windowsforms-113975-cross-platform-app-development-winforms-mvvm-concepts-behaviors.md
Behaviors add extra functionality to an object without modifying it. For instance, a close button closes a tab or a form and additionally displays a confirmation dialog. You can use behaviors in MVVM applications to accomplish this.
Note
MVVM Best Practices Demo The text below has a related example in the DevExpress ‘MVVM Best Practices’ demo.
Group: API Code Examples
Module: Behaviors
Example: Simple Behaviors
25.2 Demo Center: Launch the demo
A simple confirmation behavior can only be attached to a cancelable event (for example, form closing or edit value changing). To implement custom behavior, define a class that derives from the ConfirmationBehavior class. It contains two virtual string properties for the confirmation message box’s title and text. Override these properties to assign your text strings. The class constructor should inherit the base class constructor with a parameter equal to the name of the event that triggers this behavior.
public class FormCloseBehavior : ConfirmationBehavior<FormClosingEventArgs> {
public FormCloseBehavior() : base("FormClosing") { }
protected override string GetConfirmationCaption() {
return "Confirm exit";
}
protected override string GetConfirmationText() {
return "Do you really want to exit the application?";
}
}
Public Class FormCloseBehavior
Inherits ConfirmationBehavior(Of FormClosingEventArgs)
Public Sub New()
MyBase.New("FormClosing")
End Sub
Protected Overrides Function GetConfirmationCaption() As String
Return "Confirm exit"
End Function
Protected Overrides Function GetConfirmationText() As String
Return "Do you want to exit the application?"
End Function
End Class
Tip
The ConfirmationBehavior class’s last virtual property is a Boolean Confirm property. You can also override it to specify the related event’s cancel conditions.
//Inverted confirmation logic
protected override bool Confirm() {
if(MessageBox.Show(GetConfirmationText(), GetConfirmationCaption(), MessageBoxButtons.YesNo) == DialogResult.Yes) return false;
else return true;
}
'Inverted confirmation logic
Protected Overrides Function Confirm() As Boolean
If MessageBox.Show(GetConfirmationText(), GetConfirmationCaption(), MessageBoxButtons.YesNo) = DialogResult.Yes Then
Return False
Else
Return True
End If
End Function
Use the MvvmContext component’s API to attach this behavior to the target UI element.
//View
mvvmContext.AttachBehavior<FormCloseBehavior>(this);
'View
mvvmContext.AttachBehavior(Of FormCloseBehavior)(Me)
You can simplify this process by moving the behavior declaration from a separate class to a generic one.
mvvmContext1.AttachBehavior<ConfirmationBehavior<ChangingEventArgs>>(checkEdit1, behavior => {
behavior.Caption = "CheckEdit State Changing";
behavior.Text = "This checkEdit's checked-state is about to be changed. Are you sure?";
behavior.Buttons = ConfirmationButtons.YesNo;
behavior.ShowQuestionIcon = true;
}, "EditValueChanging");
mvvmContext1.AttachBehavior(Of ConfirmationBehavior(Of ChangingEventArgs))(checkEdit1, Sub(behavior)
behavior.Caption = "CheckEdit State Changing"
behavior.Text = "This checkEdit's checked-state is about to be changed. Are you sure?"
behavior.Buttons = ConfirmationButtons.YesNo
behavior.ShowQuestionIcon = True
End Sub, "EditValueChanging")
Fluent API is also supported.
mvvmContext1.WithEvent<ChangingEventArgs>(checkEdit1, "EditValueChanging").Confirmation(behavior => {
behavior.Caption = "CheckEdit State changing";
behavior.Text = "This checkEdit's checked-state is about to be changed. Are you sure?";
});
mvvmContext1.WithEvent(Of ChangingEventArgs)(checkEdit1, "EditValueChanging").Confirmation(Sub(behavior)
behavior.Caption = "CheckEdit State changing"
behavior.Text = "This checkEdit's checked-state is about to be changed. Are you sure?"
End Sub)
Note
MVVM Best Practices Demo The text below has a related example in the DevExpress ‘MVVM Best Practices’ demo.
