xamarin - Xamarin iOS 渲染器中使用的 "if (Control == null)"部分是什么？

Question

查看 Xamarin 的示例片段，我看到他们建议创建一个包含三个部分的自定义渲染器:

if (Control == null)

if (e.OldElement != null)

if (e.NewElement != null)

我的渲染器看起来像这样。但是，我不明白我会用什么 if (Control == null)节为。

问题 1) 谁能给我关于如何使用它的建议，以及我的实现是否有任何问题？

问题 2) 我是否正确实现了这一点，我应该如何/在哪里删除 OnTabbarControllerItemSelected处理程序？

问题 3) Control 元素从何而来？那会是我调用的同一个变量 _page或元素 e ?

tabbarController.ViewControllerSelected += OnTabbarControllerItemSelected;

public class TabbedPageRenderer : TabbedRenderer
{
    private MainPage _page;

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

        //if (Control == null)
        //{
        //    // Instantiate the native control
        //}

        if (e.OldElement != null)
        {
            // Unsubscribe from event handlers and cleanup any resources
            e.OldElement.PropertyChanged -= Current_PropertyChanged;
            return;
        }

        if (e.NewElement != null)
        {
            // Configure the control and subscribe to event handlers
            TabBar.Translucent = false;
            TabBar.BackgroundImage = new UIImage();
            _page = (MainPage)e.NewElement;
            e.NewElement.PropertyChanged += Current_PropertyChanged;
        }

        try
        {
            var tabbarController = (UITabBarController)this.ViewController;
            if (tabbarController != null)
            {
                tabbarController.ViewControllerSelected += OnTabbarControllerItemSelected;
                UpdateTheme();
            }
        }
        catch (Exception exception)
        {
            Console.WriteLine(exception);
        }
    }

    private void OnTabbarControllerItemSelected(object sender, UITabBarSelectionEventArgs eventArgs)
    {
        if (!(_page.CurrentPage is Japanese.CardsTabPage) && App.quizRunning == true)
            _page.CurrentPage = App.navCardsTabPage;
    }

    void Current_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Theme")
            UpdateTheme();
    }

    void UpdateTheme()
    {
        switch (_page.Theme)
        {
            default:
            case TH.Light:
                TabBar.SelectedImageTintColor = UIColor.Red;
                break;
            case TH.Dark: 
                TabBar.SelectedImageTintColor = UIColor.Red;
                break;
        }
    }

}

Answer 1

Question 1 & 3

该代码段用于编写自定义渲染器， Control属性(property)在 ViewRenderer类(class)。由于您的控件继承自 TabbedRenderer ，没有必要遵循代码段。

Question 2

ViewController属性返回 TabbedRenderer本身，所以你不需要每次都添加监听器 OnElementChanged调用。考虑将其移至构造函数。

像这样:

public class TabbedPageRenderer : TabbedRenderer
{
    private MainPage _page;

    public TabbedPageRenderer()
    {
        this.ViewControllerSelected += OnTabbarControllerItemSelected;
    }

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

        if (e.OldElement != null)
        {
            // Unsubscribe from event handlers and cleanup any resources
            e.OldElement.PropertyChanged -= Current_PropertyChanged;
            return;
        }

        if (e.NewElement != null)
        {
            // Configure the control and subscribe to event handlers
            TabBar.Translucent = false;
            TabBar.BackgroundImage = new UIImage();
            _page = (MainPage)e.NewElement;
            e.NewElement.PropertyChanged += Current_PropertyChanged;
        }

        if(_page != null)
            UpdateTheme();
    }

    private void OnTabbarControllerItemSelected(object sender, UITabBarSelectionEventArgs eventArgs)
    {
        if (_page != null && !(_page.CurrentPage is Japanese.CardsTabPage) && App.quizRunning == true)
            _page.CurrentPage = App.navCardsTabPage;
    }

    void Current_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Theme")
            UpdateTheme();
    }

    void UpdateTheme()
    {
        switch (_page.Theme)
        {
            default:
            case TH.Light:
                TabBar.SelectedImageTintColor = UIColor.Red;
                break;
            case TH.Dark: 
                TabBar.SelectedImageTintColor = UIColor.Red;
                break;
        }
    }
}