在 Xamarin Forms iOS 13+ 应用程序中设置导航栏按钮上的字体

Question

我正在尝试在 Xamarin iOS (13+) 应用程序中设置导航按钮（特别是后退按钮）的字体。我发现这个 this solution 声称可以工作但没有，至少对我来说：

var style = new UINavigationBarAppearance();

var a = new UIStringAttributes();
a.Font = UIFont.SystemFontOfSize(35);
a.ForegroundColor = UIColor.Yellow;
style.TitleTextAttributes = a;

var dic = new NSDictionary<NSString, NSObject>(
            new NSString[] {a.Dictionary.Keys[0] as NSString, a.Dictionary.Keys[1] as NSString }, a.Dictionary.Values
);
   
var button = new UIBarButtonItemAppearance(UIBarButtonItemStyle.Plain);
button.Normal.TitleTextAttributes = dic;
button.Highlighted.TitleTextAttributes = dic;
style.ButtonAppearance = button;

UINavigationBar.Appearance.StandardAppearance = style;

然后我将它简化了一点（看看我是否可以简单地重新着色导航栏标题）但这又什么也没做：

var style = new UINavigationBarAppearance();

var a = new UIStringAttributes()
{
    ForegroundColor = UIColor.Yellow
};
style.TitleTextAttributes = a;

UINavigationBar.Appearance.StandardAppearance = style;

玩得更远，我奇怪地发现，以下内容确实会重新着色导航栏标题：

var a = new UIStringAttributes()
{
    ForegroundColor = UIColor.Yellow
};

UINavigationBar.Appearance.TitleTextAttributes = a;

...但是使用相同的思考过程，这不会重新着色导航栏按钮：

var a = new UITextAttributes()
{
    TextColor = UIColor.Yellow
};

UIBarButtonItem.Appearance.SetTitleTextAttributes(a, UIControlState.Normal);

谁能明白为什么这些方法都不能用于修改导航栏按钮的样式？或者这可能是 Xamarin 的问题？

更新：我意识到第一个示例确实有效，但仅在 Xamarin iOS 中。但是，它在 Xamarin Forms 中不起作用。看起来 Xamarin Forms 可能会强制执行其自己的标准外观，这会覆盖我在代码中输入的内容。

Answer 1

在您的情况下，我们可以设置自定义 NavigationBar 。

在您的视图控制器中

如果你想让它在所有控制器上工作，你可以定义一个基本的 ViewController

 public override void ViewWillAppear(bool animated)
    {
        base.ViewWillAppear(animated);
        var page = Element as ContentPage;
        NavigationController.NavigationBar.Hidden = true;
        double height = IsiphoneX();
        UIView backView = new UIView()
        {
            BackgroundColor = UIColor.White,
            Frame = new CGRect(0,20,UIScreen.MainScreen.Bounds.Width, height),
        };
        UIButton backBtn = new UIButton() {
            Frame = new CGRect(20, height-44, 40, 44),
            Font = UIFont.SystemFontOfSize(18),  // set style here
        } ;
        backBtn.SetTitle("Back", UIControlState.Normal);
        backBtn.SetTitleColor(UIColor.Blue, UIControlState.Normal);
        backBtn.AddTarget(this,new Selector("GoBack"),UIControlEvent.TouchUpInside);
        UILabel titleLabel = new UILabel() {
            Frame=new CGRect(UIScreen.MainScreen.Bounds.Width/2-75, 0,150, height),
            Font = UIFont.SystemFontOfSize(20),
            Text = page.Title,
            TextColor = UIColor.Black,
            Lines = 0,
        };
        UILabel line = new UILabel() {
            Frame = new CGRect(0, height, UIScreen.MainScreen.Bounds.Width, 0.5),
            BackgroundColor = UIColor.Black,
        };

        backView.AddSubview(backBtn);
        backView.AddSubview(titleLabel);
        backView.AddSubview(line);
        View.AddSubview(backView);
    }
     double IsiphoneX()
    {
        double height = 44; //set height as you want here !!!
        if (UIDevice.CurrentDevice.CheckSystemVersion(11, 0))
        {
            if (UIApplication.SharedApplication.Delegate.GetWindow().SafeAreaInsets.Bottom > 0.0)
            {
                height = 64;  //set height as you want here !!!
            }
        }
        return height;
    }
    [Export("GoBack")]
    void GoBack()
    {
        NavigationController.PopViewController(true);
    }
    public override void ViewWillDisappear(bool animated)
    {
        base.ViewWillDisappear(animated);

        NavigationController.NavigationBar.Hidden = false;
    }

通过这种方式，您可以根据需要设置所有样式，例如 BackgroundColor 、 TextColor 甚至将后退按钮设置为图像。

Answer 2

我终于找到了一种方法，可以通过覆盖 Xamarin Forms 对其自身导航栏样式的不断执行来实现：

public class MyNavigationRenderer : NavigationRenderer
{
    public override void ViewDidAppear(bool animated)
    {
        base.ViewDidAppear(animated);

        UpdateStyle();

        Element.PropertyChanged += HandlePropertyChanged;
    }

    void UpdateStyle()
    {
        var navigationBarAppearance = NavigationBar.StandardAppearance;
        var titleTextAttributes = new UIStringAttributes
        {
            ForegroundColor = UIColor.Yellow,
            Font = UIFont.SystemFontOfSize(35)
        };

        var dic = new NSDictionary<NSString, NSObject>(
            new NSString[] { titleTextAttributes.Dictionary.Keys[0] as NSString, titleTextAttributes.Dictionary.Keys[1] as NSString }, titleTextAttributes.Dictionary.Values
        );

        var button = new UIBarButtonItemAppearance(UIBarButtonItemStyle.Plain);
        button.Normal.TitleTextAttributes = dic;
        button.Highlighted.TitleTextAttributes = dic;

        navigationBarAppearance.ButtonAppearance = button;

        NavigationBar.StandardAppearance = navigationBarAppearance;
        NavigationBar.CompactAppearance = navigationBarAppearance;
        NavigationBar.ScrollEdgeAppearance = navigationBarAppearance;
    }

    void HandlePropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        UpdateStyle();
    }
}