为Xamarin.Forms的导航栏增加搜索功能

    <ContentPage.ToolbarItems>
        <ToolbarItem/>
        <ToolbarItem/>
    </ContentPage.ToolbarItems>

    public interface ISearchPage
    {
        void OnSearchBarTextChanged(string text);
    }

     public class SearchPageRender : PageRenderer
    {
        public SearchPageRender(Context context) : base(context)
        {

        }

        protected override void OnAttachedToWindow()
        {
            base.OnAttachedToWindow();

            if (Element is ISearchPage
                && Element is Page page
                && page.Parent is NavigationPage navigationPage)
            {
                //Workaround to re-add the SearchView when navigating back to an ISearchPage, because Xamarin.Forms automatically removes it
                navigationPage.Popped += HandleNavigationPagePopped;
                navigationPage.PoppedToRoot += HandleNavigationPagePopped;
            }
        }

        //Adding the SearchBar in OnSizeChanged ensures the SearchBar is re-added after the device is rotated, because Xamarin.Forms automatically removes it
        protected override void OnSizeChanged(int w, int h, int oldw, int oldh)
        {
            base.OnSizeChanged(w, h, oldw, oldh);

            if (Element is ISearchPage && Element is Page page && page.Parent is NavigationPage navigationPage && navigationPage.CurrentPage is ISearchPage)
            {
                AddSearchToToolbar(page.Title);
            }
        }

        protected override void Dispose(bool disposing)
        {
            if (GetToolbar() is Android.Support.V7.Widget.Toolbar toolBar)
                toolBar.Menu?.RemoveItem(Resource.Menu.mainmenu);

            base.Dispose(disposing);
        }

        //Workaround to re-add the SearchView when navigating back to an ISearchPage, because Xamarin.Forms automatically removes it
        void HandleNavigationPagePopped(object sender, NavigationEventArgs e)
        {
            if (sender is NavigationPage navigationPage
                && navigationPage.CurrentPage is ISearchPage)
            {
                AddSearchToToolbar(navigationPage.CurrentPage.Title);
            }
        }

        void AddSearchToToolbar(string pageTitle)
        {
            if (GetToolbar() is Android.Support.V7.Widget.Toolbar toolBar
            && toolBar.Menu?.FindItem(Resource.Id.action_search)?.ActionView?.
            JavaCast<Android.Support.V7.Widget.SearchView>().GetType()
            != typeof(Android.Support.V7.Widget.SearchView))
            {
                toolBar.Title = pageTitle;
                toolBar.InflateMenu(Resource.Menu.mainmenu);

                if (toolBar.Menu?.FindItem(Resource.Id.action_search)?.ActionView?.
                    JavaCast<Android.Support.V7.Widget.SearchView>() is
                     Android.Support.V7.Widget.SearchView searchView)
                {
                    searchView.QueryTextChange += SearchView_QueryTextChange;
                    searchView.ImeOptions = (int)ImeAction.Search;
                    searchView.InputType = (int)InputTypes.TextVariationFilter;
                    searchView.MaxWidth = int.MaxValue;
                }
            }
        }

        private void SearchView_QueryTextChange(object sender, Android.Support.V7.Widget.SearchView.QueryTextChangeEventArgs e)
        {
            if (Element is ISearchPage searchPage)
                searchPage.OnSearchBarTextChanged(e.NewText);
        }

        private static Android.Support.V7.Widget.Toolbar GetToolbar() => (CrossCurrentActivity.Current?.Activity as MainActivity)?.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
    }

    <?xml version="1.0" encoding="utf-8" ?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto">
	<item android:id="@+id/action_search"
		  android:title="Search"
		  android:icon="@android:drawable/ic_menu_search"
		  app:showAsAction="always|collapseActionView"
		  app:actionViewClass="android.support.v7.widget.SearchView"/>
</menu>