Viewmodel 的多个实例答案

【问题标题】：Multiple instances of a ViewmodelViewmodel 的多个实例
【发布时间】：2011-03-21 15:40:45
【问题描述】：

我对 WPF 和 MVVM 非常陌生（这是我在 WPF 中的第一个项目）。我正在处理的项目应该接受一些搜索条件，并在网格中显示结果。为了构造查询，我使用了动态 LINQ 查询。我似乎在管理我的 ProjectSearchViewModel 实例时遇到问题，该实例对应于负责收集搜索条件和执行查询的视图。创建 MainWindowViewModel 时会创建一个实例。这将创建所有其他视图模型实例。这是我所期望的。但是当显示 MainWindow 时，我得到了另一个 ProjectSearchViewModel，我猜是从绑定中获得的。

大致思路是这样的：

在ProjectSearchView中填写搜索条件。
按下加载命令时，我使用响应式扩展方法发送 SearchResultMessage。
消息被 MainWindowViewModel 拾取
MainWindowViewModel 正在查询 ProjectSearchViewModel.SearchResult 并将 IObservable 列表分配给 AllProjectsViewModel.AllProjects，该列表绑定到数据网格以显示结果（AllProjectView 负责显示带有结果项目列表的网格）

问题是 SearchResultMessage 的参数填充和发送发生在 ProjectSearchViewModel 的一个实例中，而从 MainWindowViewModel 实际查询 SearchResult 发生在另一个实例中，其中所有搜索条件均为空。

我想我别无选择，只能发布我的代码：这里是它的精简版，省略了一些 iDisposable 管道等。对于我的模型，我使用 Entity Framework 4。

正如我所提到的，我是一个完全的新手，所以如果有人看到任何公然无视常识的行为，请直言不讳。



Imports Cheminator.ViewModel

Partial Public Class App

    Inherits Application


    Private viewModel As MainWindowViewModel

    Private window As MainWindow

    Protected Overrides Sub OnStartup(ByVal e As StartupEventArgs)

        MyBase.OnStartup(e)

        window = New MainWindow

        viewModel = New MainWindowViewModel '1st instance of ProjectSearchViewModel created Here

        window.DataContext = viewModel

        window.Show() '2nd instance of ProjectSearchViewModel created Here

    End Sub

End Class

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Cheminator"
    xmlns:vm="clr-namespace:Cheminator.ViewModel"
    xmlns:vw="clr-namespace:Cheminator.Views"
    xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
xmlns:dxd="http://schemas.devexpress.com/winfx/2008/xaml/docking"
xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
xmlns:dxnb="http://schemas.devexpress.com/winfx/2008/xaml/navbar"
xmlns:dxg="http://schemas.devexpress.com/winfx/2008/xaml/grid"
xmlns:collections="clr-namespace:System.Collections;assembly=mscorlib"
    Title="DXWpfApplication" Height="600" Width="800"
    dx:ThemeManager.ThemeName="Office2007Blue"
>

<Window.Resources>
    <ResourceDictionary Source="MainWindowResources.xaml" />
</Window.Resources>

<dxd:DockLayoutManager>
    <dxd:LayoutGroup>
        <dxd:LayoutGroup Orientation="Vertical"  Width="3*">
            <dxd:DocumentGroup Height="3*" SelectedTabIndex="0">
                <dxd:DocumentPanel Caption="Document1" Height="3*" >
                     <ContentControl
                      Content="{Binding Path=ProjectsVM}"
                      />
                </dxd:DocumentPanel>
            </dxd:DocumentGroup>
            <dxd:LayoutPanel Caption="Search Criteria" Height="*"  CaptionImage="Images/Icons/DetailView.png">                  

                <ContentControl
                      Content="{Binding Path=ProjectsSearchVM}"
                      />
            </dxd:LayoutPanel>
        </dxd:LayoutGroup>
    </dxd:LayoutGroup>
</dxd:DockLayoutManager>

