If you don't want to use CurrentPageChanged event of TabbedPage, you could convert it to command using:
https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/behaviors/reusable/event-to-command-behavior
But if you want to collapse the control when the page reopens you could handle the OnAppearing of content page. Collapse all when it fires.
Try to convert it to command too due to your requirements.

BackgroundImageSource is a property of Page and it changes the content page's background image.
Shell is not a normal content page so it changes nothing when you used it.
If you want to change the flyout page's background image, please use FlyoutBackgroundImage as you mentioned.
You could also set it on a specific page like:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShellSample.Views.AboutPage"
             xmlns:vm="clr-namespace:ShellSample.ViewModels"
             Title="{Binding Title}"
             BackgroundImageSource="bgimage.png">

And use this page in shell:

<Tab Title="About" Icon="tab_about.png">
    <ShellContent ContentTemplate="{DataTemplate local:AboutPage}" />
</Tab>

Then this image will be displayed on the about page.
Moreover, do not forget to integrate this image in the drawable folder with action behavior AndroidResource.

Here we need webview custom renderer , we could detect the url in method OnPageFinished inside class WebViewClient ,the method triggers when webview finish loading .

Simple sample

    [assembly: ExportRenderer(typeof(Xamarin.Forms.WebView), typeof(MyRenderer))]
    namespace App1.Droid
    {
        class MyRenderer : WebViewRenderer
        {
            readonly Context c = null;
            public MyRenderer(Context context):base(context)
            {
                c = context;
            }

            protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.WebView> e)
            {
                base.OnElementChanged(e);
                if (e.OldElement == null)
                {
                    // lets get a reference to the native control
                    var webView = (global::Android.Webkit.WebView)Control;
                    webView.SetWebViewClient(new MyWebViewClient());
                    webView.SetInitialScale(0);
                    webView.Settings.JavaScriptEnabled = true;
                }
            }
        }

        public class MyWebViewClient : WebViewClient
        {
            public override void OnPageFinished(Android.Webkit.WebView view, string url)
            {
                base.OnPageFinished(view, url);

                if (url == "example.com")
                {
                    view.Settings.SetSupportZoom(true);
                }      
                else
                {
                    view.Settings.SetSupportZoom(false);
                }              
            }
        }
    }

So how can I invoke a function when the app is in the background? Are there any NuGet packages for this or we need to do this using the dependency service?

For this feature, try to achieve it on the native project. On Android, you could create a service and make the service is always running(use the foreground service on the api 26 or above). Add the function code in the OnStartCommand method of the custom service class. Then use DependencyService to call the code in pcl.

[assembly: Dependency(typeof(StartServiceDroid))]
namespace DependencyServiceDemos.Droid
{
    public class StartServiceDroid : IStartService
    {
        public void StartForegroundServiceCompat()
        {
            var intent = new Intent(MainActivity.Instance, typeof(CustomService));

            if (Android.OS.Build.VERSION.SdkInt >= Android.OS.BuildVersionCodes.O)
            {
                MainActivity.Instance.StartForegroundService(intent);
            }
            else
            {
                MainActivity.Instance.StartService(intent);
            }
        }
    }

    [Service]
    public class CustomService : Service
    {
        ...
        public override StartCommandResult OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
        {
            CreateNotificationChannel();
            string messageBody = "service starting";

            var notification = new Notification.Builder(this, "10111")
            .SetContentTitle("Foreground")
            .SetContentText(messageBody)
            .SetSmallIcon(Resource.Drawable.main)
            .SetOngoing(true)
            .Build();
            StartForeground(SERVICE_RUNNING_NOTIFICATION_ID, notification);

            //=======you can do you always running work here.=====
            var startTimeSpan = TimeSpan.Zero;
            var periodTimeSpan = TimeSpan.FromSeconds(10);

            var timer = new System.Threading.Timer((e) =>
            {
                MyFunction();
            }, null, startTimeSpan, periodTimeSpan);

            return StartCommandResult.Sticky;
        }
    }
}

In ios, it could not work well. Because tasks in ios are under a strict time limit, and typically get about 600 seconds (10 minutes) of processing time after an application has moved to the background.

Check the tutorial:
https://docs.microsoft.com/en-us/xamarin/ios/app-fundamentals/backgrounding/ios-backgrounding-techniques/ios-backgrounding-with-tasks

This is a known issue and reported here : https://github.com/xamarin/Xamarin.Forms/issues/3230.

Unfortunately there is no response until now , you could add a comment below to raise the notice from Developing Teams.

The only workaround is that delete GroupShortNameBinding statement to show proper group title and remove the tapping event.

Yes, it is just the case . And this is a known issue .

You can follow up it here: https://github.com/xamarin/Xamarin.Forms/issues/4077

MediaElement does not support m3u8 video format (at least till now) , you can search keyword supported media formats in this docs , it provides the supported video format on Android and UWP .

Will the MediaElement support playing .m3u8 files in the future?

Consider raising issue on github to ask this feature .

Or is there another Xamarin Forms component that could handle m3u8 files?

Try XamarinMediaManager , i see there is a sample playing m3u8 .

@vmetodiev said:
Hi,

When using the RuntimePlatform attribute in some files, the VS cannot resolve the symbol. I have grouped my code in two folders (called "Misc" and "Models").

1. Inside the Models directory, the Device.RuntimePlatform is found and works as expected.

2. Inside the Misc directory (manually created by me), I get "The name 'Device.RuntimePlatform' does not exist in the current context." and the "using Xamarin.Forms" statement stays greyed out.

The Xamarin.Forms."Device" assembly is present and the RuntimePlatform attribute is there.

May you please advise how to figure this out?

For one of the files in which "using Xamarin.Forms" stays greyed out, expand the project in Solution Explorer that that file is part of. Right click on the project, and select "Manage NuGet Packages" from the pop-up menu. Select "Installed". Check that "Xamarin.Forms" appears in the list that appears. If it doesn't, that's the problem - Xamarin.Forms is not associated with the project containing the file that you want to use Xamarin.Forms.

BTW - why do your Models use Device.RuntimePlatform? Or was that just a test and they don't actually use Device.RuntimePlatform?

Highlight the image xamarin forms

You could use the Frame to wrap the image, make the frame's size is larger than the size of the image. Then you could specify a value to the background color of the frame, the highlight will be shown. For the clik function, you could add a tap gesture to the image.

Check the code:

<Frame x:Name="frame" Padding="0" HeightRequest="93" WidthRequest="93" HorizontalOptions="Center">
    <Image Source="grid_" HeightRequest="80" WidthRequest="80" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand">
        <Image.GestureRecognizers>
            <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped"/>
        </Image.GestureRecognizers>
    </Image>
</Frame>

Tapped event:

private void TapGestureRecognizer_Tapped(object sender, EventArgs e)
{
    frame.BackgroundColor = Color.Red;
}

@mhmqi said:
is there anyway many for several listview to make many
then i determine which one i chose?

It's not entirely clear what you mean. It sounds like you want a DataTemplateSelector. See https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/templates/data-templates/selector , which contains samples for both XAML and C#.

@mhmqi said:
but i didn't used xaml in my app
because i'm using C# markup is there anyway to represent that by C# markup

When using the forums, it's worth mentioning in questions that you are using C# markup for UI stuff, as most people will provide answers using XAML by default.