Xamarin形式Android自定义TabbedPage ActionBar返回null

Question

我在我的项目中使用custom TabbedPage在Tab bar中显示badges(count)。我正在使用下面的代码来显示custom tab bar，但它总是返回null值CustomTabbedPageRenderer.cs类OnWindowVisibilityChanged方法的activity.ActionBar。我已经尝试了许多解决办法如改变Theme为Theme.AppCompat.Light.DarkActionBar和加入Window.RequestFeature(WindowFeatures.ActionBar);MainActivity.cs下面的线，但遗憾的是这些并没有帮助我。

MainActivity.cs

namespace Bakery.Droid 
    { 
     [Activity(Label = "Bakery.Droid", Icon = "@mipmap/icon_launcher", Theme = "@style/MyTheme", MainLauncher = false, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation, ScreenOrientation = ScreenOrientation.Portrait)] 
     public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity 
     { 
       protected override void OnCreate(Bundle bundle) 
       { 

        base.OnCreate(bundle); 

        global::Xamarin.Forms.Forms.Init(this, bundle);   

        LoadApplication(new App()); 
       } 


      } 

     } 

Home.cs

using System; 
using System.Collections.Generic; 
using Xamarin.Forms; 

namespace Bakery 
{ 
    public class Home : CustomTabPage 
    { 
     public Home() 
     { 

      NavigationPage.SetHasNavigationBar(this, false); 

      var burger = new NavigationPage(new Burger()); 
      burger.Icon = "burger.png"; 
      burger.Title = "Burger"; 

      var sandwich = new NavigationPage(new Sandwich()); 
      sandwich.Icon = "sandwich.png"; 
      sandwich.Title = "Sandwich"; 

      var pizza = new NavigationPage(new Pizza()); 
      pizza.Icon = "pizza.png"; 
      pizza.Title = "Pizza"; 


      var roll = new NavigationPage(new Roll()); 
      roll.Icon = "roll.png"; 
      roll.Title = "Roll"; 

      //adding childrens into the tab 
      Children.Clear(); 
      Children.Add(burger); 
      Children.Add(sandwich); 
      Children.Add(pizza); 
      Children.Add(roll); 
     } 

     protected override void OnAppearing() 
     { 
      base.OnAppearing();  
     } 
    } 
} 

CustomTabPage.cs

using System; 
using Xamarin.Forms; 

namespace Bakery 
{ 
    public class CustomTabPage : TabbedPage 
    { 

    } 
} 

个styles.xml

<?xml version="1.0" encoding="UTF-8"?> 
<resources> 
    <style name="MyTheme" parent="MyTheme.Base"> 
    </style> 

    <style name="MyTheme.Base" parent="Theme.AppCompat.Light.DarkActionBar"> 

     <item name="windowNoTitle">true</item> 

     <item name="windowActionBar">true</item>   

     <item name="windowActionModeOverlay">true</item>   


    </style> 

</resources> 

CustomTabbedPageRenderer.cs

[assembly: ExportRenderer(typeof(TabbedPage), typeof(CustomTabbedPageRenderer))] 
    namespace Bakery.Droid 
    { 
     public class CustomTabbedPageRenderer : TabbedRenderer 
     { 
      Activity activity; 
      List<string> filenames; 

      protected override void OnElementChanged(ElementChangedEventArgs<TabbedPage> e) 
      { 
       base.OnElementChanged(e); 
       filenames = e.NewElement.Children.Select(t => t.Icon.File).ToList(); 
      } 

      protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e) 
      { 
       base.OnElementPropertyChanged(sender, e); 
       activity = this.Context as Activity; 
      } 

      protected override void OnWindowVisibilityChanged(Android.Views.ViewStates visibility) 
      { 
       try 
       { 
        base.OnWindowVisibilityChanged(visibility); 
        var actionBar = activity.ActionBar; 
        var colorDrawable = new ColorDrawable(Android.Graphics.Color.Yellow); 
        actionBar.SetStackedBackgroundDrawable(colorDrawable); 
        System.Diagnostics.Debug.WriteLine("Onwindow visible"); 
        ActionBarTabsSetup(actionBar); 
       } 
       catch (Exception Exception) 
       { 
        System.Diagnostics.Debug.WriteLine("Exception: " + Exception.ToString()); 
       } 
      } 

      void ActionBarTabsSetup(ActionBar actionBar) 
      { 
       for (var i = 0; i < actionBar.NavigationItemCount; ++i) 
       { 
        var tab = actionBar.GetTabAt(i); 
        var id = GetImageFromFilename(i); 
        if (id != 0) 
         TabSetup(tab, id); 
       } 
      } 

      void TabSetup(ActionBar.Tab tab, int resID) 
      { 
       var relLay = new Android.Widget.RelativeLayout(activity) 
       { 
        LayoutParameters = new LayoutParams(LayoutParams.WrapContent, 180) 

