OPC Studio User's Guide and Reference
SubscribeMultipleItems Method (_EasyDAClient)
Example 



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OpcLabs.EasyOpcClassic Assembly > OpcLabs.EasyOpc.DataAccess.ComTypes Namespace > _EasyDAClient Interface : SubscribeMultipleItems Method
Array of arguments, one element per each OPC item involved in the operation.

The value of this parameter cannot be null (Nothing in Visual Basic).

The individual elements of the parameter value cannot be null (Nothing in Visual Basic).

Subscribe to changes of multiple OPC items. The OpcLabs.EasyOpc.DataAccess.IEasyDAClient.ItemChanged event will be generated for each significant item change. Callback methods will be invoked if they are specified in the argument objects.
Syntax
'Declaration
 
<NotNullAttribute()>
Function SubscribeMultipleItems( _
   ByVal itemSubscriptionArgumentsArray As Object _
) As Object()
 
'Usage
 
Dim instance As _EasyDAClient
Dim itemSubscriptionArgumentsArray As Object
Dim value() As Object
 
value = instance.SubscribeMultipleItems(itemSubscriptionArgumentsArray)

Parameters

itemSubscriptionArgumentsArray
Array of arguments, one element per each OPC item involved in the operation.

The value of this parameter cannot be null (Nothing in Visual Basic).

The individual elements of the parameter value cannot be null (Nothing in Visual Basic).

Return Value

The function returns an array of integer handles. Each such handle uniquely identifies the item subscription. The indices of elements in the output array are the same as those in the input array, itemSubscriptionArgumentsArray.

This method never returns null (Nothing in Visual Basic).

The individual elements of the returned value are never null (Nothing in Visual Basic).

Exceptions
ExceptionDescription

A null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.

This is a usage error, i.e. it will never occur (the exception will not be thrown) in a correctly written program. Your code should not catch this exception.

Remarks
The size of the input array will become the size of the output array. The element positions (indices) in the output array are the same as in the input array.

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.

The server(s) can be local or can be remotely accessed via DCOM.

Example

COM

COM

// This example shows how to subscribe to changes of a single OPC XML-DA 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('*** Failure: ', eventArgs.ErrorMessageBrief);
end;

class procedure SubscribeItem.MainXml;
var
  Arguments: OleVariant;
  Client: TEasyDAClient;
  ClientEventHandlers: TSubscribeItem_ClientEventHandlers;
  HandleArray: OleVariant;
  ItemSubscriptionArguments1: _EasyDAItemSubscriptionArguments;
begin
  ItemSubscriptionArguments1 := CoEasyDAItemSubscriptionArguments.Create;
  ItemSubscriptionArguments1.ServerDescriptor.UrlString := 'http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx';
  ItemSubscriptionArguments1.ItemDescriptor.ItemID := 'Dynamic/Analog Types/Int';
  ItemSubscriptionArguments1.GroupParameters.RequestedUpdateRate := 1000;

  Arguments := VarArrayCreate([0, 0], varVariant);
  Arguments[0] := ItemSubscriptionArguments1;

  // Instantiate the client object and hook events
  Client := TEasyDAClient.Create(nil);
  ClientEventHandlers := TSubscribeItem_ClientEventHandlers.Create;
  Client.OnItemChanged := ClientEventHandlers.OnItemChanged;

  WriteLn('Subscribing...');
  TVarData(HandleArray).VType := varArray or varVariant;
  TVarData(HandleArray).VArray := PVarArray(
    Client.SubscribeMultipleItems(Arguments));

