Xamarin.ios 的 CustomTabRenderer

Question

我已经使用 Xamarin 几个月了，但仍然运行时不时遇到一些麻烦。我正在使用自定义渲染器来更改选项卡的外观。在不同的选项卡之间切换时，它工作得很好。但是，当我使用以下方式导航到我的 tabbedPage 时：

Children.Add(new UnitMapPage { Title = "Map", Icon = "map.png" });
CurrentPage = mapPage;

页面在正确的选项卡上打开，但 TabbarItem 的文本看起来与未选中的项目完全一样。只要您点击任何选项卡，它就会更改为正确的样式。这是我的自定义渲染器。

class CustomTabRenderer : TabbedRenderer
{
    private UnitViewModel _unitViewModel;
    private TabbedPage _unitPage;

    protected override void OnElementChanged(VisualElementChangedEventArgs e)
    {
        base.OnElementChanged(e);

        if (e.NewElement != null) {
            var unitPage = (TabbedPage)e.NewElement;
            _unitPage = unitPage;
            _unitViewModel = (UnitViewModel)_unitPage.BindingContext;
            _unitViewModel.PropertyChanged += OnElementPropertyChanged;
        }

        // Set Text Font for unselected tab states
        UITextAttributes normalTextAttributes = new UITextAttributes();
        normalTextAttributes.Font = UIFont.SystemFontOfSize(10.0F, UIFontWeight.Regular); // unselected
        normalTextAttributes.TextColor = UIColor.White;

        UITabBarItem.Appearance.SetTitleTextAttributes(normalTextAttributes, UIControlState.Normal);
    }

    void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "AlertCount")
        {
            if (TabBar.Items != null)
            {
                foreach (var item in TabBar.Items)
                {
                    item.Image = item.Image.ImageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal);
                    if (item.Title == "History" && _unitViewModel != null)
                    {
                        if (_unitViewModel.AlertCount > 0)
                            item.BadgeValue = _unitViewModel.AlertCount.ToString();
                        else
                            item.BadgeValue = null;
                    }
                }
            }
        }
    }

    public override void ViewWillAppear(bool animated)
    {
        if (TabBar.Items != null)
        {
            foreach (var item in TabBar.Items)
            {
                item.Image = item.Image.ImageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal);
                if (item.Title == "History" && _unitViewModel != null)
                {
                    if (_unitViewModel.AlertCount > 0)
                        item.BadgeValue = _unitViewModel.AlertCount.ToString();
                    else
                        item.BadgeValue = null;
                }
            }
        }
        base.ViewWillAppear(animated);
    }

    public override UIViewController SelectedViewController
    {
        get
        {
            UITextAttributes selectedTextAttributes = new UITextAttributes();
            selectedTextAttributes.Font = UIFont.SystemFontOfSize(12.0F, UIFontWeight.Heavy); // SELECTED
            if (base.SelectedViewController != null)
            {
                base.SelectedViewController.TabBarItem.SetTitleTextAttributes(selectedTextAttributes, UIControlState.Normal);
            }
            return base.SelectedViewController;
        }
        set
        {
            base.SelectedViewController = value;

            foreach (UIViewController viewController in base.ViewControllers)
            {
                UITextAttributes normalTextAttributes = new UITextAttributes();
                normalTextAttributes.Font = UIFont.SystemFontOfSize(10.0F, UIFontWeight.Regular); // unselected
                normalTextAttributes.TextColor = UIColor.White;

                viewController.TabBarItem.SetTitleTextAttributes(normalTextAttributes, UIControlState.Normal);
            }
        }
    }
}

Answer 1

我相信你只需要交换：

CurrentPage = yourPage;

和

SelectedItem = yourPage;

Answer 2

当我将正在测试的设备从 ios9 升级到 ios10 时，问题似乎已经出现（无法确认这一点，因为我无法找到 ios9 我可以测试的设备）。

通读关于这个 post 的答案：How do I override the Xamarin Forms TabbedPage item fonts for iOS?，让我自己尝试了各种东西 ViewWillAppear。

通过更改我的代码以在 ViewWillAppear 中设置 selectedTextAttributes 解决了这个问题。不确定这是否是最好的方法，但它成功了！

 public override void ViewWillAppear(bool animated)
    {
        if (TabBar.Items != null)
        {
            foreach (var item in TabBar.Items)
            {
                item.Image = item.Image.ImageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal);
                if (item.Title == "History" && _unitViewModel != null)
                {
                    if (_unitViewModel.AlertCount > 0)
                        item.BadgeValue = _unitViewModel.AlertCount.ToString();
                    else
                        item.BadgeValue = null;
                }

                if (item.Title.Equals(_unitPage.CurrentPage.Title))
                {
                    UITextAttributes selectedTextAttributes = new UITextAttributes();
                    selectedTextAttributes.Font = UIFont.SystemFontOfSize(12.0F, UIFontWeight.Heavy); // SELECTED
                    if (base.SelectedViewController != null)
                    {
                        base.SelectedViewController.TabBarItem.SetTitleTextAttributes(selectedTextAttributes, UIControlState.Normal);
                    }
                }
            }
        }
        base.ViewWillAppear(animated);
    }

Xamarin.ios 的 CustomTabRenderer

CustomTabRenderer for Xamarin.ios

custom-renderer

xamarin.ios

xamarin

xamarin.forms

tabbedpage