Group: API Code Examples
Module: Behaviors
Example: Event-To-Command Behavior
25.2 Demo Center: Launch the demo
Event-to-command behaviors fit all the remaining events if a confirmation behavior requires an event that receives CancelEventArgs type arguments. This behavior binds a command to a target UI element, and when this element fires the required event, the command executes. This can be used for:
Event-to-command behaviors are implemented similarly to confirmation behaviors. Define a separate class that derives from the DevExpress EventToCommandBehavior class. In the class constructor, specify the target event name and the command that should be executed on this event.
public class ClickToSayHello : DevExpress.Utils.MVVM.EventToCommandBehavior<ViewModel, EventArgs> {
public ClickToSayHello()
: base("Click", x => x.SayHello()) {
}
}
Public Class ClickToSayHello
Inherits DevExpress.Utils.MVVM.EventToCommandBehavior(Of ViewModel, EventArgs)
Public Sub New()
MyBase.New("Click", Function(x) x.SayHello())
End Sub
End Class
Attaching event-to-command behaviors is identical to confirmation behaviors.
mvvmContext.AttachBehavior<ClickToSayHello>(thirdPartyButton);
mvvmContext.AttachBehavior(Of ClickToSayHello)(thirdPartyButton)
Fluent API allows you to implement event-to-command behaviors without creating separate classes.
mvvmContext.WithEvent<ViewModel, EventArgs>(thirdPartyButton, "Click").EventToCommand(x => x.SayHello());
mvvmContext.WithEvent(Of ViewModel, EventArgs)(thirdPartyButton, "Click").EventToCommand(Function(x) x.SayHello())
Note
MVVM Best Practices Demo The text below has a related example in the DevExpress ‘MVVM Best Practices’ demo.
Group: API Code Examples
Module: Behaviors
Example: Key-To-Command and Keys-To-Command Behaviors
25.2 Demo Center: Launch the demo
These behaviors allow you to execute commands when end-users press specific keyboard keys.
Binding a single keyboard shortcut
The sample ViewModel below defines the “OnAKey” and “OnAltKey” commands that display service-based notifications.
public class KeyAwareViewModel {
protected IMessageBoxService MessageBoxService {
get { return this.GetService<IMessageBoxService>(); }
}
public void OnAKey() {
MessageBoxService.ShowMessage("Key Command: A");
}
public void OnAltAKey() {
MessageBoxService.ShowMessage("Key Command: Alt+A");
}
}
Public Class KeyAwareViewModel
Protected ReadOnly Property MessageBoxService() As IMessageBoxService
Get
Return Me.GetService(Of IMessageBoxService)()
End Get
End Property
Public Sub OnAKey()
MessageBoxService.ShowMessage("Key Command: A")
End Sub
Public Sub OnAltAKey()
MessageBoxService.ShowMessage("Key Command: Alt+A")
End Sub
End Class
Use the following fluent API methods of the MvvmContext component to bind these commands to related keys:
|
WithKey
|
The first method parameter is a UI element that must be focused when end-users press certain keys. The second parameter is a key combination.
| |
KeyToCommand
|
Refers to a command that should be executed.
|
The code below binds the “OnAKey” and “OnAltKey” commands to “A” and “Alt+A” keys respectively. Pressing both key combinations fires related commands only if the memo edit control currently has focus.
mvvmContext.ViewModelType = typeof(KeyAwareViewModel);
// Binding the "A" key
mvvmContext.OfType<KeyAwareViewModel>()
.WithKey(memo, Keys.A)
.KeyToCommand(x => x.OnAKey());
// Binding the "Alt+A" shortcut
mvvmContext.OfType<KeyAwareViewModel>()
.WithKey(memo, Keys.A | Keys.Alt)
.KeyToCommand(x => x.OnAltAKey());
mvvmContext.ViewModelType = GetType(KeyAwareViewModel)
' Binding the "A" key
mvvmContext.OfType(Of KeyAwareViewModel)().WithKey(memo, Keys.A).KeyToCommand(Function(x) x.OnAKey())
' Binding the "Alt+A" shortcut
mvvmContext.OfType(Of KeyAwareViewModel)().WithKey(memo, Keys.A Or Keys.Alt).KeyToCommand(Function(x) x.OnAltAKey())
Binding multiple keys to the same command
You want to bind multiple keys to the same command and pass these keys as parameters, use the WithKeys and KeysToCommands methods. Below is the sample KeyAwareViewModel View Model that contains parameterized “OnKey” and “OnKeyArgs” commands. These commands notify users what keys have been pressed.
public class KeyAwareViewModel {
protected IMessageBoxService MessageBoxService {
get { return this.GetService<IMessageBoxService>(); }
public void OnKey(Keys keys) {
MessageBoxService.ShowMessage("Key Command:" + keys.ToString());
}
public void OnKeyArgs(KeyEventArgs args) {
string message = string.Join(", ",
"KeyValue: " + args.KeyValue.ToString(),
"KeyData: " + args.KeyData.ToString(),
"KeyCode: " + args.KeyCode.ToString(),
"Modifiers: " + args.Modifiers.ToString());
MessageBoxService.ShowMessage("Args = {" + message + "}");
}
}
Public Class KeyAwareViewModel
Protected ReadOnly Property MessageBoxService() As IMessageBoxService
Get
Return Me.GetService(Of IMessageBoxService)()
End Get
public void OnKey(Keys keys)
MessageBoxService.ShowMessage("Key Command:" & keys.ToString())
public void OnKeyArgs(KeyEventArgs args)
Dim message As String = String.Join(", ", "KeyValue: " & args.KeyValue.ToString(), "KeyData: " & args.KeyData.ToString(), "KeyCode: " & args.KeyCode.ToString(), "Modifiers: " & args.Modifiers.ToString())
MessageBoxService.ShowMessage("Args = {" & message & "}")
End Property
The code below binds these commands to multiple hotkeys simultaneously.
