How does EventHandler work? IConnectionPoint!

The EventHandler function allows you to connect some code to an object’s event interface. For example, Word allows you to run code when the user changes the Selected text in a document. An ADODB Recordset will fire events, such as WillMove and MoveComplete when moving from record to record.

The EventHandler function takes 2 objects as parameters: the event source, like Word or a RecordSet, and the event sink, like the code you’d like to run. Your event sink code must Implement an interface defined by the source.

EventHandler then does some handshaking between the two to get the events working.

  • It queries the Source for IConnectionPointContainer,
  • then uses that to call EnumConnectionPoints to enumerate the connection points.
  • The sink object is queried for the interface of each found connection point to see if it’s implemented.
  • If found, then the IConnectionPoint->Advise method is called to start the event handling.

The sample below creates an ADODB Recordset and makes a few calls to MoveNext, with and without an event sink attached. You can change the event sink connection to be the VFP native EventHandler function, or you can use the generated machine code to show how it works. Both ways return the same results.

Here are a couple issues:

  • We need a COM interface to the Event Sink (“myclass”) , which is not a COM object. We get that from the _VFP.Objects collection, which is using COM

  • We need a way to call generated machine code. I used the EnumChildWindows function, which is relatively painless. I just pass it the address of the code to execute as the EnumChildWindowProc, which returns False, so that it’s only called once.

  • The code has a lot of conditional jumps and some error checking. The jumping needs to be resolved in a 2 pass assembler. That’s what the Jump table does. It records the definitions and references of labels, then after the code is generated, the table is scanned for jump address fixups. Most assemblers use two passes.

  • The sample code is a little different from the native VFP EventHandler function: it doesn’t enumerate all the connection points, but rather does a FindConnectionPoint for the particular RecordSetEvents interface.

You can modify the sample to bind events in other scenarios. For example, I’ve found some objects may not work properly with EnumConnectionPoints, but work only with FindConnectionPoint. Other people have reported that the object may not have a Source interface, but do implement IConnectionPointContainer on other interfaces.

For example, oleview and navigate to Microsoft Word 11.0 Object library and dbl-click. Navigate to CoClass, CoClass Application

coclass Application {

    [default] interface _Application;

    [source] dispinterface ApplicationEvents;

    [source] dispinterface ApplicationEvents2;

    [source] dispinterface ApplicationEvents3;

    [default, source] dispinterface ApplicationEvents4;

};

The Word typelibrary shows that the Application interface sources some event interfaces.

If you look at c:\windows\system32\msfeeds.dll (which comes from installing IE7 beta 2) via OleView, there are no Source interfaces. However, you can call the GetWatcher method (the 24th vTable entry (offset 0x60)) of the IFeedFolder object to get a ConnectionPointContainer object.

As an exercise to the reader (Craig: this means you!), modify the sample code to connect events for RSSFeeds.

See also:

Binding to Internet Explorer Instances

Binding to Internet Explorer events: bug in Typelibrary

For a full sample of generating machine code on a background thread, see Webcrawl a blog to retrieve all entries locally: RSS on steroids

CLEAR ALL

CLEAR

LOCAL oEvents

LOCAL oRS AS adodb.recordset

LOCAL oConn AS adodb.Connection

SET ASSERTS ON

oEvents = NEWOBJECT("MyClass")

oConn = NEWOBJECT("adodb.connection")

oConn.Open("Provider=VFPOLEDB.1;Data Source="+HOME(2)+"northwind")

oRS = oConn.Execute("select * from customers")

fUseMyHandler=.t. && change this to use the custom handler below or the native VFP handler

IF fUseMyHandler

oEventEx=CREATEOBJECT("EventHandlerEx")

oEventEx.EVENTHANDLER(oRS, oEvents)

ELSE

? EVENTHANDLER(oRS, oEvents)

ENDIF

?

? PADR(oRS.Fields(0).Value,20)

IF fUseMyHandler

oEventEx.EVENTHANDLER(oRS, oEvents,.t.) && unbind

ELSE

? EVENTHANDLER (oRS, oEvents, .T.)

