当 ContentPage 具有 SizeChanged 事件侦听器时，Xamarin.Forms Java.Lang.NullPointerException

Question

I'm getting the following error message when I try to overwrite a Xamarin.Forms ContentPage.Content .

I just get the message if the new ContentPage has a SizeChanged event listener.

Java.Lang.NullPointerException
  Message=Attempt to read from field 'int android.view.ViewGroup$LayoutParams.width' on a null object reference

Code of the ContentPage I want to load

    public class ShowSizePage : ContentPage
    {
        readonly Label label;
        public ShowSizePage()
        {
            label = new Label
            {
                FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center,
                Text = "initialized",
            };

            SizeChanged += OnPageSizeChanged;
            Content = label;
        }
        void OnPageSizeChanged (object sender, EventArgs args)
        {
            label.Text = String.Format("{0} \u00D7 {1}", Width, Height);
        }

    }

Code from the caller of above ContentPage

// Part of an other ContentPage
        void OnShowSizeClicked (object sender, EventArgs args)
        {
            ContentPage c = new ShowSizePage();

            Content = c.Content; // Exception is caused here
        }

I don't get the Exception if I call ShowSizePage directly from my MainPage .

What do I have to change to avoid the Exception?

Edit minimal reproducible example code:

using System;
using Xamarin.Forms;

namespace NP_Exception
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            // Change this to false to see that ShowSizePage works properly if not loaded through MenuPage
            bool showMenu = true;

            if (showMenu)
                MainPage = new MenuPage(); // Throws exception
            else
                MainPage = new ShowSizePage(); // Works
        }
    }

    public class MenuPage : ContentPage
    {
        readonly StackLayout menu = new StackLayout();
        readonly ScrollView menuView = new ScrollView();
        public MenuPage ()
        {
            Button showSizeButton = new Button { Text = "Show Size" };
            showSizeButton.Clicked += OnShowSizeClicked;
            menu.Children.Add(showSizeButton);

            Button showLabelButton = new Button { Text = "Show Label" };
            showLabelButton.Clicked += OnShowLabelClicked;
            menu.Children.Add(showLabelButton);

            menuView.Content = menu;

            Content = menuView;
        }    
        void OnShowSizeClicked(object sender, EventArgs args)
        {
            ContentPage c = new ShowSizePage();

            Content = c.Content; // Exception is caused here
        }

        void OnShowLabelClicked(object sender, EventArgs args)
        {
            ContentPage c = new ShowLabelPage();

            Content = c.Content;
        }

        protected override bool OnBackButtonPressed()
        {
            Content = menuView;

            return true;
        }
    }
    public class ShowSizePage : ContentPage
    {
        readonly Label label;
        public ShowSizePage()
        {
            label = new Label
            {
                FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center,
                Text = "initialized",
            };

            SizeChanged += OnPageSizeChanged;
            Content = label;
        }
        void OnPageSizeChanged(object sender, EventArgs args)
        {
            label.Text = String.Format("{0} \u00D7 {1}", Width, Height);
        }

    }
    public class ShowLabelPage : ContentPage {
        public ShowLabelPage ()
        {
            Content = new Label { Text = "Hello World! Press hardware back button for menu." };
        }
    }
}

Update: The exception occurs on Xamarin.Forms v4.4.0.991265 but seems to be fixed in v4.6.0.726

Answer 1

I found a solution to get rid of the Exception while getting the desired result.

I had to wrap the Page Content into a ContentView and bind the SizeChanged event to the ContentView

Working Code Sample :

    public class ShowSizePage : ContentPage
    {
        readonly Label label;
        public ShowSizePage()
        {
            label = new Label
            {
                FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center,
                Text = "initialized",
            };

            // Wrap Content in ContentView to avoid Exception
            ContentView view = new ContentView { Content = label };

            // Bind SizeChanged event to view
            view.SizeChanged += OnPageSizeChanged;

            // Set View as Content
            Content = view;
        }
        void OnPageSizeChanged(object sender, EventArgs args)
        {
            // Use Width & Height of the sender object
            View view = (View)sender;

            label.Text = String.Format("{0} \u00D7 {1}", view.Width, view.Height);
        }
    }

Answer 2

This issue has been fixed in the latest version of Xamrin.forms v4.6.0.726 . Anyone who meet this issue with earlier version can update your Xamarin.Forms to solve it.