第一次调用更改后，UIView alpha不会更新。 （快速4.2）

Question

我有一个UIView，当用户打开滑出菜单时，我想在其中将alpha值从0更改为0.5。当用户点击变暗的区域时，alpha值应回到0。当前，当轻按菜单按钮时，alpha值将变为0.5，从而为视图添加了暗淡效果。但是，一个断点和打印语句显示，当点击UIView时，将alpha更改回0的行将运行，但是UI仍显示0.5 alpha。我看过的所有地方的代码都完全相同，所以我不确定自己在做什么错。

let dimView = UIView()
func setupMenuButton() {
    let menuButton = UIBarButtonItem(title: "Menu", style: .plain, target: self, action: #selector(showMenu))
    navigationItem.rightBarButtonItem = menuButton
}

@objc func showMenu() {
    //TODO: present menu and dim background
    if let window = UIApplication.shared.keyWindow {
        let dimView = UIView()
        dimView.backgroundColor = UIColor.black
        dimView.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(dismissDimView)))
        window.addSubview(dimView)
        dimView.frame = window.frame
        dimView.alpha = 0
        UIView.animate(withDuration: 0.5, animations: {
            dimView.alpha = 0.5
        })
    }
}

@objc func dismissDimView() {
    UIView.animate(withDuration: 0.5, animations: {
        self.dimView.alpha = 0
        print("dim view is not transparent")
    })
}

Answer 1

在dimView中创建的showMenu与第一行中创建的dimView不同。您正在dimView中创建一个全新的showMenu。

解决此问题的一种方法是不在dimView中创建新的showMenu，而改用在外部声明的变量：

@objc func showMenu() {
    //TODO: present menu and dim background
    if let window = UIApplication.shared.keyWindow {
        // notice I deleted a line here
        dimView.backgroundColor = UIColor.black
        dimView.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(dismissDimView)))
        window.addSubview(dimView)
        dimView.frame = window.frame
        dimView.alpha = 0
        UIView.animate(withDuration: 0.5, animations: {
            dimView.alpha = 0.5
        })
    }
}

@objc func dismissDimView() {
    UIView.animate(withDuration: 0.5, animations: {
        self.dimView.alpha = 0
        // here I remove the dimView from the window so that it can be added back in the next time showMenu is called
    }, completion: { [weak self] _ in self?.dimView.removeFromSuperView() })
}