Predefined State Transitions Created in Code

• Dec 12, 2023
• 7 minutes to read

This topic describes how to create a predefined State Machine in code. A predefined State Machine cannot be changed by users. For information how to define a state machine at runtime, refer to the User-Defined State Transitions Specified at Runtime topic instead.

Important

In certain scenarios, creating a custom Controller with the SingleChoiceAction Action or a set of SimpleAction Actions can be simpler and more straightforward than defining a coded State Machine. So, if you need to define a static state management process that should not be changed by users, consider this approach first instead of using the State Machine module.

See Also:

Add a Simple Action

Add a Parametrized Action

How to: Include an Action to a Detail View Layout

Note

To see this example in action, refer to the State Machine section of the Feature Center demo located in the %PUBLIC%\Documents\DevExpress Demos 23.2\Components\XAF\FeatureCenter.NETFramework.XPO folder by default, or refer to the Feature Center demo online.

The following code defines a simple Task business class, which can have different states such as not started, in progress, completed, etc.

C# (EF Core)

using DevExpress.Persistent.Base;
using DevExpress.Persistent.BaseImpl.EF;
using System.ComponentModel.DataAnnotations;
//...
public enum TaskStatus { Draft, NotStarted, InProgress, Paused, Completed, Dropped }

[DefaultClassOptions,ImageName("BO_Task")]
public class Task : BaseObject {
    public virtual string Subject { get; set; }
    public virtual TaskStatus Status { get; set; }
}

// Make sure that you use options.UseChangeTrackingProxies() in your DbContext settings.

C# (XPO)

using DevExpress.Persistent.Base;
using DevExpress.Persistent.BaseImpl;
using DevExpress.Xpo;
//...
public enum TaskStatus { Draft, NotStarted, InProgress, Paused, Completed, Dropped }

[DefaultClassOptions, ImageName("BO_Task")]
public class Task : BaseObject {
    public Task(Session session) : base(session) { }
    public string Subject {
        get { return GetPropertyValue<string>(nameof(Subject)); }
        set { SetPropertyValue<string>(nameof(Subject), value); }
    }
    public TaskStatus Status {
        get { return GetPropertyValue<TaskStatus>(nameof(Status)); }
        set { SetPropertyValue<TaskStatus>(nameof(Status), value); }
    }
}

VB.NET (XPO)

Imports DevExpress.Persistent.Base
Imports DevExpress.Persistent.BaseImpl
Imports DevExpress.Xpo
'...
Public Enum TaskStatus
    Draft
    NotStarted
    InProgress
    Paused
    Completed
    Dropped
End Enum

<DefaultClassOptions, ImageName("BO_Task")> _
Public Class Task
    Inherits BaseObject
    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub
    Public Property Subject() As String
        Get
            Return GetPropertyValue(Of String)(NameOf(Subject))
        End Get
        Set(ByVal value As String)
            SetPropertyValue(Of String)(NameOf(Subject), value)
        End Set
    End Property
    Public Property Status() As TaskStatus
        Get
            Return GetPropertyValue(Of TaskStatus)(NameOf(Status))
        End Get
        Set(ByVal value As TaskStatus)
            SetPropertyValue(Of TaskStatus)(NameOf(Status), value)
        End Set
    End Property
End Class

To create a state machine for this class, perform the following steps.

1. Declare a StateMachine<T> class descendant with the generic type parameter specifying the target business class. Override the Name property to specify a textual description for a state machine.

   C#

   using DevExpress.ExpressApp.StateMachine.NonPersistent;
   //...
   public class TaskStatusStateMachine : StateMachine<Task> {
       public override string Name { 
           get { return "Change status to"; } 
       }
   }
   

   VB.NET

   Imports DevExpress.ExpressApp.StateMachine.NonPersistent
   '...
   Public Class TaskStatusStateMachine
       Inherits StateMachine(Of Task)
       Public Overrides ReadOnly Property Name() As String
           Get
               Return "Change status to"
           End Get
       End Property
   End Class
   

2. Decide which business class property should be used as a state property. To function, a state machine must be able to distinguish between object states. This is why you need to specify a property whose values will represent different object states. This can be either an enumeration-typed property or a reference property. In this example, the Status property is a perfect candidate. Thus, it is used as a state property, and consequently, its values will be used as state markers. Note that different states must use different marker objects. To specify a state property, override the StatePropertyName property.