       }; 

       var linLay = new LinearLayout(activity) 
       { 
        LayoutParameters = new LayoutParams(LayoutParams.WrapContent, 180), 
        Orientation = Orientation.Vertical, 
       }; 
       linLay.SetHorizontalGravity(Android.Views.GravityFlags.Center); 
       var imageView = new ImageView(activity); 
       imageView.SetImageResource(resID); 
       imageView.SetPadding(-35, 4, -35, 0); 
       imageView.SetMinimumWidth(60); 

       var textView = new TextView(activity) 
       { 
        Text = tab.Text 
       }; 


       linLay.AddView(imageView); 
       linLay.AddView(textView);  

       relLay.AddView(linLay); 

       var badgeView = new TextView(activity) 
       { 
        Text = "2" 
       }; 

       var badgeImageView = new ImageView(activity); 
       badgeImageView.SetImageResource(Resource.Drawable.red); 
       badgeImageView.SetMinimumWidth(5); 
       badgeImageView.SetMinimumHeight(5); 
       badgeImageView.SetPadding(77, 5, 0, 0); 
       badgeView.SetPadding(85, 0, 0, 0); 

       relLay.AddView(badgeImageView); 
       relLay.AddView(badgeView); 
       tab.SetCustomView(relLay); 
      } 

      int GetImageFromFilename(int n) 
      { 
       var filename = filenames[n].Split('.'); 
       var id = Resources.GetIdentifier(filename[0], "drawable", activity.PackageName); 
       return id; 
      } 
     } 
    } 

Answer 1

我使用下面的代码显示自定义标签栏，但它总是返回null值CustomTabbedPageRenderer.cs类OnWindowVisibilityChanged方法的activity.ActionBar。

该ActionBar是null，因为它从来没有在活动中设置。您需要先设置动作条：

1. 在MainActivity.cs设置SupportActionBar象下面这样：

   //Create your own theme "MyTheme" 
   [Activity(Label = "CustomTabbedPageDemo", Icon = "@drawable/icon", Theme = "@style/MyTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)] 
   public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity 
   { 
       protected override void OnCreate(Bundle bundle) 
       { 
        //Get the ActionBar Layout in Resouce\layout\Toolbar.axml 
        var toolbar=(Toolbar)LayoutInflater.Inflate(Resource.Layout.Toolbar, null); 
        //Set the Support ActionBar 
        SetSupportActionBar(toolbar); 
        SupportActionBar.Title = "My ActionBar"; 
        base.OnCreate(bundle); 
   
        global::Xamarin.Forms.Forms.Init(this, bundle); 
       LoadApplication(new App()); 
       } 
   } 
   

2. 创建自己的主题替换默认的一个。 Xamarin.Forms要求主题从Theme.AppCompat.XXX.XXX继承：

   <style name="MyTheme" parent="Theme.AppCompat.Light.DarkActionBar"> 
       <item name="windowNoTitle">true</item> 
       <item name="windowActionBar">false</item> 
       <item name="colorPrimary">#5A8622</item> 
   </style> 
   

3. 基本Toolbar.axml例如：

   <?xml version="1.0" encoding="utf-8"?> 
   <android.support.v7.widget.Toolbar 
       xmlns:android="http://schemas.android.com/apk/res/android" 
       android:id="@+id/toolbar" 
       android:layout_width="match_parent" 
       android:layout_height="wrap_content" 
       android:background="?attr/colorPrimary" 
       android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/> 
   

4. 修改您CustomTabPageRenderer.cs从Android.App改变ActionBar的命名空间using Android.Support.V7.App和检索ActionBar通过activity.SupportActionBar：

   using Android.Support.V7.App; 
   ... 
   protected override void OnWindowVisibilityChanged(Android.Views.ViewStates visibility) 
   { 
       try 
       { 
        base.OnWindowVisibilityChanged(visibility); 
        //get the support actionbar 
        var actionBar = activity.SupportActionBar; 
   
        var colorDrawable = new ColorDrawable(Android.Graphics.Color.Yellow); 
        actionBar.SetStackedBackgroundDrawable(colorDrawable); 
        System.Diagnostics.Debug.WriteLine("Onwindow visible"); 
   
        ActionBarTabsSetup(actionBar); 
       } 
       catch (Exception Exception) 
       { 
        System.Diagnostics.Debug.WriteLine("Exception: " + Exception.ToString()); 
       } 
   }