我有一个 MDPage，它是一个 MasterDetailPage，它将侧面菜单项列表页面称为主页面。此详细信息是通过主页的新导航页面添加的。
我的代码是
public MDPage(){
Master = new SideMenuPage();
InitializeComponent();
masterPage.ListView.ItemSelected += OnItemSelected;
Detail = new NavigationPage(new HomePage())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};}
我有一个要求，导航页面包含底部菜单，这样在单击底部菜单时，需要重新初始化 MDPage 以设置不同页面（例如“关于我们”页面）的新导航页面。所以我为 MDPage 添加了一个参数化构造函数，然后单击我正在调用的底部菜单项App.Current.MainPage = new MDPage("AboutUs").下面是参数化构造函数。
public MDPage(string bottomMenuItem)
{
Master = new SideMenuPage();
InitializeComponent();
masterPage.ListView.ItemSelected += OnItemSelected;
if("AboutUs" == bottomMenuItem)
{
Detail = new NavigationPage(new AboutUs())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}}
当我打开应用程序时，首先调用 MDPage 构造函数。然后，从侧面菜单中，我可以选择打开作为详细信息添加的主页。请注意，此主页包含一个底部子菜单，它只是一个图像。点击此按钮后，它应该再次重新初始化 MDPage。这在 Andorid 中运行良好。但在 iOS 中，它会抛出 null 异常。它不允许我设置Master = new SideMenuPage();我通过反复试验找到了根本原因，因为此代码不会抛出异常，而是在 iOS Main.cs 文件中抛出。请帮我。
主页 - XAML
<?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:local="clr-namespace:ClubApp.Views;assembly=ClubApp"
x:Class="ClubApp.Views.MainPage">
<MasterDetailPage.Master>
<local:MasterPage x:Name="masterPage" />
</MasterDetailPage.Master>
<MasterDetailPage.Detail>
<NavigationPage>
<x:Arguments>
</x:Arguments>
</NavigationPage>
</MasterDetailPage.Detail>
</MasterDetailPage>
MainPage.xaml.cs
using System.Linq;using System.Text;using System.Threading.Tasks;using Xamarin.Forms;namespace Test.Views{
public partial class MainPage : MasterDetailPage
{
public MainPage()
{
Master = new MasterPage();
InitializeComponent();
masterPage.ListView.ItemSelected += OnItemSelected;
Detail = new NavigationPage(new Views.HomePage())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
public MainPage(string bottomMenu)
{
Master = new MasterPage();
InitializeComponent();
masterPage.ListView.ItemSelected += OnItemSelected;
if ("News" == bottomMenu)
{
Detail = new NavigationPage(new Views.HomePage())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
else if ("Profile" == bottomMenu)
{
Detail = new NavigationPage(new Views.Profile())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
}
async void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
{
try
{
var item = e.SelectedItem as MasterPageItem;
if (item != null)
{
if (item.Title == "News")
{
Detail = new NavigationPage(new Views.NewsPage())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
if (item.Title == "Home")
{
Detail = new NavigationPage(new Views.HomePage())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
if (item.Title == "Profile")
{
Detail = new NavigationPage(new Views.Profile())
{
BarBackgroundColor = Color.Black,
BarTextColor = Color.White,
};
}
masterPage.ListView.SelectedItem = null;
IsPresented = false;
}
}
catch (Exception ex)
{
}
}
}}
母版页.xaml
<?xml version="1.0" encoding="utf-8" ?><ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Test.Views.MasterPage"> <ContentPage.Content>
<StackLayout VerticalOptions="FillAndExpand" BackgroundColor="#10000c" Padding = "0,50,0,0" >
<StackLayout x:Name="slUserProfile" Orientation="Vertical" Spacing = "0"
VerticalOptions="FillAndExpand">
<Image Source="{Binding member_image}" x:Name="imgSideImage"
HorizontalOptions="CenterAndExpand" Aspect="AspectFill" HeightRequest="100"
WidthRequest="100" />
<Label Text="{Binding name}" TextColor="#efa747"
FontSize ="17"
HorizontalOptions="CenterAndExpand"/>
</StackLayout >
<ListView x:Name="lvSideMenu" VerticalOptions="FillAndExpand" SeparatorVisibility="None"
BackgroundColor="Black">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Padding="15,5,0,0" Orientation="Horizontal">
<Image Source="{Binding IconSource}"/>
<StackLayout Padding = "10,0,0,0">
<local:CustomLabel Text="{Binding Title}" VerticalOptions="CenterAndExpand"
TextColor="#dac6ac" FontSize ="14"/>
</StackLayout>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout></ContentPage.Content></ContentPage>
MasterPage.xaml.cs
using Xamarin.Forms;namespace Test.Views{
public partial class MasterPage : ContentPage
{
public ListView ListView { get { return lvSideMenu; } }
public Page Detail { get; set; }
public MasterPage()
{
InitializeComponent();
var masterPageItems = new List<MasterPageItem>();
//Fills side menu items.
