IOS 实现 Toast 的原理

    @objc func touchDown(){
        if let button = button {
            button.backgroundColor = UIColor.cyan
        }
        let toast = UILabel.init(frame: CGRect.init(x: UIScreen.main.bounds.width / 4, y: UIScreen.main.bounds.height/2, width: UIScreen.main.bounds.width/2, height:50))
        if let keyWindow = UIApplication.shared.keyWindow {
            toast.backgroundColor = UIColor.white
            keyWindow.addSubview(toast)
            toast.textColor = UIColor.black
            toast.text = "This is toast!"
            toast.baselineAdjustment = UIBaselineAdjustment.alignCenters
            toast.textAlignment = NSTextAlignment.center
            toast.alpha = 0.2
            keyWindow.bringSubviewToFront(toast)
            UIView.animate(withDuration: 0.3, animations: {
                toast.alpha = 1
            }, completion: {
                f in
                UIView.animate(withDuration: 0.3, animations: {
                    toast.alpha = 0
                }, completion: {
                    f in
                    if f {
                        keyWindow.willRemoveSubview(toast)
                    }
                    
                })
            })
            
        }
    }