1. dispatch_once替代方案
OC中用來保證代碼塊只執(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,需要某個代碼塊執(zhí)行一次呢?
private lazy var takeOnceTime: Void = {
// 代碼塊...
}()
_ = takeOnceTime
定義一個懶加載的變量,防止在初始化的時候被執(zhí)行.后面加一個void,為了在_ = takeOnceTime賦值時不耗性能,返回一個Void類型.
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 = {
// 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中兩個方法+load()和+initialize().
+load(): app啟動的時候會加載所有的類,此時就會調(diào)用每個類的load方法.
+initialize(): 第一次初始化這個類的時候會被調(diào)用.
然而在目前的swift版本中這兩個方法都不可用了,那現(xiàn)在我們要在這個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next屬性,它會在applicationDidFinishLaunching之前被調(diào)用,這個時候通過runtime獲取到所有類的列表,然后向所有遵循SelfAware協(xié)議的類發(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)議實現(xiàn)的+awake()方法在這個階段都會被調(diào)用.
3. 交換方法 Method Swizzling
黑魔法Method Swizzling在swift中實現(xiàn)的兩個困難點
- swizzling 應(yīng)該保證只會執(zhí)行一次.
- swizzling 應(yīng)該在加載所有類的時候調(diào)用.
分別在上面給出了解決方案.
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做了默認(rèn)實現(xiàn),相當(dāng)于一層封裝.
1. 給按鈕添加點擊計數(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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