masterPageItems.Add(new MasterPageItem
{
Title = "Profile",
IconSource = "profile_sidemenu.png"
});
masterPageItems.Add(new MasterPageItem
{
Title = "Home",
IconSource = "home_smenu.png"
});
masterPageItems.Add(new MasterPageItem
{
Title = "News",
IconSource = "news_smenu.png"
});
lvSideMenu.ItemsSource = masterPageItems;
Icon = "menu.png";
Title = "Menu";
}
}
public class MenuItems
{
public string MenuTitle { get; set; }
}
public class MasterPageItem
{
public string Title { get; set; }
public string IconSource { get; set; }
}}
主页.xaml
<?xml version="1.0" encoding="utf-8" ?><ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Test.Views.HomePage" Title="Home"><ContentPage.Content><StackLayout> <Label Text="This is Home page"/></StackLayout></ContentPage.Content></ContentPage>
配置文件.xaml
<?xml version="1.0" encoding="utf-8" ?><ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Test.Views.Profile" Title="Profile"><ContentPage.Content> <StackLayout>
<StackLayout>
<Label Text="This is Profile page" />
</StackLayout>
<StackLayout HeightRequest="80">
<Grid RowSpacing="0" ColumnSpacing="0" VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<StackLayout x:Name="tProfile" BackgroundColor="Red" Grid.Column="0" Spacing="0" Padding="0,10,0,10">
<Image Source="bprofileSel.png" HorizontalOptions="Center" VerticalOptions="End"/>
<Label Text="Profile" FontSize="10" TextColor="White" HorizontalOptions="CenterAndExpand" VerticalTextAlignment="Center"/>
</StackLayout>
<StackLayout x:Name="tNews" BackgroundColor="Black" Grid.Column="1" Spacing="0" Padding="0,10,0,10">
<Image Source="bNewsUnsel.png" HorizontalOptions="Center" VerticalOptions="End"/>
<local:CustomLabel Text="News" FontSize="10" TextColor="White" HorizontalOptions="CenterAndExpand" VerticalTextAlignment="Center"/>
</StackLayout>
</Grid>
</StackLayout>
</StackLayout></ContentPage.Content></ContentPage>
配置文件.xaml.cs
Using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;using Xamarin.Forms;namespace Test.Views{
public partial class Profile : ContentPage
{
public Profile()
{
InitializeComponent();
try
{
tProfile.GestureRecognizers.Add(new TapGestureRecognizer
{
Command = new Command(() => ProfileClicked()),
});
tNews.GestureRecognizers.Add(new TapGestureRecognizer
{
Command = new Command(() => NewsClicked()),
});
}
catch (Exception ex)
{
}
}
private void ProfileClicked()
{
App.Current.MainPage = new MainPage("Profile");
}
private void NewsClicked()
{
App.Current.MainPage = new MainPage("News");
}
}}
新闻页面可以像个人资料页面一样设置。在 app.cs 中调用 MainPage = new MainPage()
当应用程序启动时，它将显示带有侧面菜单的主页。现在，当我单击菜单（例如侧面菜单中的个人资料）时，它将带我进入个人资料页面，根据设计，可以在那里看到底部子菜单。点击它会导致iOS代码中出现null异常。这在安卓上运行良好。我的假设是根本原因是由于 MasterPage 重新初始化为 Master。但我需要像这样飞行的要求。请帮我。
This is the error in iOS Main.cs
None