Can't change UINavigationBar prompt color

问题:

I am unable to change the prompt color on my navigation bar. I've tried the code below in viewDidLoad, but nothing happens.

self.navigationController?.navigationBar.titleTextAttributes = [NSAttributedStringKey.foregroundColor: UIColor.white]

Am I missing something? Is the code above wrong?

回答1:

It seems like you're right about this one. You need to use UIAppearance to style the prompt text on iOS 11.

I've filed radar #34758558 that the titleTextAttributes property just stopped working for prompt in iOS 11.

The good news is that there are a couple of workarounds, which we can uncover by using Xcode's view hierarchy debugger:

// 1. This works, but potentially changes *all* labels in the navigation bar. 
// If you want this, it works.
UILabel.appearance(whenContainedInInstancesOf: [UINavigationBar.self]).textColor = UIColor.white

The prompt is just a UILabel. If we use UIAppearance's whenContainedInInstancesOf:, we can pretty easily update the color the way we want.

If you look closely, you'll notice that there's also a wrapper view on the UILabel. It has its own class that might respond to UIAppearance...

// 2. This is a more precise workaround but it requires using a private class.

if let promptClass = NSClassFromString("_UINavigationBarModernPromptView") as? UIAppearanceContainer.Type
{
  UILabel.appearance(whenContainedInInstancesOf: [promptClass]).textColor = UIColor.white
}

I'd advise sticking to the more general solution, since it doesn't use private API. (App review, etc.) Check out what you get with either of these two solutions:

回答2:

I was able to make the prompt color white on iOS 11 was setting the barStyle to black. I set the other color attributes (like the desired background color) using the appearance proxy:

myNavbar.barStyle = UIBarStyleBlack; // Objective-C
myNavbar.barStyle = .black // Swift

回答3:

You may use

for view in self.navigationController?.navigationBar.subviews ?? [] {
    let subviews = view.subviews
    if subviews.count > 0, let label = subviews[0] as? UILabel {
        label.textColor = UIColor.white
        label.backgroundColor = UIColor.red
    }
}

It will be a temporary workaround until they'll fix it

回答4:

More complicated version to support old and new iOS

    func updatePromptUI(for state: State) {
    if (state != .Online) {
        //workaround for SOFT-7019 (iOS 11 bug - Offline message has transparent background)
        if #available(iOS 11.0, *) {
            showPromptView()
        } else {
            showOldPromptView()
        }
    }
    else {
        self.navigationItem.prompt = nil
        if #available(iOS 11.0, *) {
            self.removePromptView()
        } else {
            self.navigationController?.navigationBar.titleTextAttributes = nil
            self.navigationController?.navigationBar.titleTextAttributes = [NSAttributedStringKey.foregroundColor:UIColor.lightGray]
        }
    }
}

private func showOldPromptView() {
    self.navigationItem.prompt = "Network Offline. Some actions may be unavailable."
    let navbarFont = UIFont.systemFont(ofSize: 16)
    self.navigationController?.navigationBar.titleTextAttributes = [NSAttributedStringKey.font: navbarFont, NSAttributedStringKey.foregroundColor:UIColor.white]
}

private func showPromptView() {
    self.navigationItem.prompt = String()
    self.removePromptView()

    let promptView = UIView(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: 18))
    promptView.backgroundColor = .red
    let promptLabel = UILabel(frame: CGRect(x: 0, y: 2, width: promptView.frame.width, height: 14))
    promptLabel.text = "Network Offline. Some actions may be unavailable."
    promptLabel.textColor = .white
    promptLabel.textAlignment = .center
    promptLabel.font = promptLabel.font.withSize(13)
    promptView.addSubview(promptLabel)
    self.navigationController?.navigationBar.addSubview(promptView)
}

private func removePromptView() {
    for view in self.navigationController?.navigationBar.subviews ?? [] {
        view.removeFromSuperview()
    }
}

回答5:

I suggest using a custom UINavigationBar subclass and overriding layoutSubviews:

- (void)layoutSubviews {
    [super layoutSubviews];

    if (self.topItem.prompt) {
        UILabel *promptLabel = [[self recursiveSubviewsOfKind:UILabel.class] selectFirstObjectUsingBlock:^BOOL(UILabel *label) {
            return [label.text isEqualToString:self.topItem.prompt];
        }];
        promptLabel.textColor = self.tintColor;
    }
}

Basically I'm enumerating all UILabels in the subview hierarchy and check if their text matches the prompt text. Then we set the textColor to the tintColor (feel free to use a custom color). That way, we don't have to hardcode the private _UINavigationBarModernPromptView class as the prompt label's superview. So the code is be a bit more future-proof.

Converting the code to Swift and implementing the helper methods recursiveSubviewsOfKind: and selectFirstObjectUsingBlock: are left as an exercise to the reader