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

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

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

  VarClear(HandleArray);
  VarClear(Arguments);
  FreeAndNil(Client);
  FreeAndNil(ClientEventHandlers);
end;
// This example shows how to subscribe to changes of multiple items 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
  TSubscribeMultipleItems_ClientEventHandlers = class
    // Item changed event handler
    procedure OnItemChanged(
      ASender: TObject;
      sender: OleVariant;
      const eventArgs: _EasyDAItemChangedEventArgs);
  end;

procedure TSubscribeMultipleItems_ClientEventHandlers.OnItemChanged(
  ASender: TObject;
  sender: OleVariant;
  const eventArgs: _EasyDAItemChangedEventArgs);
begin
  if eventArgs.Succeeded then
    WriteLn(eventArgs.Arguments.ItemDescriptor.ItemId, ': ', eventArgs.Vtq.ToString)
  else
    WriteLn(eventArgs.Arguments.ItemDescriptor.ItemId, ' *** Failure: ', eventArgs.ErrorMessageBrief);
end;

class procedure SubscribeMultipleItems.Main;
var
  Arguments: OleVariant;
  Client: TEasyDAClient;
  ClientEventHandlers: TSubscribeMultipleItems_ClientEventHandlers;
  HandleArray: OleVariant;
  ItemSubscriptionArguments1: _EasyDAItemSubscriptionArguments;
  ItemSubscriptionArguments2: _EasyDAItemSubscriptionArguments;
  ItemSubscriptionArguments3: _EasyDAItemSubscriptionArguments;
  ItemSubscriptionArguments4: _EasyDAItemSubscriptionArguments;
begin
  ItemSubscriptionArguments1 := CoEasyDAItemSubscriptionArguments.Create;
  ItemSubscriptionArguments1.ServerDescriptor.ServerClass := 'OPCLabs.KitServer.2';
  ItemSubscriptionArguments1.ItemDescriptor.ItemID := 'Simulation.Random';
  ItemSubscriptionArguments1.GroupParameters.RequestedUpdateRate := 1000;

  ItemSubscriptionArguments2 := CoEasyDAItemSubscriptionArguments.Create;
  ItemSubscriptionArguments2.ServerDescriptor.ServerClass := 'OPCLabs.KitServer.2';
  ItemSubscriptionArguments2.ItemDescriptor.ItemID := 'Trends.Ramp (1 min)';
  ItemSubscriptionArguments2.GroupParameters.RequestedUpdateRate := 1000;

  ItemSubscriptionArguments3 := CoEasyDAItemSubscriptionArguments.Create;
  ItemSubscriptionArguments3.ServerDescriptor.ServerClass := 'OPCLabs.KitServer.2';
  ItemSubscriptionArguments3.ItemDescriptor.ItemID := 'Trends.Sine (1 min)';
  ItemSubscriptionArguments3.GroupParameters.RequestedUpdateRate := 1000;

  ItemSubscriptionArguments4 := CoEasyDAItemSubscriptionArguments.Create;
  ItemSubscriptionArguments4.ServerDescriptor.ServerClass := 'OPCLabs.KitServer.2';
  ItemSubscriptionArguments4.ItemDescriptor.ItemID := 'Simulation.Register_I4';
  ItemSubscriptionArguments4.GroupParameters.RequestedUpdateRate := 1000;

  Arguments := VarArrayCreate([0, 3], varVariant);
  Arguments[0] := ItemSubscriptionArguments1;
  Arguments[1] := ItemSubscriptionArguments2;
  Arguments[2] := ItemSubscriptionArguments3;
  Arguments[3] := ItemSubscriptionArguments4;

  // Instantiate the client object and hook events
  Client := TEasyDAClient.Create(nil);
  ClientEventHandlers := TSubscribeMultipleItems_ClientEventHandlers.Create;
  Client.OnItemChanged := ClientEventHandlers.OnItemChanged;

  TVarData(HandleArray).VType := varArray or varVariant;
  TVarData(HandleArray).VArray := PVarArray(
    Client.SubscribeMultipleItems(Arguments));

  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.');
  VarClear(HandleArray);
  VarClear(Arguments);
  FreeAndNil(Client);
  FreeAndNil(ClientEventHandlers);
end;
Requirements

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

See Also