   C#

   public class TaskStatusStateMachine : StateMachine<Task> {
       //...
       public override string StatePropertyName { 
           get { return "Status"; }
       }
   }
   

   VB.NET

   Public Class TaskStatusStateMachine
       Inherits StateMachine(Of Task)
       '...
       Public Overrides ReadOnly Property StatePropertyName() As String
           Get
               Return "Status"
           End Get
       End Property
   End Class
   

3. Declare a set of allowed states and transitions between them. States are State class instances and transitions are Transition class instances. A state belongs to a state machine and has an associated caption and a marker. Each state has a collection of allowed transitions that specify possible target states. In this example, it should not be possible for a task to become completed until it is in progress, so the in progress state should not contain a completed transition.

   C#

   using DevExpress.ExpressApp.StateMachine;
   //...
   public class TaskStatusStateMachine : StateMachine<Task> {
       //...
       private IState startState;
       public TaskStatusStateMachine(IObjectSpace objectSpace) : base(objectSpace) {
           startState = new State(this, TaskStatus.Draft);
   
           IState notStartedState = new State(this, "Not Started", TaskStatus.NotStarted);
           IState inProgressState = new State(this, "In Progress", TaskStatus.InProgress);
           IState pausedState = new State(this, TaskStatus.Paused);
           IState completedState = new State(this, TaskStatus.Completed);
           IState droppedState = new State(this, TaskStatus.Dropped);
   
           startState.Transitions.Add(new Transition(notStartedState));
           notStartedState.Transitions.Add(new Transition(startState));
           notStartedState.Transitions.Add(new Transition(inProgressState));            
           inProgressState.Transitions.Add(new Transition(pausedState));
           inProgressState.Transitions.Add(new Transition(completedState));
           inProgressState.Transitions.Add(new Transition(droppedState));
           pausedState.Transitions.Add(new Transition(inProgressState));
           droppedState.Transitions.Add(new Transition(notStartedState));
   
           States.Add(startState);
           States.Add(notStartedState);
           States.Add(inProgressState);
           States.Add(pausedState);
           States.Add(completedState);
           States.Add(droppedState);
       }
       public override IState StartState { 
           get { return startState; } 
       }
   }
   

   VB.NET

   Imports DevExpress.ExpressApp.StateMachine
   '...
   Public Class TaskStatusStateMachine
       Inherits StateMachine(Of Task)
       '...
       Private startState_Renamed As IState
       Public Sub New(ByVal objectSpace As IObjectSpace)
           MyBase.New(objectSpace)
           startState_Renamed = New State(Me, TaskStatus.Draft)
   
           Dim notStartedState As IState = New State(Me, "Not Started", TaskStatus.NotStarted)
           Dim inProgressState As IState = New State(Me, "In Progress", TaskStatus.InProgress)
           Dim pausedState As IState = New State(Me, TaskStatus.Paused)
           Dim completedState As IState = New State(Me, TaskStatus.Completed)
           Dim droppedState As IState = New State(Me, TaskStatus.Dropped)
   
           startState_Renamed.Transitions.Add(New Transition(notStartedState))
           notStartedState.Transitions.Add(New Transition(startState_Renamed))
           notStartedState.Transitions.Add(New Transition(inProgressState))
           inProgressState.Transitions.Add(New Transition(pausedState))
           inProgressState.Transitions.Add(New Transition(completedState))
           inProgressState.Transitions.Add(New Transition(droppedState))
           pausedState.Transitions.Add(New Transition(inProgressState))
           droppedState.Transitions.Add(New Transition(notStartedState))
   
           States.Add(startState_Renamed)
           States.Add(notStartedState)
           States.Add(inProgressState)
           States.Add(pausedState)
           States.Add(completedState)
           States.Add(droppedState)
       End Sub
       Public Overrides ReadOnly Property StartState() As IState
           Get
               Return startState_Renamed
           End Get
       End Property
   End Class
   

   After you have defined states, transitions, and appearance rules in a state machine constructor, add them to the machine’s States collection. Override the StartState property to specify the initial state for newly created objects.

