視圖模型管理

---

立即下載DevExpress WinForms

本文描述了如何在運(yùn)行時(shí)檢索ViewModel實(shí)例，注意如果您在設(shè)計(jì)時(shí)使用MvvmContext組件來(lái)構(gòu)建 MVVM-applications，該組件將自動(dòng)管理ViewModels。

如果ViewModel遵循POCO概念，MVVM框架將動(dòng)態(tài)地把該ViewModel轉(zhuǎn)換為包含必要基礎(chǔ)結(jié)構(gòu)(例如，支持簡(jiǎn)化的數(shù)據(jù)綁定)的新類?？蚣芘c動(dòng)態(tài)創(chuàng)建的類實(shí)例一起工作，這意味著您無(wú)法在運(yùn)行時(shí)初始訪問(wèn)這些實(shí)例，因?yàn)樗鼈兊念愋蜕形创_定。

使用以下選項(xiàng)來(lái)檢索工作的ViewMode：

• The ViewModelSource.Create method

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

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(shí)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(shí)例。注意，此時(shí)還需調(diào)用SetViewModel方法來(lái)指定框架在需要ViewModel時(shí)應(yīng)該使用這個(gè)實(shí)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?yàn)槟鷮⑹OCO模型提供的所有特性。

• ViewModelCreate events

這種方法被設(shè)計(jì)為與依賴注入框架(如Ninject)一起工作，下面的例子說(shuō)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?jiǎng)討B(tài)生成Viewmodel并將它們綁定到接口(當(dāng)將接口直接綁定到POCO時(shí)，MVVM框架特性將丟失)。要獲得所需的實(shí)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(gè)weak event，如果它包含一個(gè)變量的閉包(上面示例中的" kernel "或下面 Autofac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(huì)過(guò)早地收集該變量，并且事件處理程序可能永遠(yuǎn)不會(huì)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(wèn)題，請(qǐng)?jiān)趫?zhí)行訂閱的對(duì)象級(jí)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