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Cheminator"
xmlns:vm="clr-namespace:Cheminator.ViewModel"
xmlns:vw="clr-namespace:Cheminator.Views"    >

<DataTemplate DataType="{x:Type vm:AllProjectsViewModel}">
    <vw:AllProjectsView />
</DataTemplate>

<DataTemplate DataType="{x:Type vm:ProjectSearchViewModel}">
    <vw:ProjectSearchView />
</DataTemplate>

<UserControl x:Class="Cheminator.Views.AllProjectsView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
         xmlns:dxd="http://schemas.devexpress.com/winfx/2008/xaml/docking"
         xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
         xmlns:dxg="http://schemas.devexpress.com/winfx/2008/xaml/grid"       
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:vm="clr-namespace:Cheminator.ViewModel"
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">

    <dxg:GridControl AutoPopulateColumns="True" ShowBorder="False" >
        <dxg:GridControl.DataSource>
            <Binding Path="AllProjects"/>                     
        </dxg:GridControl.DataSource>
        <dxg:GridControl.View>
            <dxg:TableView>

        </dxg:TableView>
        </dxg:GridControl.View>
   </dxg:GridControl>
</UserControl>

<UserControl x:Class="Cheminator.Views.ProjectSearchView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:vm="clr-namespace:Cheminator.ViewModel"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="160" d:DesignWidth="470">
<Grid Height="160" Width="470">
    <Grid.DataContext>
        <vm:ProjectSearchViewModel />
    </Grid.DataContext>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="175*" />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Label Content="Cotation ID:" Height="28" Margin="49,12,32,0" Name="Label1" VerticalAlignment="Top" />
    <TextBox Grid.Column="1" Height="23" Margin="0,14,159,0" Name="CotationIDTextBox" VerticalAlignment="Top" Text="{Binding Path=CotationID, UpdateSourceTrigger=LostFocus}"/>

    <Label Grid.Row="1" Content="Cotation Name:" Height="28" Margin="49,6,6,0" Name="Label2" VerticalAlignment="Top" />
    <TextBox Grid.Row="1" Grid.Column="1" Height="23" Margin="0,8,159,0" Name="CotationNameTextBox" VerticalAlignment="Top" Text="{Binding Path=ProjectSummary, UpdateSourceTrigger=PropertyChanged}"/>

        <Label Grid.Row="2" Content="User:" Height="28" Margin="49,6,32,0" Name="Label3" VerticalAlignment="Top" />
    <TextBox Grid.Row="2" Grid.Column="1" Height="23" Margin="0,8,159,0" Name="UserTextBox" VerticalAlignment="Top" Text="{Binding Path=UserName, UpdateSourceTrigger=PropertyChanged}"/>
    <Button 
                    Command="{Binding Path=LoadCommand}"
                    Content="_Load"
                    HorizontalAlignment="Right"
                    Margin="0,10,51,12" 
                    MinWidth="60" Grid.Row="3" Width="72" Grid.Column="1" />
</Grid>
</UserControl>

Public Class MainWindowViewModel
    Inherits ViewModelBase

    Private _commands As ReadOnlyCollection(Of CommandViewModel)
    Private _ProjectsVM As AllProjectsViewModel
    Private _ProjectsSearchVM As ProjectSearchViewModel

    Public Sub New()
        MyBase.DisplayName = "Cheminator"
        _ProjectsSearchVM = New ProjectSearchViewModel

        Messenger.[Default].OfType(Of SearchResultMessage) _
            .Subscribe(Sub(param As SearchResultMessage)
                           ProjectsVM.AllProjects = ProjectsSearchVM.SearchResult
                       End Sub)
        _ProjectsVM = New AllProjectsViewModel
    End Sub

    Public ReadOnly Property ProjectsVM As AllProjectsViewModel
        Get

            If (_ProjectsVM IsNot Nothing) Then
                Return _ProjectsVM
            End If
            Return Nothing
        End Get
    End Property


