Handle Object Events

Object events are used to respond to the opening, adding, modifying, and erasing of objects from a drawing database. There are two types of object related events: object and database level. Object level events are defined to respond to a specific object in a database, whereas Database level events respond to all objects in a database.

You define an object level event by registering an event handler with the event of a database object. Database level object events are defined by registering an event handler with one of the events of an open Database object.

The following events are available for DBObjects:

Cancelled: Triggered when the opening of the object is cancelled text.
Copied: Triggered after the object is cloned.
Erased: Triggered when the object is flagged to be erased or is unerased.
Goodbye: Triggered when the object is about to be deleted from memory because its associated database is being destroyed.
Modified: Triggered when the object is modified.
ModifiedXData: Triggered when the XData attached to the object is modified.
ModifyUndone: Triggered when previous changes to the object are being undone.
ObjectClosed: Triggered when the object is closed.
OpenedForModify: Triggered before the object is modified.
Reappended: Triggered when the object is removed from the database after an Undo operation and then re-appended with a Redo operation.
SubObjectModified: Triggered when a subobject of the object is modified.
Unappended: Triggered when the object is removed from the database after an Undo operation.

The following are some of the events used to respond to object changes at the Database level:

ObjectAppended: Triggered when an object is added to a database.
ObjectErased: Triggered when an object is erased or unerased from a database.
ObjectModified: Triggered when an object has been modified.
ObjectOpenedForModify: Triggered before an object is modified.
ObjectReappended: Triggered when an object is removed from a database after an Undo operation and then re-appended with a Redo operation.
ObjectUnappended: Triggered when an object is removed from a database after an Undo operation.

Enable an Object event

This example creates a lightweight polyline with events. The event handler for the polyline then displays the new area whenever the polyline is changed. To trigger the event, simply change the size of the polyline in AutoCAD. Remember that you must run the CreatePLineWithEvents subroutine before the event handler is activated.

VB.NET

Imports Autodesk.AutoCAD.Runtime

Imports Autodesk.AutoCAD.ApplicationServices

Imports Autodesk.AutoCAD.DatabaseServices

Imports Autodesk.AutoCAD.Geometry

'' Global variable for polyline object

Dim acPoly As Polyline = Nothing

<CommandMethod("AddPlObjEvent")> _

Public Sub AddPlObjEvent()

  '' Get the current document and database, and start a transaction

  Dim acDoc As Document = Application.DocumentManager.MdiActiveDocument

  Dim acCurDb As Database = acDoc.Database

  Using acTrans As Transaction = acCurDb.TransactionManager.StartTransaction()

      '' Open the Block table record for read

      Dim acBlkTbl As BlockTable

      acBlkTbl = acTrans.GetObject(acCurDb.BlockTableId, _

                                   OpenMode.ForRead)

      '' Open the Block table record Model space for write

      Dim acBlkTblRec As BlockTableRecord

      acBlkTblRec = acTrans.GetObject(acBlkTbl(BlockTableRecord.ModelSpace), _

                                      OpenMode.ForWrite)

      '' Create a closed polyline

      acPoly = New Polyline()

      acPoly.AddVertexAt(0, New Point2d(1, 1), 0, 0, 0)

      acPoly.AddVertexAt(1, New Point2d(1, 2), 0, 0, 0)

      acPoly.AddVertexAt(2, New Point2d(2, 2), 0, 0, 0)

      acPoly.AddVertexAt(3, New Point2d(3, 3), 0, 0, 0)

      acPoly.AddVertexAt(4, New Point2d(3, 2), 0, 0, 0)

      acPoly.Closed = True

      '' Add the new object to the block table record and the transaction

      acBlkTblRec.AppendEntity(acPoly)

      acTrans.AddNewlyCreatedDBObject(acPoly, True)

      AddHandler acPoly.Modified, AddressOf acPolyMod

      '' Save the new object to the database

      acTrans.Commit()

  End Using

End Sub

<CommandMethod("RemovePlObjEvent")> _

Public Sub RemovePlObjEvent()

  If acPoly <> Nothing Then

      '' Get the current document and database, and start a transaction

      Dim acDoc As Document = Application.DocumentManager.MdiActiveDocument

      Dim acCurDb As Database = acDoc.Database

      Using acTrans As Transaction = acCurDb.TransactionManager.StartTransaction()

          '' Open the polyline for read

          acPoly = acTrans.GetObject(acPoly.ObjectId, _

                                     OpenMode.ForRead)

          If acPoly.IsWriteEnabled = False Then

              acPoly.UpgradeOpen()

          End If

          RemoveHandler acPoly.Modified, AddressOf acPolyMod

          acPoly = Nothing

      End Using

  End If

End Sub

Public Sub acPolyMod(ByVal senderObj As Object, _

                     ByVal evtArgs As EventArgs)

  Application.ShowAlertDialog("The area of " & _

                              acPoly.ToString() & " is: " & _

                              acPoly.Area)

End Sub

using Autodesk.AutoCAD.Runtime;

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.DatabaseServices;

using Autodesk.AutoCAD.Geometry;

// Global variable for polyline object

Polyline acPoly = null;

[CommandMethod("AddPlObjEvent")]

public void AddPlObjEvent()

  // Get the current document and database, and start a transaction

  Document acDoc = Application.DocumentManager.MdiActiveDocument;

  Database acCurDb = acDoc.Database;

  using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())

      // Open the Block table record for read

      BlockTable acBlkTbl;

      acBlkTbl = acTrans.GetObject(acCurDb.BlockTableId,

                                   OpenMode.ForRead) as BlockTable;

      // Open the Block table record Model space for write

      BlockTableRecord acBlkTblRec;

      acBlkTblRec = acTrans.GetObject(acBlkTbl[BlockTableRecord.ModelSpace],

                                      OpenMode.ForWrite) as BlockTableRecord;

      // Create a closed polyline

      acPoly = new Polyline();

      acPoly.AddVertexAt(0, new Point2d(1, 1), 0, 0, 0);

      acPoly.AddVertexAt(1, new Point2d(1, 2), 0, 0, 0);

      acPoly.AddVertexAt(2, new Point2d(2, 2), 0, 0, 0);

      acPoly.AddVertexAt(3, new Point2d(3, 3), 0, 0, 0);

      acPoly.AddVertexAt(4, new Point2d(3, 2), 0, 0, 0);

      acPoly.Closed = true;

      // Add the new object to the block table record and the transaction

      acBlkTblRec.AppendEntity(acPoly);

      acTrans.AddNewlyCreatedDBObject(acPoly, true);

      acPoly.Modified += new EventHandler(acPolyMod);

      // Save the new object to the database

      acTrans.Commit();

[CommandMethod("RemovePlObjEvent")]

public void RemovePlObjEvent()

  if (acPoly != null)

      // Get the current document and database, and start a transaction

      Document acDoc = Application.DocumentManager.MdiActiveDocument;

      Database acCurDb = acDoc.Database;

      using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())

          // Open the polyline for read

          acPoly = acTrans.GetObject(acPoly.ObjectId,

                                     OpenMode.ForRead) as Polyline;

          if (acPoly.IsWriteEnabled == false)

              acPoly.UpgradeOpen();

          acPoly.Modified -= new EventHandler(acPolyMod);

          acPoly = null;

public void acPolyMod(object senderObj,

                      EventArgs evtArgs)

  Application.ShowAlertDialog("The area of " +

                              acPoly.ToString() + " is: " +

                              acPoly.Area);

VBA/ActiveX Code Reference