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立即下載DevExpress WinForms

 本文描述了如何在運行時檢索ViewModel實例，注意如果您在設計時使用MvvmContext組件來構(gòu)建 MVVM-applications，該組件將自動管理ViewModels。

 如果ViewModel遵循POCO概念，MVVM框架將動態(tài)地把該ViewModel轉(zhuǎn)換為包含必要基礎結(jié)構(gòu)(例如，支持簡化的)的新類。框架與動態(tài)創(chuàng)建的類實例一起工作，這意味著您無法在運行時初始訪問這些實例，因為它們的類型尚未確定。

使用以下選項來檢索工作的ViewMode：

• The ViewModelSource.Create method

 在這種方法中，您首先創(chuàng)建一個ViewModel實例，然后調(diào)用SetViewModel方法將該實例與特定的ViewModel類型關(guān)聯(lián)起來。

C#：

var mainViewModel = ViewModelSource.Create<MainViewModel>();
mvvmContext1.SetViewModel(typeof(MainViewModel), mainViewModel);

VB.NET：

Dim mainViewModel = ViewModelSource.Create(Of MainViewModel)()
mvvmContext1.SetViewModel(GetType(MainViewModel), mainViewModel)

• The ViewModelBase class

 您可以從實現(xiàn)MVVM框架特性的ViewModelBase類中繼承ViewModels，在這種情況下，是可以直接創(chuàng)建ViewModel實例。注意，此時還需調(diào)用SetViewModel方法來指定框架在需要ViewModel時應該使用這個實例。

C#：

public class ViewModel : ViewModelBase {
//. . .
}
var myViewModel = new ViewModel();
mvvmContext1.SetViewModel(typeof(ViewModel), myViewModel);

VB.NET：

Public Class ViewModel
Inherits ViewModelBase

'. . .
End Class
Private myViewModel = New ViewModel()
mvvmContext1.SetViewModel(GetType(ViewModel), myViewModel)

我們不推薦這種方法，因為您將失去POCO模型提供的所有特性。

• ViewModelCreate events

 這種方法被設計為與依賴注入框架(如)一起工作，下面的例子說明了這種注入是如何與Ninject 框架一起工作的。

C#：

public class SamuraiViewModel {
public IWeapon Weapon { get; private set; }
public SamuraiViewModel(IWeapon weapon) {
this.Weapon = weapon;
}
public void Attack() {
Weapon.Hit();
}
}

// Bind a dependency for IWeapon
kernel.Bind<IWeapon>().To<Sword>();

// Set up MVVMContext
var fluent = mvvmContext1.OfType<SamuraiViewModel>();
fluent.BindCommand(simpleButton1, x => x.Attack());

VB.NET：

Public Class SamuraiViewModel
Private privateWeapon As IWeapon
Public Property Weapon() As IWeapon
Get
Return privateWeapon
End Get
Private Set(ByVal value As IWeapon)
privateWeapon = value
End Set
End Property
Public Sub New(ByVal weapon As IWeapon)
Me.Weapon = weapon
End Sub
Public Sub Attack()
Weapon.Hit()
End Sub
End Class

' Bind a dependency for IWeapon
kernel.Bind(Of IWeapon)().To(Of Sword)()

' Set up MVVMContext
Dim fluent = mvvmContext1.OfType(Of SamuraiViewModel)()
fluent.BindCommand(simpleButton1, Function(x) x.Attack())

在這種情況下，您需要動態(tài)生成Viewmodel并將它們綁定到接口(當將接口直接綁定到POCO時，MVVM框架特性將丟失)。要獲得所需的實例，處理常規(guī)(本地)或靜態(tài)(全局)ViewModelCreate事件如下：

C#：

// Retrieve the live POCO ViewModel instance with the Ninject kernel
//regular event
mvvmContext1.ViewModelCreate += MVVMContext_ViewModelCreate;
void MVVMContext_ViewModelCreate(object sender, DevExpress.Utils.MVVM.ViewModelCreateEventArgs e) {
// kernel.Bind<SamuraiViewModel>().To(e.RuntimeViewModelType);
// e.ViewModel = kernel.Get<SamuraiViewModel>();
e.ViewModel = kernel.Get(e.RuntimeViewModelType);
}
//static event
MVVMContextCompositionRoot.ViewModelCreate += (s,e)=> {
e.ViewModel = kernel.Get(e.RuntimeViewModelType);
};

