NSTimer：防止内存泄漏

思路：创建一个Timer的分类，添加两个类方法，一个是创建实例的方法（target设置为Timer类对象，传入block）；
另一个是timerAction的方法（执行block）；
这样另外一个持有timer的对象和timer就没有相互引用，就可以在deinit中执行timer的invalidate方法。

1. 创建Timer的Extension

import Foundation

class Block<T> {
    let f : T
    init(_ f: T) { self.f = f }
}

extension Timer {
    class func app_scheduledTimer(withTimeInterval interval: TimeInterval, repeats: Bool, block: @escaping (Timer) -> Swift.Void) -> Timer {
        if #available(iOS 10.0, *) {
            return Timer.scheduledTimer(withTimeInterval: interval, repeats: repeats, block: block)
        }
        return Timer.scheduledTimer(timeInterval: interval, target: self, selector: #selector(app_timerAction), userInfo: Block(block), repeats: repeats)
    }
    
    @objc class func app_timerAction(_ sender: Timer) {
        if let block = sender.userInfo as? Block<(Timer) -> Swift.Void> {
            block.f(sender)
        }
    }
}

2. 使用

var time: Int = 60
var timer: Timer?

func creatTimer() {
    timer = Timer.app_scheduledTimer(withTimeInterval: 1, repeats: true, block: { [weak self] (_) in
            guard let `self` = self else { return }
            self.time -= 1
            if self.time == 0 {
                print("timer结束")
                self.timer?.invalidate()
                self.timer = nil
            } else {
                print("timer执行中 -- \(self.time)s")
            }
        })
}

deinit {
    // deinit中需要将timer释放    
    timer?.invalidate()
    timer = nil
}