mvvmContext.ViewModelType = typeof(KeyAwareViewModel);
// Binding to the OnKey command
mvvmContext.OfType<KeyAwareViewModel>()
.WithKeys(memo, new Keys[] { Keys.A, Keys.B, Keys.C })
.KeysToCommand(x => x.OnKey(Keys.None), args => args.KeyCode);
// Binding to the OnKeyArgs command
mvvmContext.OfType<KeyAwareViewModel>()
.WithKeys(memo, new Keys[] { Keys.Shift | Keys.A, Keys.Shift | Keys.B, Keys.Shift | Keys.C})
.KeysToCommand(x => x.OnKeyArgs((KeyEventArgs)null), args => (KeyEventArgs)args);
mvvmContext.ViewModelType = GetType(KeyAwareViewModel)
' Binding to the OnKey command
mvvmContext.OfType(Of KeyAwareViewModel)().WithKeys(memo, New Keys() { Keys.A, Keys.B, Keys.C }).KeysToCommand(Function(x) x.OnKey(Keys.None), Function(args) args.KeyCode)
' Binding to the OnKeyArgs command
mvvmContext.OfType(Of KeyAwareViewModel)().WithKeys(memo, New Keys() { Keys.Shift Or Keys.A, Keys.Shift Or Keys.B, Keys.Shift Or Keys.C}).KeysToCommand(Function(x) x.OnKeyArgs(DirectCast(Nothing, KeyEventArgs)), Function(args) CType(args, KeyEventArgs))
You can implement a custom behavior if there is no ready DevExpress behavior for your needs. For instance, the Ctrl+C keyboard combination copies the entire GridView row by default. You can attach your own behavior to copy the value of the selected row cell only.
public class ControlCBehavior : EventTriggerBase {
public ControlCBehavior()
: base("KeyDown") {
}
protected override void OnEvent() {
if (Args.Control && Args.KeyCode == System.Windows.Forms.Keys.C) {
var cbService = this.GetService<Services.IClipboardService>();
GridView View = Source as GridView;
if (View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn) != null && View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn).ToString() != String.Empty)
cbService.SetText(View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn).ToString());
else
XtraMessageBox.Show("The value in the selected cell is null or empty!");
Args.Handled = true;
}
}
}
Public Class ControlCBehavior
Inherits EventTriggerBase(Of System.Windows.Forms.KeyEventArgs)
Public Sub New()
MyBase.New("KeyDown")
End Sub
Protected Overrides Sub OnEvent()
If Args.Control AndAlso Args.KeyCode = System.Windows.Forms.Keys.C Then
Dim cbService = Me.GetService(Of Services.IClipboardService)()
Dim View As GridView = TryCast(Source, GridView)
If View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn) IsNot Nothing AndAlso View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn).ToString() <> String.Empty Then
cbService.SetText(View.GetRowCellValue(View.FocusedRowHandle, View.FocusedColumn).ToString())
Else
XtraMessageBox.Show("The value in the selected cell is null or empty!")
End If
Args.Handled = True
End If
End Sub
End Class
This behavior uses the custom IClipboardService service that copies text to a clipboard.
public class ClipboardService : IClipboardService {
public void SetText(string text) {
Clipboard.SetText(text);
}
}
public interface IClipboardService {
void SetText(string text);
}
Public Class ClipboardService
Implements IClipboardService
Public Sub SetText(ByVal text As String) Implements IClipboardService.SetText
Clipboard.SetText(text)
End Sub
End Class
Public Interface IClipboardService
Sub SetText(ByVal text As String)
End Interface
After the custom behavior is ready, register the service and call the AttachBehavior method to attach the behavior to the grid view.
mvvmContext1.RegisterService(new Services.ClipboardService());
mvvmContext1.AttachBehavior<Behaviors.ControlCBehavior>(gridView1);
mvvmContext1.RegisterService(New Services.ClipboardService())
mvvmContext1.AttachBehavior(Of Behaviors.ControlCBehavior)(gridView1)
The custom behavior now suppresses the default shortcut handler and copies only the selected cell value.