VB.NET：

' Retrieve the live POCO ViewModel instance with the Ninject kernel
'regular event
AddHandler mvvmContext1.ViewModelCreate, AddressOf MVVMContext_ViewModelCreate
void MVVMContext_ViewModelCreate(Object sender, DevExpress.Utils.MVVM.ViewModelCreateEventArgs e)
' kernel.Bind<SamuraiViewModel>().To(e.RuntimeViewModelType);
' e.ViewModel = kernel.Get<SamuraiViewModel>();
e.ViewModel = kernel.Get(e.RuntimeViewModelType)
'static event
AddHandler MVVMContextCompositionRoot.ViewModelCreate, Sub(s,e) e.ViewModel = kernel.Get(e.RuntimeViewModelType)

 靜態(tài)ViewModelCreate事件是一個weak event，如果它包含一個變量的閉包(上面示例中的" kernel "或下面示例中的" rootContainer "范圍)，其生命周期短于父容器的生命周期，那么VS垃圾收集器可能會過早地收集該變量，并且事件處理程序可能永遠不會被調(diào)用。

C#：

[STAThread]
static void Main() {
//rootContainer is declared at the Main method level
IContainer rootContainer;
var builder = new ContainerBuilder();
builder.RegisterType<TestViewModel>();
builder.RegisterType<MyClass>().As<IService>();
rootContainer = builder.Build();
MVVMContextCompositionRoot.ViewModelCreate += (s, e) => {
using (var scope = rootContainer.BeginLifetimeScope(
b => b.RegisterType(e.RuntimeViewModelType).As(e.ViewModelType))) {
e.ViewModel = scope.Resolve(e.ViewModelType);
}
};
}

VB.NET：

<STAThread>
Shared Sub Main()
'rootContainer is declared at the Main method level
Dim rootContainer As IContainer
Dim builder = New ContainerBuilder()
builder.RegisterType(Of TestViewModel)()
builder.RegisterType(Of [MyClass])().As(Of IService)()
rootContainer = builder.Build()
AddHandler MVVMContextCompositionRoot.ViewModelCreate, Sub(s, e)
Using scope = rootContainer.BeginLifetimeScope(Function(b) b.RegisterType(e.RuntimeViewModelType).As(e.ViewModelType))
e.ViewModel = scope.Resolve(e.ViewModelType)
End Using
End Sub
End Sub

要解決潛在的問題，請在執(zhí)行訂閱的對象級別將變量聲明為屬性/字段。

C#：

//rootContainer is declared at the root level
private static IContainer rootContainer { get; set; }
[STAThread]
static void Main()
{
var builder = new ContainerBuilder();
builder.RegisterType<TestViewModel>();
builder.RegisterType<MyClass>().As<IService>();
rootContainer = builder.Build();
MVVMContextCompositionRoot.ViewModelCreate += (s, e) =>
{
using (var scope = rootContainer.BeginLifetimeScope(
b => b.RegisterType(e.RuntimeViewModelType).As(e.ViewModelType))) {
e.ViewModel = scope.Resolve(e.ViewModelType);
}
};
}

VB.NET：

'rootContainer is declared at the root level
Private Shared Property rootContainer() As IContainer
<STAThread>
Shared Sub Main()
Dim builder = New ContainerBuilder()
builder.RegisterType(Of TestViewModel)()
builder.RegisterType(Of [MyClass])().As(Of IService)()
rootContainer = builder.Build()
AddHandler MVVMContextCompositionRoot.ViewModelCreate, Sub(s, e)
Using scope = rootContainer.BeginLifetimeScope(Function(b) b.RegisterType(e.RuntimeViewModelType).As(e.ViewModelType))
e.ViewModel = scope.Resolve(e.ViewModelType)
End Using
End Sub
End Sub