    Public ReadOnly Property ProjectsSearchVM As ProjectSearchViewModel
        Get

            If (_ProjectsSearchVM IsNot Nothing) Then
                Return _ProjectsSearchVM
            End If
            Return Nothing
        End Get
    End Property

End Class

Public Class AllProjectsViewModel
    Inherits ViewModelBase


    Private m_ProjectsList As ObservableCollection(Of xGMV_Cotation)


    Public Sub New()
        MyBase.DisplayName = "Temp AllProjectsViewModel Name" 'Strings.AllProjectsViewModel_DisplayName

    End Sub


    Public Property AllProjects() As ObservableCollection(Of xGMV_Cotation)
        Get
            Return m_ProjectsList
        End Get
        Set(ByVal value As ObservableCollection(Of xGMV_Cotation))
            m_ProjectsList = value
            OnPropertyChanged("AllProjects")
        End Set
    End Property

End Class

Public Class ProjectSearchViewModel
    Inherits ViewModelBase


    Private m_ProjectsList As ObservableCollection(Of xGMV_Cotation)

    Public Sub New()
        MyBase.DisplayName = "Cheminator.ProjectSearchViewModel"




    End Sub

    Dim _CotationID As Integer
    Public Property CotationID As Integer
        Get
            Return _CotationID
        End Get
        Set(ByVal value As Integer)
            _CotationID = value
            MyBase.OnPropertyChanged("CotationID")
        End Set
    End Property
    Public Property ProjectSummary As String
    Public Property UserName As String


    Private m_LoadCommand As RelayCommand
    Public ReadOnly Property LoadCommand As ICommand
        Get
            If m_LoadCommand Is Nothing Then
                Dim LoadAction As New Action(Of Object)(AddressOf Me.Load)
                m_LoadCommand = New RelayCommand(LoadAction)
            End If


            Return m_LoadCommand
        End Get
    End Property

    Public ReadOnly Property SearchResult() As ObservableCollection(Of xGMV_Cotation)
        Get

            Dim xWhere As String = ""
            Dim i As Integer = 0
            Dim parameterList As New ArrayList
            If Not String.IsNullOrEmpty(CotationID) Then
                xWhere = String.Format("CotationID = @{0}", i)
                parameterList.Add(CotationID)
                i += 1
            End If

            If Not String.IsNullOrEmpty(ProjectSummary) Then
                If i > 0 Then
                    xWhere = xWhere & " AND "
                End If
                xWhere = xWhere & String.Format("ProjectSummary = '@{0}'", i)
                i += 1
                parameterList.Add(ProjectSummary)
            End If

            If Not String.IsNullOrEmpty(UserName) Then
                If i > 0 Then
                    xWhere = xWhere & " AND "
                End If
                xWhere = xWhere & String.Format("UserName = '@{0}'", i)
                i += 1
                parameterList.Add(UserName)
            End If


            Return New ObservableCollection(Of xGMV_Cotation)(DataContext.DBEntities.xGMV_Cotations.Where(xWhere, parameterList.ToArray))
        End Get

    End Property

    Private Sub Load()


        OnPropertyChanged("SearchResult")
        Messenger.Default.Send(New SearchResultMessage())

    End Sub

结束类

【问题讨论】：

第 5 个代码块，行，这段代码很可疑，我会删除它。尝试这样做并说出之后发生的事情。

标签： wpf mvvm

【解决方案1】：

每次您创建 ProjectSearchView 用户控件时，您也会创建一个 ProjectSearchViewModel。您应该从您的 usercontrl xaml 中删除以下内容。

<Grid.DataContext>
    <vm:ProjectSearchViewModel />
</Grid.DataContext>

因为你的datatemplate你不需要为ProjectSearchView设置datacontext，它已经有了正确的datacontext。

【讨论】：

原来如此。谢谢。