delphi - NPAPI plugin framework Error

Question

I am trying to use NPAPI Framework from Yury Sidorov by following this answer:

How to embed Delphi VCL form into HTML page using NPAPI

but I get an error with NPPlugin.pas. I am using delphi XE7 and here what i did by following Krom Stern instructions

procedure DefDebugOut(const Msg: string);
begin
  OutputDebugStringA(PAnsiChar(Msg + #13#10)); // Changed From Pchar To PAnsiChar
end;

but still getting error with compiler with this message

[dcc32 Error] NPPlugin.pas(2215): E2010 Incompatible types: 'PWideChar' and 'PAnsiChar'

Error raised on this procedure

procedure TPlugin.SetException(const msg: string);
var
  s: String;
begin
  try
    {$ifopt D+} NPP_DebugOut('Exception: ' + msg); {$endif}
    s:=UTF8Encode(msg);
    NPN_SetException(m_pScriptableObject, PAnsiChar(s));
  except
    { prevent any exception from leaking out of DLL }
  end;
end;

here is the procedure NPN_SetException

procedure NPN_SetException(npobj: PNPObject; msg: PNPUTF8);
begin
  NavigatorFuncs.SetException(npobj, msg);
end;

Answer 1

I'll start with a piece by piece breakdown of what we can see. Bear in mind that we don't have NPPlugin.pas at hand and have to infer its contents from the information in the question. All the same, I think it's possible for us to do that accurately.

---

s := UTF8Encode(msg);

Here s is of type string. That's an alias for UnicodeString, encoded as UTF-16. So you convert from UTF-16 to UTF-8 and then back to UTF16.

You need it like this:

NPN_SetException(m_pScriptableObject, PAnsiChar(UTF8Encode(msg)));

Alternatively, if you need a variable to hold UTF-8 encoded text, declare it to be UTF8String which is AnsiString(65001). If you changed the type of s to be UTF8String then the code in the question would be correct. Although somewhat more verbose than it would need to be.

---

Another problem is here:

OutputDebugStringA(PAnsiChar(Msg + #13#10));

Your cast doesn't make Msg actually be 8 bit encoded. However, you don't want to use the ANSI version of the function. You need this:

OutputDebugString(PChar(Msg + sLineBreak));

---

Your exception handler is misguided. It is the DLL's job not to leak exceptions. If you attempt to catch and suppress them all you will simply mask errors in your own code. You need to remove that exception handler and check for errors by following the instructions given by the library documentation.

---

Now to the bigger picture. None of the above explains your reported error. The only sound explanation for that is that your declaration for NPN_SetException accepts wide text. In which case you could make the code compile simply by writing this:

NPN_SetException(m_pScriptableObject, PChar(msg));

Of course, that makes the appearance of UTF-8 somewhat inexplicable. In fact the library Mozilla does accept 8 bit text, UTF-8 encoded. So why would NPN_SetException expect to be passed UTF-16 text? Well it doesn't. The explanation is that you have declared NPN_SetException incorrectly. So, just to be clear, whilst PChar(msg) would make your code compile, it would not resolve your problem. You would be left with code that failed at runtime.

So, how did this happen? You've taken a working piece of code that used PChar aliased to PAnsiChar onto a Delphi with PChar aliased to PWideChar and not translated correctly. Even when you get your code to compile, it will not work correctly. You started with code like this:

function NPN_SetException(..., Msg: PChar): ...;

On older Delphi versions where PChar was PAnsiChar then this was correct. You are compiling this now on XE7 where PChar is PWideChar and so this is not correct. It needs to be:

function NPN_SetException(..., Msg: PAnsiChar): ...;

Then the calling code can be:

NPN_SetException(m_pScriptableObject, PAnsiChar(UTF8Encode(msg)));

---

My advice is that you:

1. Step back and revisit the handling of Unicode in Delphi.
2. Go back to you original code and change all the Mozilla interface code that uses PChar to PAnsiChar.
3. Whenever you need to provide PAnsiChar do it with PAnsiChar(UTF8Encode(str)).