SubscribeItem Method (

Subscribe to OPC item. Specify machine name, server class, item ID, and requested update rate. No state object is specified (this overload is suitable for simple scenarios only).

Syntax

int SubscribeItem( 
   string machineName,
   string serverClass,
   string itemId,
   int requestedUpdateRate
)

int SubscribeItem( 
   String^ machineName,
   String^ serverClass,
   String^ itemId,
   int requestedUpdateRate
)

'Declaration
 
Function SubscribeItem( _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemId As String, _
   ByVal requestedUpdateRate As Integer _
) As Integer

'Usage
 
Dim instance As _EasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemId As String
Dim requestedUpdateRate As Integer
Dim value As Integer
 
value = instance.SubscribeItem(machineName, serverClass, itemId, requestedUpdateRate)

Parameters

machineName

Name of the machine (empty string for local computer).

The value represents a UNC or DNS computer name. Any string can be passed to this parameter (i.e. will not cause System.ArgumentException), but not all values make sense and will work when an operation using them is attempted. IPv6 addresses are normally enclosed between '[' and ']'.

The value of this parameter can be null (Nothing in Visual Basic).

serverClass

Contains ProgID of the OPC server.

The value of this parameter can be null (Nothing in Visual Basic).

itemId

ID of the item that will be subscribed to.

The value of this parameter can be null (Nothing in Visual Basic).

requestedUpdateRate

How often should the updates be received (number of milliseconds)

Return Value

The method returns an integer handle that uniquely identifies the item subscription.

Remarks

This member or type is for use from COM. It is not meant to be used from .NET or Python. Refer to the corresponding .NET member or type instead, if you are developing in .NET or Python.

Example

COM

// This example shows how to subscribe to changes of a single item and display the value of the item with each change.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in Object Pascal (Delphi) on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-OP .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

type
  TSubscribeItem_ClientEventHandlers = class
    // Item changed event handler
    procedure OnItemChanged(
      ASender: TObject;
      sender: OleVariant;
      const eventArgs: _EasyDAItemChangedEventArgs);
  end;

procedure TSubscribeItem_ClientEventHandlers.OnItemChanged(
  ASender: TObject;
  sender: OleVariant;
  const eventArgs: _EasyDAItemChangedEventArgs);
begin
  if eventArgs.Succeeded then
    WriteLn(eventArgs.Vtq.ToString)
  else
    WriteLn(Format('*** Failure: %s', [eventArgs.ErrorMessageBrief]));
end;

class procedure SubscribeItem.Main;
var
  Client: TEasyDAClient;
  ClientEventHandlers: TSubscribeItem_ClientEventHandlers;
begin
  // Instantiate the client object and hook events
  Client := TEasyDAClient.Create(nil);
  ClientEventHandlers := TSubscribeItem_ClientEventHandlers.Create;
  Client.OnItemChanged := ClientEventHandlers.OnItemChanged;

  Client.SubscribeItem('', 'OPCLabs.KitServer.2', 'Simulation.Random', 1000);

  WriteLn('Processing item changed events for 1 minute...');
  PumpSleep(60*1000);

  WriteLn('Unsubscribing...');
  Client.UnsubscribeAllItems;

  WriteLn('Waiting for 5 seconds...');
  PumpSleep(5*1000);

  WriteLn('Finished.');
  FreeAndNil(Client);
  FreeAndNil(ClientEventHandlers);
end;

// This example shows how to subscribe to changes of a single item and display the value of the item with each change.
//
// Some related documentation: http://php.net/manual/en/function.com-event-sink.php . Pay attention to the comment that says 
// "Be careful how you use this feature; if you are doing something similar to the example below, then it doesn't really make 
// sense to run it in a web server context.". What they are trying to say is that processing a web request should be 
// a short-lived code, which does not fit well with the idea of being subscribed to events and received them over longer time. 
// It is possible to write such code, but it is only useful when processing the request is allowed to take relatively long. Or, 
// when you are using PHP from command-line, or otherwise - not to serve a web page directly.
// 
// Subscribing to QuickOPC-COM events in the context of PHP Web application, while not imposing the limitations to the request 
// processing time, has to be "worked around", e.g. using the "event pull" mechanism.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in PHP on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-PHP .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

class DEasyDAClientEvents {
    function ItemChanged($varSender, $varE)
    {
        if ($varE->Succeeded)
        {
        print $varE->Vtq->ToString();
            print "\n";
        }
        else
        {
            printf("*** Failure: %s\n", $varE->ErrorMessageBrief);
        }
    }
}

$Client = new COM("OpcLabs.EasyOpc.DataAccess.EasyDAClient");
$Events = new DEasyDAClientEvents();
com_event_sink($Client, $Events, "DEasyDAClientEvents");

$Client->SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 1000);

print "Processing item changed events for 1 minute...\n";
$startTime = time(); do { com_message_pump(1000); } while (time() < $startTime + 60);

REM This example shows how to subscribe to changes of a single item and display the value of the item with each change.
REM
REM Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
REM OPC client and subscriber examples in Visual Basic on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VB .
REM Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
REM a commercial license in order to use Online Forums, and we reply to every post.

' The client object, with events
'Public WithEvents Client1 As EasyDAClient

Private Sub SubscribeItem_Main_Command_Click()
    OutputText = ""
    
    ' Instantiate the client object and hook events
    Set Client1 = New EasyDAClient

    OutputText = OutputText & "Subscribing..." & vbCrLf
    Call Client1.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 1000)

    OutputText = OutputText & "Processing item changed events for 1 minute..." & vbCrLf
    Pause 60000

    OutputText = OutputText & "Unsubscribing..." & vbCrLf
    Client1.UnsubscribeAllItems

    OutputText = OutputText & "Waiting for 5 seconds..." & vbCrLf
    Pause 5000

    OutputText = OutputText & "Finished." & vbCrLf
    Set Client1 = Nothing
End Sub

Public Sub Client1_ItemChanged(ByVal sender As Variant, ByVal eventArgs As EasyDAItemChangedEventArgs)
    If eventArgs.Succeeded Then
        OutputText = OutputText & eventArgs.vtq & vbCrLf
    Else
        OutputText = OutputText & "*** Failure: " & eventArgs.ErrorMessageBrief & vbCrLf
    End If
End Sub

Rem This example shows how to subscribe to changes of a single item and display the value of the item with each change.
Rem
Rem Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
Rem OPC client and subscriber examples in VBScript on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBScript .
Rem Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
Rem a commercial license in order to use Online Forums, and we reply to every post.

Option Explicit

Dim Client: Set Client = CreateObject("OpcLabs.EasyOpc.DataAccess.EasyDAClient")
WScript.ConnectObject Client, "Client_"

Client.SubscribeItem "", "OPCLabs.KitServer.2", "Simulation.Random", 1000

WScript.Echo "Processing item changed events for 1 minute..."
WScript.Sleep 60*1000



Sub Client_ItemChanged(Sender, e)
    If Not (e.Succeeded) Then
        WScript.Echo "*** Failure: " & e.ErrorMessageBrief
        Exit Sub
    End If

    WScript.Echo e.Vtq.ToString
End Sub

Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2016, Windows Server 2022; .NET: Linux, macOS, Microsoft Windows

Reference

_EasyDAClient Interface
_EasyDAClient Members

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Missing some example? Ask us for it on our Online Forums! You do not have to own a commercial license in order to use Online Forums, and we reply to every post.