AsyncCommand Class

An asynchronous command without a command parameter.

Namespace: DevExpress.Mvvm

Assembly: DevExpress.WinUI.Mvvm.v23.2.dll

NuGet Package: DevExpress.WinUI

Declaration

C#

public class AsyncCommand :
    AsyncCommand<object>

VB.NET

Public Class AsyncCommand
    Inherits AsyncCommand(Of Object)

Remarks

You can use asynchronous commands to run a time-consuming operation in a separate thread and keep the UI responsive.

For example, you can bind a button to an asynchronous command. When a user clicks the button, the command is invoked and the button becomes disabled. The rest of the interface does not freeze. When the process is done, the button is enabled.

Run Demo: AsyncCommand Module in the WinUI MVVM Demo

The DevExpress WinUI MVVM Framework includes the following asynchronous commands:

AsyncCommand<T>

A command whose Execute(T) and CanExecute(T) delegates accept a command parameter of the T type.

AsyncCommand

A command whose Execute and CanExecute delegates do not have a command parameter.

Create and Run Asynchronous Commands

The following code sample creates AsyncCommand and AsyncCommand<T> instances and binds them to the Button.Command property.

XAML

<Button Content="..." Command="{x:Bind MyAsyncCommand}"/>
<Button Content="..." Command="{x:Bind MyAsyncCommand}" CommandParameter="..."/>

C#

public MyViewModel() {
    MyAsyncCommand = new AsyncCommand<string>(Calculate, CanCalculate);
}

public AsyncCommand<string> MyAsyncCommand { get; }
async Task Calculate(string parameter) {
    //...
}
bool CanCalculate(string parameter) {
    //...
}

VB.NET

Public Sub New()
    MyAsyncCommand = New AsyncCommand(Of String)(AddressOf Calculate, AddressOf CanCalculate)
End Sub

Private privateMyAsyncCommand As AsyncCommand(Of String)
Public Property MyAsyncCommand() As AsyncCommand(Of String)
    Get
        Return privateMyAsyncCommand
    End Get
    Private Set(ByVal value As AsyncCommand(Of String))
        privateMyAsyncCommand = value
    End Set
End Property
Private Async Function Calculate(ByVal parameter As String) As Task
    '...
End Function
Private Function CanCalculate(ByVal parameter As String) As Boolean
    '...
End Function

Cancel a Command Execution

Do any of the following to cancel a command’s execution:

• Call a Cancel method.

• Call the CancelCommand.

  XAML

  <Button Content="Calculate" Command="{x:Bind MyAsyncCommand}"/>
  <Button Content="Cancel" Command="{x:Bind MyAsyncCommand.CancelCommand}"/>
  <ProgressBar Value="{x:Bind Progress, Mode=OneWay}"/>
  

• Pass a CancellationToken to the AsyncCommand constructor and call the CancellationToken.ThrowIfCancellationRequested method.

  C#

  public AsyncCommand MyAsyncCommand { get; private set; }
  
  public MyViewModel() {
      MyAsyncCommand = new AsyncCommand(Calculate);
  }
  
  async Task Calculate(CancellationToken cancellationToken) {
      for(int i = 0; i <= 100; i++) {
          cancellationToken.ThrowIfCancellationRequested();
          // ...
      }
  }
  

  VB.NET

  Public Property MyAsyncCommand As AsyncCommand
  
  Public Sub New()
      MyAsyncCommand = New AsyncCommand(Calculate)
  End Sub
  
  Function Async Calculate(ByVal cancellationToken As CancellationToken) As Task
      For i = 0 To 100
          cancellationToken.ThrowIfCancellationRequested()
          ' ...
      Next
  End Sub
  

When a command’s cancellation process is started, the IsCancellationRequested property is set to true. When a command is cancelled, the IsCancellationRequested property is set to false.

Access the UI Thread from the Command

Use the IDispatcherService to access the UI thread during the command execution process.

C#

IDispatcherService DispatcherService => GetUIService<IDispatcherService>();
async Task Calculate() {
    ...
    await DispatcherService.InvokeAsync(() => { ... });

    // Or if you do not need to wait until the command is executed:
    DispatcherService.BeginInvoke(() => { ... });
    ...
}

VB.NET

ReadOnly Property DispatcherService As IDispatcherService
    Get
        Return GetUIService(Of IDispatcherService)()
    End Get
End Property
Async Sub Calculate() {
    ...
    Await DispatcherService.InvokeAsync(Sub() ...)

    'or if do not need to wait for your dispatcher operation is executed:
    DispatcherService.BeginInvoke(Sub() ...)
    ...
}

Simultaneous Command Execution

The AsyncCommand and AsyncCommand<T> classes contain the IsExecuting property. When the command execution is in progress, this property is set to true and the CanExecute(T) method returns false. If CanExecute(T) is false, you can disable a control that is bound to the command until the previous command execution is completed.

Set the AsyncCommand.AllowMultipleExecution constructor parameter to true to allow a user to execute a command multiple times simultaneously. In this case, the AsyncCommand.CanExecute method returns a value based on your CanExecute delegate implementation.

C#

MyAsyncCommand = new AsyncCommand<string>(Calculate, CanCalculate, allowMultipleExecution: true);

VB.NET

MyAsyncCommand = New AsyncCommand(Of String)(Calculate, CanCalculate, allowMultipleExecution:= True)

Implements

ICommand<Object>

IAsyncCommand

IDelegateCommand

Inheritance

Object

CommandBase<Object>

AsyncCommand<Object>

AsyncCommand