在block中对弱引用进行强引用

typealias Closer = ((_ string1 : String, _ string2 : String) -> Void)
var block:Closer?

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    
    let vc = ViewController()
    weak var weakSelf = vc
    
    self.block = { (string1 : String,  string2 : String) -> Void in
        
        print(weakSelf)
        let strong = weakSelf
        
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 5, execute: {
            
            print(strong)
        })
    }
    
    self.block?("", "")
}

打印结果：

Optional(<Demo.ViewController: 0x7fd93a40ace0>)
Optional(<Demo.ViewController: 0x7fd93a40ace0>)

如果block中不进行强引用，而直接强行拆包，会引起奔溃。这在OC中更能体现。
打印结果：

Optional(<Demo.ViewController: 0x7fd93a40ace0>)
nil