4. A state machine can optionally have Conditional Appearance rules associated with its states. These rules are represented by StateAppearance instances.

   C#

   public class TaskStatusStateMachine : StateMachine<Task> {
       //...
       public TaskStatusStateMachine(IObjectSpace objectSpace) : base(objectSpace) {
           //...        
           StateAppearance inProgressAppearance = new StateAppearance(inProgressState);
           inProgressAppearance.TargetItems = "Subject";
           inProgressAppearance.Enabled = false;
           StateAppearance completedAppearance = new StateAppearance(completedState);
           completedAppearance.TargetItems = "Subject";
           completedAppearance.Enabled = false;
           StateAppearance pausedAppearance = new StateAppearance(pausedState);
           pausedAppearance.TargetItems = "*";
           pausedAppearance.BackColor = System.Drawing.Color.Yellow;
       }
   }
   

   VB.NET

   Public Class TaskStatusStateMachine
       Inherits StateMachine(Of Task)
       Private startState_Renamed As IState
       '...
       Public Sub New(ByVal objectSpace As IObjectSpace)
           MyBase.New(objectSpace)
           '...
           Dim inProgressAppearance As New StateAppearance(inProgressState)
           inProgressAppearance.TargetItems = "Subject"
           inProgressAppearance.Enabled = False
           Dim completedAppearance As New StateAppearance(completedState)
           completedAppearance.TargetItems = "Subject"
           completedAppearance.Enabled = False
           Dim pausedAppearance As New StateAppearance(pausedState)
           pausedAppearance.TargetItems = "*"
           pausedAppearance.BackColor = System.Drawing.Color.Yellow
       End Sub
   End Class
   

5. Implement the IStateMachineProvider interface in the target business class. The interface’s only member - GetStateMachines method - returns a list of all state machines available for the business class.

   C# (EF Core)

   using System.Collections.Generic;
   using DevExpress.ExpressApp.EFCore;
   using DevExpress.ExpressApp.StateMachine;
   using DevExpress.ExpressApp.EFCore;
   //...
   public class Task : BaseObject, IStateMachineProvider {
       //...
       public IList<IStateMachine> GetStateMachines() {
           List<IStateMachine> result = new List<IStateMachine>();
           result.Add(new TaskStatusStateMachine(EFCoreObjectSpace.FindObjectSpaceByObject(this)));
           return result;
       }
   }
   
   // Make sure that you use options.UseChangeTrackingProxies() in your DbContext settings.
   

   C# (XPO)

   using System.Collections.Generic;
   //...
   public class Task : BaseObject, IStateMachineProvider {
       //...
       public IList<IStateMachine> GetStateMachines() {
           List<IStateMachine> result = new List<IStateMachine>();
           result.Add(new TaskStatusStateMachine(XPObjectSpace.FindObjectSpaceByObject(this)));
           return result;
       }
   }
   

   VB.NET (XPO)

   Imports System.Collections.Generic
   '...
   Public Class Task
       Inherits BaseObject
       Implements IStateMachineProvider
       '...
       Public Function GetStateMachines() As IList(Of IStateMachine)
           Dim result As New List(Of IStateMachine)()
           result.Add(New TaskStatusStateMachine(XPObjectSpace.FindObjectSpaceByObject(Me)))
           Return result
       End Function
   End Class
   

The State Machine module will display the ChangeStateAction Action in Task Views. The Action is provided by the StateMachineController.

Note

The StateMachineController is not designed to execute additional logic using its TransitionExecuting and TransitionExecuted events. Instead, place your custom code in the setter of the desired business class property.

You can optionally implement the IStateMachineUISettings interface and set its ExpandActionsInDetailView property to true.

C#

public class TaskStatusStateMachine : StateMachine<Task>, IStateMachineUISettings {
    //...
    public bool ExpandActionsInDetailView {
        get { return true; }
    }
}

VB.NET

Public Class TaskStatusStateMachine
    Inherits StateMachine(Of Task)
    Implements IStateMachineUISettings
    '...
    Public ReadOnly Property ExpandActionsInDetailView() As Boolean
        Get
            Return True
        End Get
    End Property
End Class

In this instance, the target business class Detail Views will contain separate Simple Actions corresponding to available state transitions.