MVVM框架以自己的方式處理您的應用程序代碼并解釋特定的代碼片段，例如如果語法正確，則屬性可以被視為可綁定，這些語法規則稱為慣例。慣例允許您避免編寫額外的代碼，因為框架將“理解”您對它的期望并自動生成所需的一切。本文檔收集了您在構建MVVM應用程序時需要注意的所有MVVM框架慣例。

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可綁定屬性

所有公共自動實現的虛擬屬性都被視為可綁定的。

C#

public virtual string Test { get; set; }

VB.NET

Public Overridable Property Test() As String

要禁止為此類屬性生成可綁定屬性，請按如下方式使用 Bindable 屬性。

C#

[Bindable(false)]
public virtual string Test { get; set; }

VB.NET

<Bindable(False)>
Public Overridable Property Test() As String

框架會忽略帶有支持字段的屬性，您可以使用 BindableProperty 特性顯式標記此類屬性，以便仍然能夠將它們用于數據綁定。

C#

using DevExpress.Mvvm.DataAnnotations;
//. . .
string test;
[BindableProperty]
public virtual string Test
{
get { return test; }
set { test = value; }
}

VB.NET

Imports DevExpress.Mvvm.DataAnnotations
'. . .
Private testField As String
<BindableProperty>
Public Overridable Property Test() As String
Get
Return testField
End Get
Set(ByVal value As String)
testField = value
End Set
End Property

屬性依賴

屬性可以在應用程序運行時更改其值，要跟蹤這些更改并對其做出響應，請聲明屬性依賴項。屬性依賴是一種在其相關屬性發生變化或即將發生變化時自動執行的方法。 要實現此操作，必須調用On<Related_Property_Name>Changing或On<Related_Property_Name>Changed方法。

C#

public virtual string Test { get; set; }
protected void OnTestChanged() {
//do something
}

VB.NET

Public Overridable Property Test() As String
Protected Sub OnTestChanged()
'do something
End Sub

On...Changed 和 On..Changing 方法也可以有一個參數，在這種情況下，參數將分別接收舊的或新的屬性值。

C#

public virtual string Test { get; set; }
protected void OnTestChanging(string newValue) {
//do something
}
protected void OnTestChanged(string oldValue) {
//do something
}

VB.NET

Public Overridable Property Test() As String
Protected Sub OnTestChanging(ByVal newValue As String)
'do something
End Sub
Protected Sub OnTestChanged(ByVal oldValue As String)
'do something
End Sub

BindableProperty 屬性也允許您使用具有不同名稱的方法。

C#

[BindableProperty(OnPropertyChangingMethodName = "BeforeChange", OnPropertyChangedMethodName = "AfterChange")]
public virtual string Test { get; set; }

protected void BeforeChange() {
//. . .
}
protected void AfterChange() {
//. . .
}

VB.NET

<BindableProperty(OnPropertyChangingMethodName := "BeforeChange", OnPropertyChangedMethodName := "AfterChange")>
Public Overridable Property Test() As String

Protected Sub BeforeChange()
'. . .
End Sub
Protected Sub AfterChange()
'. . .
End Sub

命令

在 POCO ViewModels 中聲明的所有具有零個或一個參數的公共無效方法都被視為命令。

C#

public void DoSomething(object p) {
MessageBox.Show(string.Format("The parameter passed to command is {0}.", p));
}

VB.NET

Public Sub DoSomething(ByVal p As Object)
MessageBox.Show(String.Format("The parameter passed to command is {0}.", p))
End Sub

名稱以 ...Command 結尾的方法會引發異常，您可以通過使用 Command 屬性標記這些方法并通過 Name 參數設置適當的名稱來強制框架將這些方法視為有效命令。

C#

using DevExpress.Mvvm.DataAnnotations;
[Command(Name="DoSomething")]
public void DoSomethingCommand(object p) {
//do something
}

VB.NET

Imports DevExpress.Mvvm.DataAnnotations
<Command(Name := "DoSomething")>
Public Sub DoSomethingCommand(ByVal p As Object)
'do something
End Sub

對于每個命令方法，框架都會生成相應的支持屬性。 默認情況下，此屬性以相關方法加上“Command”后綴命名。 您可以使用 Command 屬性的 Name 參數為這個自動生成的支持屬性保留另一個名稱。

C#

[Command(Name = "MyMvvmCommand")]
public void DoSomething(object p) {
//do something
}

