问题
I am trying to add a tabbed page as shown in the xamarin documentation but I keep getting the following error, System.NullReferenceException "Object reference not set to an instance of an object." Do I need to declare a new object for each page or is this done automatically when using xaml?
Here is my StackTrace,
at Xamarin.Forms.Platform.Android.AppCompat.Platform.LayoutRootPage(Xamarin.Forms.P age page, Int32 width, Int32 height) [0x0005d] in C:\BuildAgent\work\aad494dc9bc9783\Xamarin.Forms.Platform.Android\AppCompat\Plat form.cs:279
at Xamarin.Forms.Platform.Android.AppCompat.Platform.Xamarin.Forms.Platform.Android.IPlatformLayout.OnLayout (Boolean changed, Int32 l, Int32 t, Int32 r, Int32 b) [0x00003] in C:\BuildAgent\work\aad494dc9bc9783\Xamarin.Forms.Platform.Android\AppCompat\Platform.cs:196
at Xamarin.Forms.Platform.Android.PlatformRenderer.OnLayout (Boolean changed, Int32 l, Int32 t, Int32 r, Int32 b) [0x0000e] in C:\BuildAgent\work\aad494dc9bc9783\Xamarin.Forms.Platform.Android\PlatformRenderer.cs:73
at Android.Views.ViewGroup.n_OnLayout_ZIIII (IntPtr jnienv, IntPtr native__this, Boolean changed, Int32 l, Int32 t, Int32 r, Int32 b) [0x00009] in /Users/builder/data/lanes/3415/7db2aac3/source/monodroid/src/Mono.Android/platforms/android-23/src/generated/Android.Views.ViewGroup.cs:3427
at (wrapper dynamic-method) System.Object:d59f31f1-b9b6-40ea-9254-a422fe409750 (intptr,intptr,bool,int,int,int,int)
Here is my code,
using Xamarin.Forms;
namespace BSi.Mobile.Events.Views
{
public partial class MainMasterDetailPage : MasterDetailPage
{
public MainMasterDetailPage()
{
InitializeComponent();
}
}
}
<?xml version="1.0" encoding="utf-8" ?>
<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
xmlns:local="clr-namespace:BSi.Mobile.Events.Views;assembly=BSi.Mobile.Events"
prism:ViewModelLocator.AutowireViewModel="True"
x:Class="BSi.Mobile.Events.Views.MainMasterDetailPage">
<MasterDetailPage.Master>
<ContentPage Title="Default" Padding="10">
<StackLayout>
<Button Text="Home" Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage\MainPage"/>
<Button Text="Event Schedule" Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage\EventPage"/>
<Button Text="Floor Plan" Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage\FloorPlanPage"/>
<Button Text="Sponsor/Media Partners" Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage\SponsorsPage"/>
<Button Text="Where To Eat" Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage\FoodNearbyPage"/>
</StackLayout>
</ContentPage>
</MasterDetailPage.Master>
<MasterDetailPage.Detail>
<TabbedPage>
<TabbedPage.Children>
<local:MainPage Title="Info" Icon="http://icons.iconarchive.com/icons/graphicloads/100-flat/256/home-icon.png"/>
<local:EventPage Title="Sessions" Icon="http://icons.iconarchive.com/icons/graphicloads/100-flat/256/home-icon.png"/>
<local:SpeakersPage Title="Speakers" Icon="http://icons.iconarchive.com/icons/graphicloads/100-flat/256/home-icon.png"/>
<local:ExhibitsPage Title="Exhibits" Icon="http://icons.iconarchive.com/icons/graphicloads/100-flat/256/home-icon.png"/>
</TabbedPage.Children>
</TabbedPage>
</MasterDetailPage.Detail>
</MasterDetailPage>
UPDATE:
I made some changes as suggested by SushiHangover, I now recieve no errors but my tabbed menu is not appearing.
回答1:
You can add TabbedPage Children like this in XAML:
<MasterDetailPage.Detail>
<TabbedPage>
<TabbedPage.Children>
<ContentPage Title="Page 1" Icon="ja.png" />
<ContentPage Title="Page 2" Icon="ja.png" />
<ContentPage Title="Page 3" Icon="ja.png" />
</TabbedPage.Children>
</TabbedPage>
</MasterDetailPage.Detail>
Just substitute your "local" custom pages for ContentPage
Or via code:
public TabsPage ()
{
this.Children.Add (new ContentPage () { Title = "Page 1", Icon = "ja.png" });
this.Children.Add (new ContentPage () { Title = "Page 2", Icon = "ja.png" });
this.Children.Add (new ContentPage () { Title = "Page 3", Icon = "ja.png" });
}
回答2:
The answer was to use command parameter that is relative to the pages to be displayed in the tabbed page.
