View source for First experiences using QuickOPC with Free Pascal (Lazarus)

[[Category:Application note]] [[Category:COM]] [[Category:Free Pascal]] [[Category:Lazarus]] 

== Introduction ==
Performed with Lazarus installation package '''lazarus-1.6.0-fpc-3.0.0-win64.exe''', on Windows 7 (x64). Later tested the pre-made code with '''lazarus-1.6.2-fpc-3.0.0-win64.exe''', on Windows 7 (x64), and Windows 10 2015 (x64). Seen crashes when '''lazarus-1.6.2-fpc-3.0.0-win32.exe''' was used.

Lazarus downloads page is: https://www.lazarus-ide.org/index.php?page=downloads .

QuickOPC versions used: 2016.2 (5.41) "in the works", later 2018.1 (5.52) Beta.

== 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. {{Style=menu|Package -> Install/Uninstall Packages ...}}, select {{Style=item|LazActiveX 0.1}} under {{Style=label|Available for installation}}, press {{Style=button|Install selection}}, press {{Style=button|Save and rebuild IDE}}. Press {{Style=button|Continue}}.

Then, use the procedures described on the LazActiveX page under '''TActiveXContainer early binding'''. {{Style=menu|Tools -> Import Type Library}}, select tab {{Style=label|ActiveX References}}, type {{Style=keyboard|OPC Labs}} into the {{Style=label|Search}} box, select one of our libraries (their names all start with "OPC Labs"), check {{Style=label|Convert dependant typelibs}}, press {{Style=button|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'''. You need to do this in both '''mscorlib''' version imported (see further down).

{{Note|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.}}

{{Note|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:

<pre>
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>"
</pre>

In order to resolve it, it is necessary to go back to the code generated for '''mscorlib''' (in both of them, again), and comment out the line with declaration of '''Pointer''', as such:
<pre>
 // Pointer = _Pointer;
</pre>

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'''.

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.

{{Note|The code generated by type library importer did not compile for a property getter that was of type ' '''object[]''' ' in .NET. We have resolved this by changing its type to '''Int32Collection''', which was acceptable at that place ('''AEAttributeSetDictionary.Item''').}}

{{Warning|Due to keywords conflicts, '''LazActiveX''' importer renames methods like '''Read''' to '''Read_''' and '''Write''' to '''Write_'''. Obviously, simply using the right method names is a solution - just don't be surprised when something as "primitive" as '''Read''' seems to be missing. '''Text''' gets renamed to '''Text_''', '''Reset''' to '''Reset_'''.}}

== Examples ==

A working example:

<syntaxhighlight lang="pascal">
// 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;
</syntaxhighlight>

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

<syntaxhighlight lang="pascal">
// 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;                
</syntaxhighlight>

== 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 ==

QuickOPC works with Free Pascal (Lazarus) well - after overcoming the issues described. Examples in Free Pascal will be included with QuickOPC version 2016.2. We will have all examples for OPC UA that were present in version 5.40 for Object Pascal (Delphi), and some more.

== See also ==
Related links:
* http://www.lazarus-ide.org/
* http://wiki.freepascal.org/
* http://wiki.freepascal.org/LazActiveX