VB.NET

'this command will be executed only if "p" equals 4
Public Sub DoSomething(ByVal p As Integer)
MessageBox.Show(String.Format("The parameter passed to command is {0}.", p))
End Sub
Public Function CanDoSomething(ByVal p As Integer) As Boolean
Return (2 + 2) = p
End Function

具有其他名稱的 CanExecute 方法仍然可以通過使用 Command 屬性的 CanExecuteMethodName 參數綁定到命令。

C#

[Command(CanExecuteMethodName = "DoSomethingCriteria")]
public void DoSomething(int p) {
MessageBox.Show(string.Format("The parameter passed to command is {0}.", p));
}
public bool DoSomethingCriteria(int p) {
return (2 + 2) == p;
}

VB.NET

<Command(CanExecuteMethodName := "DoSomethingCriteria")>
Public Sub DoSomething(ByVal p As Integer)
MessageBox.Show(String.Format("The parameter passed to command is {0}.", p))
End Sub
Public Function DoSomethingCriteria(ByVal p As Integer) As Boolean
Return (2 + 2) = p
End Function

當命令剛剛綁定到它的目標（以獲取目標的初始狀態）時，首先檢查 CanExecute 子句。 稍后，每次 CanExecuteChanged 事件通知命令的目標有關命令狀態更改時，都會重新計算此條件。 此事件是在底層命令對象級別聲明的，要從 ViewModel 級別發送此類通知，請按如下方式調用 RaiseCanExecuteChanged 擴展方法。

C#

//a bindable property
public virtual bool IsModified { get; protected set; }

//a command
public void Save() {
//. . .
}

//a CanExecute condition
public bool CanSave() {
return IsModified;
}

//the OnChanged method calls the RaiseCanExecuteChanged method for the "Save" command
//this forces the command to update its CanExecute condition
public void OnIsModifiedChanged() {
this.RaiseCanExecuteChanged(x=>x.Save());
}

VB.NET

'a bindable property
Private privateIsModified As Boolean
Public Overridable Property IsModified() As Boolean
Get
Return privateIsModified
End Get
Protected Set(ByVal value As Boolean)
privateIsModified = value
End Set
End Property

'a command
Public Sub Save()
'. . .
End Sub

'a CanExecute condition
Public Function CanSave() As Boolean
Return IsModified
End Function

'the OnChanged method calls the RaiseCanExecuteChanged method for the "Save" command
'this forces the command to update its CanExecute condition
Public Sub OnIsModifiedChanged()
Me.RaiseCanExecuteChanged(Sub(x) x.Save())
End Sub

服務

為了解析服務，框架會覆蓋接口類型的虛擬屬性，這些屬性的名稱必須以 ...Service 結尾。

C#

public virtual IMyNotificationService MyService {
get { throw new NotImplementedException(); }
}

public virtual IMyNotificationService AnotherService {
get { throw new NotImplementedException(); }
}

VB.NET

Public Overridable ReadOnly Property MyService() As IMyNotificationService
Get
Throw New NotImplementedException()
End Get
End Property

Public Overridable ReadOnly Property AnotherService() As IMyNotificationService
Get
Throw New NotImplementedException()
End Get
End Property

您還可以使用 ServiceProperty 屬性顯式標記具有其他名稱的服務屬性。

C#

using DevExpress.Mvvm.DataAnnotations;
//. . .
[ServiceProperty]
public virtual IMyNotificationService MyProvider {
get { throw new NotImplementedException(); }
}

VB.NET

Imports DevExpress.Mvvm.DataAnnotations
'. . .
<ServiceProperty>
Public Overridable ReadOnly Property MyProvider() As IMyNotificationService
Get
Throw New NotImplementedException()
End Get
End Property

當框架覆蓋服務屬性時，它會生成相應的 GetService<> 擴展方法調用。 ServiceProperty 屬性允許您為此方法指定其他參數（例如，服務密鑰）。

C#

[ServiceProperty(Key="Service1")]
public virtual IMyNotificationService Service {
get { throw new NotImplementedException(); }
}

[ServiceProperty(Key = "Service2")]
public virtual IMyNotificationService AnotherService {
get { throw new NotImplementedException(); }
}

VB.NET

<ServiceProperty(Key:="Service1")>
Public Overridable ReadOnly Property Service() As IMyNotificationService
Get
Throw New NotImplementedException()
End Get
End Property

<ServiceProperty(Key := "Service2")>
Public Overridable ReadOnly Property AnotherService() As IMyNotificationService
Get
Throw New NotImplementedException()
End Get
End Property

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