<?xml version="1.0" encoding="utf-8"?>
<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
prism:ViewModelLocator.AutowireViewModel="True"
x:Class="BSi.Mobile.Events.Views.MainMasterDetailPage">
<MasterDetailPage.Master>
<ContentPage Title="Default">
<StackLayout>
<StackLayout BackgroundColor="{StaticResource iconGray}" Padding="15" HeightRequest="100">
<Image Source="canfitpro_logo2" HeightRequest="60" HorizontalOptions="Start" VerticalOptions="EndAndExpand" />
</StackLayout>
<TableView Intent="Form" HasUnevenRows="True">
<TableRoot>
<TableSection Title="Vancouver Fitness Expo">
<ViewCell>
<StackLayout Margin="15, 10, 10, 10" Orientation="Horizontal">
<StackLayout.GestureRecognizers>
<TapGestureRecognizer Command="{Binding MenuSelectedCommand}" CommandParameter="EventTabbedPage/NavigationPage/MainPage"/>
</StackLayout.GestureRecognizers>
<Image Source="icon_home_circle" HeightRequest="40"/>
<Label Text="Home" VerticalTextAlignment="Center" Margin="20, 0, 0, 0" Style="{DynamicResource SubtitleStyle}"/>
</StackLayout>
</ViewCell>
<ViewCell>
<StackLayout Margin="15, 10, 10, 10" Orientation="Horizontal">
<StackLayout.GestureRecognizers>
<TapGestureRecognizer Command="{Binding MenuSelectedCommand}" CommandParameter="EventTabbedPage/NavigationEventPage"/>
</StackLayout.GestureRecognizers>
<Image Source="icon_sessions_circle" HeightRequest="40"/>
<Label Text="Sessions" VerticalTextAlignment="Center" Margin="20, 0, 0, 0" Style="{DynamicResource SubtitleStyle}"/>
</StackLayout>
</ViewCell>
</TableSection>
<TableSection Title="Resources">
<ViewCell>
<StackLayout Margin="15, 10, 10, 10" Orientation="Horizontal">
<StackLayout.GestureRecognizers>
<TapGestureRecognizer Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage/FloorPlanPage"/>
</StackLayout.GestureRecognizers>
<Image Source="icon_floormap_circle" HeightRequest="40"/>
<Label Text="Floor Plan" VerticalTextAlignment="Center" Margin="20, 0, 0, 0" Style="{DynamicResource SubtitleStyle}"/>
</StackLayout>
</ViewCell>
<ViewCell>
<StackLayout Margin="15, 10, 10, 10" Orientation="Horizontal">
<StackLayout.GestureRecognizers>
<TapGestureRecognizer Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage/SponsorsPage"/>
</StackLayout.GestureRecognizers>
<Image Source="icon_partners_circle" HeightRequest="40"/>
<Label Text="Sponsor / Media Partners" VerticalTextAlignment="Center" Margin="20, 0, 0, 0" Style="{DynamicResource SubtitleStyle}"/>
</StackLayout>
</ViewCell>
<ViewCell>
<StackLayout Margin="15, 10, 10, 10" Orientation="Horizontal">
<StackLayout.GestureRecognizers>
<TapGestureRecognizer Command="{Binding MenuSelectedCommand}" CommandParameter="NavigationPage/FoodNearbyPage"/>
</StackLayout.GestureRecognizers>
<Image Source="icon_restaurant_circle" HeightRequest="40"/>
<Label Text="Where To Eat" VerticalTextAlignment="Center" Margin="20, 0, 0, 0" Style="{DynamicResource SubtitleStyle}"/>
</StackLayout>
</ViewCell>
</TableSection>
</TableRoot>
</TableView>
</StackLayout>
</ContentPage>
</MasterDetailPage.Master>
</MasterDetailPage>
来源：https://stackoverflow.com/questions/38662871/tabbed-page-inside-master-detail-page-xamarin