1. dispatch_once替代方案

OC中用來(lái)保證代碼塊只執(zhí)行一次的dispatch_once在swfit中已經(jīng)被廢棄了,取而代之的是使用static let,let本身就帶有線程安全性質(zhì)的.

例如單例的實(shí)現(xiàn).

final public class MySingleton {
    static let shared = MySingleton()
    private init() {}
}

但如果我們不想定義常量,需要某個(gè)代碼塊執(zhí)行一次呢?

private lazy var takeOnceTime: Void = {
    // 代碼塊...
}()

_ = takeOnceTime

定義一個(gè)懶加載的變量,防止在初始化的時(shí)候被執(zhí)行.后面加一個(gè)void,為了在_ = takeOnceTime賦值時(shí)不耗性能,返回一個(gè)Void類(lèi)型.

lazy var改為static let也可以,為了使用方便,我們用一個(gè)類(lèi)方法封裝下

class ClassName {
    private static let takeOnceTime: Void = {
        // 代碼塊...
    }()
    static func takeOnceTimeFunc() {
        ClassName.takeOnceTime
    }
}

// 使用
ClassName.takeOnceTimeFunc()

這樣就可以做到和dispatch_once一樣的效果了.

2. 被廢棄的+load()和+initialize()

我們都知道OC中兩個(gè)方法+load()和+initialize().

+load(): app啟動(dòng)的時(shí)候會(huì)加載所有的類(lèi),此時(shí)就會(huì)調(diào)用每個(gè)類(lèi)的load方法.
+initialize(): 第一次初始化這個(gè)類(lèi)的時(shí)候會(huì)被調(diào)用.

然而在目前的swift版本中這兩個(gè)方法都不可用了,那現(xiàn)在我們要在這個(gè)階段搞事情該怎么做? 例如method swizzling.

JORDAN SMITH大神給出了一種很巧解決方案.UIApplication有一個(gè)next屬性,它會(huì)在applicationDidFinishLaunching之前被調(diào)用,這個(gè)時(shí)候通過(guò)runtime獲取到所有類(lèi)的列表,然后向所有遵循SelfAware協(xié)議的類(lèi)發(fā)送消息.

extension UIApplication {
    private static let runOnce: Void = {
        NothingToSeeHere.harmlessFunction()
    }()
    override open var next: UIResponder? {
        // Called before applicationDidFinishLaunching
        UIApplication.runOnce
        return super.next
    }
}

protocol SelfAware: class {
    static func awake()
}
class NothingToSeeHere {
    static func harmlessFunction() {
        let typeCount = Int(objc_getClassList(nil, 0))
        let types = UnsafeMutablePointer<AnyClass>.allocate(capacity: typeCount)
        let autoreleasingTypes = AutoreleasingUnsafeMutablePointer<AnyClass>(types)
        objc_getClassList(autoreleasingTypes, Int32(typeCount))
        for index in 0 ..< typeCount {
            (types[index] as? SelfAware.Type)?.awake()
        }
        types.deallocate()
    }
}

之后任何遵守SelfAware協(xié)議實(shí)現(xiàn)的+awake()方法在這個(gè)階段都會(huì)被調(diào)用.

3. 交換方法 Method Swizzling

黑魔法Method Swizzling在swift中實(shí)現(xiàn)的兩個(gè)困難點(diǎn)

• swizzling 應(yīng)該保證只會(huì)執(zhí)行一次.
• swizzling 應(yīng)該在加載所有類(lèi)的時(shí)候調(diào)用.

分別在上面給出了解決方案.

下面給出了兩個(gè)示例供參考:

protocol SelfAware: class {
    static func awake()
    static func swizzlingForClass(_ forClass: AnyClass, originalSelector: Selector, swizzledSelector: Selector)
}

extension SelfAware {
    
    static func swizzlingForClass(_ forClass: AnyClass, originalSelector: Selector, swizzledSelector: Selector) {
        let originalMethod = class_getInstanceMethod(forClass, originalSelector)
        let swizzledMethod = class_getInstanceMethod(forClass, swizzledSelector)
        guard (originalMethod != nil && swizzledMethod != nil) else {
            return
        }
        if class_addMethod(forClass, originalSelector, method_getImplementation(swizzledMethod!), method_getTypeEncoding(swizzledMethod!)) {
            class_replaceMethod(forClass, swizzledSelector, method_getImplementation(originalMethod!), method_getTypeEncoding(originalMethod!))
        } else {
            method_exchangeImplementations(originalMethod!, swizzledMethod!)
        }
    }
}

class NothingToSeeHere {
    static func harmlessFunction() {
        let typeCount = Int(objc_getClassList(nil, 0))
        let types = UnsafeMutablePointer<AnyClass>.allocate(capacity: typeCount)
        let autoreleasingTypes = AutoreleasingUnsafeMutablePointer<AnyClass>(types)
        objc_getClassList(autoreleasingTypes, Int32(typeCount))
        for index in 0 ..< typeCount {
            (types[index] as? SelfAware.Type)?.awake()
        }
        types.deallocate()
    }
}
extension UIApplication {
    private static let runOnce: Void = {
        NothingToSeeHere.harmlessFunction()
    }()
    override open var next: UIResponder? {
        UIApplication.runOnce
        return super.next
    }
}

在SelfAware的extension中為swizzlingForClass做了默認(rèn)實(shí)現(xiàn),相當(dāng)于一層封裝.

1. 給按鈕添加點(diǎn)擊計(jì)數(shù)

extension UIButton: SelfAware {
    static func awake() {
        UIButton.takeOnceTime
    }
    private static let takeOnceTime: Void = {
        let originalSelector = #selector(sendAction)
        let swizzledSelector = #selector(xxx_sendAction(action:to:forEvent:))
        
        swizzlingForClass(UIButton.self, originalSelector: originalSelector, swizzledSelector: swizzledSelector)
    }()
    
    @objc public func xxx_sendAction(action: Selector, to: AnyObject!, forEvent: UIEvent!) {
        struct xxx_buttonTapCounter {
            static var count: Int = 0
        }
        xxx_buttonTapCounter.count += 1
        print(xxx_buttonTapCounter.count)
        xxx_sendAction(action: action, to: to, forEvent: forEvent)
    }
}

2. 替換控制器的viewWillAppear方法

extension UIViewController: SelfAware {
    static func awake() {
        swizzleMethod
    }
    private static let swizzleMethod: Void = {
        let originalSelector = #selector(viewWillAppear(_:))
        let swizzledSelector = #selector(swizzled_viewWillAppear(_:))
        
        swizzlingForClass(UIViewController.self, originalSelector: originalSelector, swizzledSelector: swizzledSelector)
    }()
    
    @objc func swizzled_viewWillAppear(_ animated: Bool) {
        swizzled_viewWillAppear(animated)
        print("swizzled_viewWillAppear")
    }
}

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