swift - UIView alpha 在第一次调用更改它后不会更新。 ( swift 4.2)

Question

我有一个 UIView，我想在用户打开滑出菜单时将 alpha 从 0 更改为 0.5。当用户点击变暗区域时，alpha 应该返回到 0。目前，当点击菜单按钮时，alpha 变为 0.5，为 View 添加暗淡效果。但是，断点和打印语句显示，当点击 UIView 时运行将 alpha 更改回 0 的行，但 UI 仍显示 0.5 alpha。在我看过的所有地方，代码都完全相同，所以我不确定我做错了什么。

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

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

解决此问题的一种方法是不在 showMenu 中创建新的 dimView，而是使用在外部声明的那个:

@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() })
}