First experiences using QuickOPC with Free Pascal (Lazarus)

Introduction

Performed with in-the-works QuickOPC version 2016.2 (5.41), and Lazarus installation package lazarus-1.6.0-fpc-3.0.0-win64.exe, on Windows 7 (x64).

Steps Taken

In order to use the COM type libraries of QuickOPC, it is needed to install the LazActiveX package (comes with Lazarus). See http://wiki.freepascal.org/LazActiveX for details and instructions. Package -> Install/Uninstall Packages ..., select LazActiveX 0.1 under "Available for installation", press "Install selection", press "Save and rebuild IDE".

Then, use the procedures described on the LazActiveX page under TActiveXContainer early binding. Tools -> Import Type Library, select tab "ActiveX References", type "OPC Labs" into the Search box, select one of our libraries, check "Convert dependant typelibs", press OK. The detection of "dependant" libraries does not fully work, though - for example, OpcLabs.Baselib (OPC Labs Core Library) is not imported (the comment in the generated file says that it was not registered), therefore it is necessary to import some of the "dependant" libraries manually.

There appears to be a problem with mscorlib import, its compilation resulting in errors like "mscorlib_2_4_tlb.pas(5108,16) Error: Type "Byte" is not completely defined". Work around this by commenting out the declarations of PByte, Byte, PDouble, Double, PInt64, Int64, PSingle and Single.

There appears to be a problem with BaseLib import, an incomplete Const declaration for EmptyEnumeration is being generated. Work around this for start by commenting out the extra Const line. As this enumeration is not currently needed for COM development, we have removed it from the QuickOPC COM type library.

Argument name sender used in QuickOPC COM event sink interfaces had conflicted with the additional Sender argument generated by Free Pascal type library importer. We have resolved this by renaming our arguments to sender0 (as aSender would cause conflict with Delphi type library importer instead).

After making these changes, we were getting error similar to the one below:

opclabs_easyopcua_5_41_tlb.pas(8917,13) Error: Incompatible types: got "TEvsEasyUAClientConfiguration.EventSinkInvoke(TObject;LongInt;const TGuid;LongInt;Word;tagDISPPARAMS;_Pointer;_Pointer;_Pointer);" expected "<procedure variable type of procedure(TObject;LongInt;const TGuid;LongInt;Word;tagDISPPARAMS;Pointer;Pointer;Pointer) of object;Register>"

In order to resolve it, it is necessary to go back to the code generated for mscorlib, and comment out the line with declaration of Pointer, as such:

 // Pointer = _Pointer;

After these changes, it should be possible to compile the .PAS code for imported type libraries. Note that due to various commonly-named types in mscorlib, be prepared that by placing that imported library into uses of your program, some types may need further qualification. For example, you may have to write SysUtils.Exception instead of simply Exception, because Exception is now also a class defined in mscorlib.

Examples

A working example:

// This example shows how to read value of a single node, and display it.

class procedure ReadValue.Main;
var
  Client: EasyUAClient;
  Value: OleVariant;
begin
  // Instantiate the client object
  Client := CoEasyUAClient.Create;

  Value := Client.ReadValue(
    'http://opcua.demo-this.com:51211/UA/SampleServer',
    'nsu=http://test.org/UA/Data/;i=10853');
  WriteLn('value: ', Value);
end;

Hooking COM events requires use of the TEvsXXXX wrappers, like this:

// This example shows how to subscribe to event notifications and display each
// incoming event.

type
  TClientEventHandlers2 = class
    procedure OnEventNotification(
      Sender: TObject;
      sender0: OleVariant;
      eventArgs: _EasyUAEventNotificationEventArgs);
  end;

procedure TClientEventHandlers2.OnEventNotification(
  Sender: TObject;
  sender0: OleVariant;
  eventArgs: _EasyUAEventNotificationEventArgs);
begin
  // Display the event
  WriteLn(eventArgs.ToString);
end;

class procedure SubscribeEvent.Main;
var
  Client: EasyUAClient;
  EvsClient: TEvsEasyUAClient;
  ClientEventHandlers2: TClientEventHandlers2;
begin
  // Instantiate the client object and hook events
  EvsClient := TEvsEasyUAClient.Create(nil);
  Client := EvsClient.ComServer;
  ClientEventHandlers2 := TClientEventHandlers2.Create;
  EvsClient.OnEventNotification := @ClientEventHandlers2.OnEventNotification;

  WriteLn('Subscribing...');
  Client.SubscribeEvent(
    'opc.tcp://opcua.demo-this.com:62544/Quickstarts/AlarmConditionServer',
    'nsu=http://opcfoundation.org/UA/;i=2253',  // UAObjectIds.Server
    1000);

  WriteLn('Processing event notifications for 30 seconds...');
  PumpSleep(30*1000);

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

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

Importing OpcLabs.BaseLibForms brings in a different version of mscorlib (2.0 instead of 2.4), therefore the fixes described earlier for mscorlib must be applied to that second import as well.

Open issues

Data returned in UInt64 variant do not display (or even cannot be further processed?), throw EVariantError. See e.g. __EasyUAClient.CallMethod.Main.inc.

Conclusion

Examples in Free Pascal will be included with QuickOPC version 2016.2.

See also

Related links:

• http://www.lazarus-ide.org/
• http://wiki.freepascal.org/
• http://wiki.freepascal.org/LazActiveX