平時使用Timer/NSTimer時候, 由于Timer會持有self, 所以釋放一直是一個麻煩.
本文不再對于Timer初始化, 需要加入Runloop等事項進(jìn)行討論了, 使用scheduledTimer默認(rèn)加入Runloop.

先來段看起來正確, 卻無法釋放的錯誤寫法

class TimerViewController: UIViewController {
    lazy var timer: Timer = {
        return Timer.scheduledTimer(timeInterval: 1, 
                                    target: self, 
                                    selector:  #selector(timeRun), 
                                    userInfo: nil, 
                                    repeats: true)
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        timer.fire()
    }

   @objc func timeRun() {
       print(#function)
    }

   deinit() {
      /// WARNING: 此處不可能執(zhí)行了, 因為timer持有了self
      /// 手動去釋放 timer.invalidate()的話, 比如remove或者點擊按鈕等, 也不是很好
      timer.invalidate()
   }
}

解決思路就是打破這個引用, 也就是Timer不再持有self.
讓Timer隨著self生命周期一起走, 盡量保證原始API

先來一個效果代碼, 注意觀察Timer的初始化.safe_scheduled

class TimerViewController: UIViewController {
    lazy var timer: Timer = {
       return Timer.safe_scheduled(interval: 1, 
                                   target: self, 
                                   selector: #selector(timeGo), 
                                   repeats: true, 
                                   userInfo: nil)
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        timer.fire()
    }

   @objc func timeRun() {
       print( #function)
    }
    
    deinit() {
       print( #function)
   }
}

具體實現(xiàn)細(xì)節(jié)

extension Timer {
  /// 此Timer不可在deinit/dealloc中執(zhí)行invalidate等任何操作, 已經(jīng)隨著target生命周期釋放了
  static func safe_scheduled(interval: TimeInterval, 
                             target: Any, 
                             selector: Selector, 
                             repeats: Bool, 
                             userInfo: Any? = nil) -> Timer {
    return WolfTimer(scheuled: interval,
                     target: target,
                     selector: selector,
                     userInfo: userInfo,
                     repeats: repeats).k_timer ??
      Timer.scheduledTimer(timeInterval: interval,
                           target: target,
                           selector: selector,
                           userInfo: userInfo,
                           repeats: repeats)
  }
}

class WolfTimer {
  
  convenience init(scheuled interval: TimeInterval, 
                   target: Any, 
                   selector: Selector, 
                   userInfo: Any?, repeats: Bool) {
    self.init()
    k_target = target as? NSObjectProtocol
    k_selector = selector
    k_timer = Timer.scheduledTimer(timeInterval: interval, target: self, selector: #selector(timer_run), userInfo: userInfo, repeats: repeats)
  }
  
  weak var k_timer: Timer?
  weak var k_target: NSObjectProtocol?
  var k_selector: Selector?
  
  @objc func timer_run() {
    guard let target = k_target, 
          let selector = k_selector,
          target.responds(to: selector) else { 
       k_timer?.invalidate()
       return
    }
   target.perform(selector, with: k_timer)
  }
  
  deinit {
    print(#function + "WolfTimer")
  }
  
}

也就是說, ViewController中的Timer被vc所retain持有,
Timer持有WolfTimer的對象.
Timer執(zhí)行計時器, 則由WolfTimer根據(jù)target和selector去perform執(zhí)行.
如果ViewController被釋放, 則由于

weak var k_target: NSObjectProtocol?
weak var k_timer: Timer?

是weak的弱引用, 而也會變?yōu)閚il, 也就釋放了.

mark注意點: 原本覺得k_timer可以不為weak, 但是考慮到timer可能并沒有執(zhí)行, 那么WolfTimer也就無法釋放了, 我就試了試, 結(jié)果發(fā)現(xiàn)另外一個坑, 如果k_timer不使用weak, 那么如果ViewController的deinit中如果執(zhí)行timer.invalidate()或者其他方法, 會觸發(fā)VC過度釋放.

從大神處學(xué)到了一種新的做法.
感謝 LEE大神
Timer初始化時候, 使用WeakObjc(self)

class TimerViewController: UIViewController {
    lazy var timer: Timer = {
        return Timer.scheduledTimer(timeInterval: 1, 
                                    target: WeakObjc(self), 
                                    selector:  #selector(timeRun), 
                                    userInfo: nil, 
                                    repeats: true)
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        timer.fire()
    }

   @objc func timeRun() {
       print(#function)
    }
}


class WeakObjc: NSObject {
    
    private weak var target: AnyObject?
    
    init(_ target: AnyObject) {
        self.target = target
        super.init()
    }
    
    override func forwardingTarget(for aSelector: Selector!) -> Any? {
        return target
    }
    
    override func responds(to aSelector: Selector!) -> Bool {
        return target?.responds(to: aSelector) ?? super.responds(to: aSelector)
    }
    
    override func method(for aSelector: Selector!) -> IMP! {
        return target?.method(for: aSelector) ?? super.method(for: aSelector)
    }
    
    override func isEqual(_ object: Any?) -> Bool {
        return target?.isEqual(object) ?? super.isEqual(object)
    }
    
    override func isKind(of aClass: AnyClass) -> Bool {
        return target?.isKind(of: aClass) ?? super.isKind(of: aClass)
    }
    
    override var superclass: AnyClass? {
        return target?.superclass
    }
    
    override func isProxy() -> Bool {
        return target?.isProxy() ?? super.isProxy()
    }
    
    override var hash: Int {
        return target?.hash ?? super.hash
    }
    
    override var description: String {
        return target?.description ?? super.description
    }
    
    override var debugDescription: String {
        return target?.debugDescription ?? super.debugDescription
    }
    
    deinit { print("deinit:\t\(classForCoder)") }
}

感謝 LEE大神