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，当用户打开滑出菜单时，我想将其 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，为视图添加暗淡效果。但是，断点和打印语句显示当点击 UIView 行以将 alpha 更改回 0 运行时，但 UI 仍然显示 0.5 alpha。我到处看代码都是完全一样的，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：

@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() })
}

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

UIView alpha does not update after first call to change it. (Swift 4.2)

alpha

uiview

swift