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using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;
[assembly: ExportRenderer(typeof(NavigationPage), typeof(CustomNavigationRenderer))]
public class CustomNavigationRenderer : NavigationRenderer
{
    private const string fontName = "フォントのPostScript名";

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();

        //ナビゲーションバーのスタイルを適用する
        //iOS13以降はAppDelegateでも実施
        CustomNavigationRenderer.SetNavigationStyle(this);
    }

    /// <summary>
    /// ナビゲーションバーのスタイルを適用する
    /// </summary>
    /// <param name="instance">NavigationRendererのインスタンス</param>
    public static void SetNavigationStyle(IVisualElementRenderer instance)
    {
        //ナビゲーションのスタイル変更
        UINavigationBar.Appearance.BarTintColor = UIColor.White;
        UINavigationBar.Appearance.TintColor = UIColor.White;
        UINavigationBar.Appearance.BackgroundColor = UIColor.Blue;

        //タイトル用のフォント
        var titleAttr = new UIStringAttributes()
        {
            Font = UIFont.FromName(fontName, 20),
            ForegroundColor = UIColor.White,
            BackgroundColor = UIColor.Blue
        };
        UINavigationBar.Appearance.TitleTextAttributes = titleAttr;

        if (UIDevice.CurrentDevice.CheckSystemVersion(13, 0))
        {
            //ナビゲーションバーボタン用のフォント
            var buttonAttr = new UIStringAttributes()
            {
                Font = UIFont.FromName(fontName, 16),
                ForegroundColor = UIColor.White,
                BackgroundColor = UIColor.Blue
            };

            var appr = new UINavigationBarAppearance();
            appr.ConfigureWithTransparentBackground();
            appr.TitleTextAttributes = titleAttr;

            var dic = new NSDictionary<NSString, NSObject>(
                                buttonAttr.Dictionary.Keys.Select(r => r as NSString).ToArray(),
                                buttonAttr.Dictionary.Values
                                );

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

            var done = new UIBarButtonItemAppearance(UIBarButtonItemStyle.Done);
            done.Normal.TitleTextAttributes = dic;
            done.Highlighted.TitleTextAttributes = dic;
            appr.DoneButtonAppearance = done;

            UINavigationBar.Appearance.StandardAppearance = appr;
            UINavigationBar.Appearance.ScrollEdgeAppearance = appr;
            UINavigationBar.Appearance.CompactAppearance = appr;

            if (instance != null)
            {
                //戻るボタンがフォント変更されない場合
                var bar = instance.NativeView?.Subviews?.Where(r => r.GetType().BaseType == typeof(UIKit.UINavigationBar)).FirstOrDefault();
                if (bar != null)
                {
                    ((UIKit.UINavigationBar)bar).StandardAppearance = appr;
                }
            }
        }
    }
}

[Register("AppDelegate")]
public partial class AppDelegate : global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate
{
    public override bool FinishedLaunching(UIApplication app, NSDictionary options)
    {
        global::Xamarin.Forms.Forms.Init();
        LoadApplication(new App());

        //ナビゲーションタイトルの色変更
        //NavigationRendererに実装している機能を呼び出す
        CustomNavigationRenderer.SetNavigationStyle(null);

        return base.FinishedLaunching(app, options);
    }
}

using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;
[assembly: ExportRenderer(typeof(Page), typeof(CustomPageRenderer))]
public class CustomPageRenderer : PageRenderer
{
    private const string fontName = "フォントのPostScript名";

    public override void ViewWillAppear(bool animated)
    {
        if (this.NavigationController == null ||
            this.NavigationController.TopViewController == null)
        {
            return;
        }

        UINavigationItem navigationItem = this.NavigationController.TopViewController.NavigationItem;
        if (navigationItem != null)
        {
            //戻るボタンもフォント変更する場合
            navigationItem.BackBarButtonItem.SetTitleTextAttributes(new UITextAttributes()
            {
                Font = UIFont.FromName(fontName, 16),
                //イメージピッカーのキャンセルボタンまで白文字になって見えないので、TextColorは指定しない
                //TextColor = UIColor.White,
            }, UIControlState.Normal);
        }
    }
}

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