比较复杂，可是能够用于动态的为对象建立属性，所以颇为强大。

此代码很差理解，也许使用Observer设计模式来替代更好。