交換方法Method Swizzling[swift]

摘要： 1. dispatch_once替代方案 OC中用來保證程式碼塊只執行一次的dispatch_once 在swfit中已經被廢棄了,取而代之的是使用static let ,let 本身就帶有執行緒安全性質的. 例如單例的實現. ...

1. dispatch_once替代方案

OC中用來保證程式碼塊只執行一次的dispatch_once 在swfit中已經被廢棄了,取而代之的是使用static let ,let 本身就帶有執行緒安全性質的.

例如單例的實現.

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

但如果我們不想定義常量,需要某個程式碼塊執行一次呢?

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

_ = takeOnceTime

定義一個懶載入的變數,防止在初始化的時候被執行.後面加一個void ,為了在_ = takeOnceTime 賦值時不耗效能,返回一個Void 型別.

lazy var 改為static let 也可以,為了使用方便,我們用一個類方法封裝下

class ClassName {
private static let takeOnceTime: Void = {
// 程式碼塊...
}()
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中兩個方法+load() 和+initialize() .

+load() :app啟動的時候會載入所有的類,此時就會呼叫每個類的load方法.

+initialize() : 第一次初始化這個類的時候會被呼叫.

然而在目前的swift版本中這兩個方法都不可用了,那現在我們要在這個階段搞事情該怎麼做? 例如method swizzling .

ofollow,noindex">JORDAN SMITH大神 給出了一種很巧解決方案.UIApplication 有一個next 屬性,它會在applicationDidFinishLaunching 之前被呼叫,這個時候通過runtime 獲取到所有類的列表,然後向所有遵循SelfAware協議的類傳送訊息.

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()
}
}

之後任何遵守SelfAware 協議實現的+awake() 方法在這個階段都會被呼叫.

3. 交換方法 Method Swizzling

黑魔法Method Swizzling 在swift中實現的兩個困難點

• swizzling 應該保證只會執行一次.
• swizzling 應該在載入所有類的時候呼叫.

分別在上面給出瞭解決方案.

下面給出了兩個示例供參考:

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 做了預設實現,相當於一層封裝.

1. 給按鈕新增點選計數

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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