ENDIF

oRS.MoveNext

? PADR(oRS.Fields(0).Value,20)

oRS.MoveNext

? PADR(oRS.Fields(0).Value,20)

CLEAR ALL

DEFINE CLASS EventHandlerEx as Custom

hProcHeap =0

dwCookie=0 && IConnectionPoint->Advise cookie

oCOMVFP=null

hr=0 && HResult

cError=0 && addr of error

PROCEDURE init

DECLARE integer LoadLibrary IN WIN32API string

DECLARE integer FreeLibrary IN WIN32API integer

DECLARE integer GetProcAddress IN WIN32API integer hModule, string procname

DECLARE integer GetProcessHeap IN WIN32API

DECLARE integer HeapAlloc IN WIN32API integer hHeap, integer dwFlags, integer dwBytes

DECLARE integer HeapFree IN WIN32API integer hHeap, integer dwFlags, integer lpMem

DECLARE integer CLSIDFromString IN ole32 string lpszProgID, string @ strClSID

DECLARE integer SysAllocString IN oleaut32 string wstr

DECLARE integer SysFreeString IN oleaut32 integer bstr

DECLARE integer EnumChildWindows IN WIN32API integer hWnd, integer lpEnumProc, integer lParam

CREATE CURSOR memAllocs (memPtr i, AllocType c(1)) && track mem allocs that need to be freed: H=Heap,B=BSTR,L=Library

this.hProcHeap = GetProcessHeap()

PROCEDURE EVENTHANDLER(oSrc as Object, oSink as Object,fUnbind as Boolean)

CREATE table jumps (cLabel c(20),cFlag c(1),sCodePos i) && cFlag="D" defined, "R", reference

INDEX on cLabel+cFlag TAG cLabel

this.hr=this.MakeStr(REPLICATE(CHR(0),4)) && allocate space for HResult

this.cError=this.MakeStr(REPLICATE(CHR(0),4)) && Allocate space for error string

nLocals=10

sCode=""

sCode = sCode + CHR(0x55) && push ebp

sCode = sCode + CHR(0x8b) + CHR(0xec) && mov ebp, esp

sCode = sCode + CHR(0x81) + CHR(0xec)+BINTOC(nLocals * 4, "4rs") && sub esp, nLocals

* sCode = sCode + CHR(0xcc) && int 3 DebugBreak() to attach a debugger

*sCode = sCode + CHR(0xb8) + CHR(5)+CHR(0x40)+CHR(0)+CHR(0x80) && mov eax, 0x80004005 && pretend error msg to test err handling

*!* sCode = sCode + CHR(0x6a) + CHR(0x00) && push 0

*!* sCode = sCode + this.CallDllFunction("MessageBeep", "user32") && MessageBeep(0)

*hr = oSrc->QueryInterface(IID_IConnectionPointContainer,&pcpc) //First QI the oSrc for IConnectionPointContainer

sCode = sCode + CHR(0x8d) + CHR(0x45)+CHR(0xf0) && lea eax, [ebp-10h] && addr to put pConnectionPointContainer

sCode = sCode + CHR(0x50) && push eax

cIid=SPACE(16)

CLSIDFromString(STRCONV("{B196B284-BAB4-101A-B69C-00AA00341D07}"+CHR(0),5),@cIid) && IID_IConnectionPointContainer

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(cIid),"4rs") && mov eax, str

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0xb8) + BINTOC(SYS(3095,oSrc),"4rs") && mov eax, oSrc: the IDispatch for oSrc for THIS pointer

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x00) && call [eax+0h] && call indirect the function at 0h in the vTable, which is QI

sCode = sCode + CHR(0x83) + CHR(0xf8) + CHR(0x00) && cmp eax, 0 && if hr = SUCCESS

* jne FailedQICPC

sCode = sCode + CHR(0x75)+CHR(0x00) && jne nBytes && nBytes calc'd below. je= 0x74, Jump if Equal, jne = 0x75

INSERT INTO jumps values ("FailedQICPC","R",LEN(sCode)) && refer to a label to jump to at this pos

*hr= pcpc->FindConnectionPoint( IID_IRecordSet,&pcp) // get the pConnectionPoint

sCode = sCode + CHR(0x8d) + CHR(0x45)+CHR(0xec) && lea eax, [ebp-14h] && addr to put pcp COM ptr

sCode = sCode + CHR(0x50) && push eax

CLSIDFromString(STRCONV("{00000266-0000-0010-8000-00AA006D2EA4}"+CHR(0),5),@cIid) && IID for RecordSetEvents

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(cIid),"4rs") && mov eax, str

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xf0) && mov eax, [ebp-10h] ; pCPC

sCode = sCode + CHR(0x50) && push eax && push the THIS ptr

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x10) && call [eax+4*4h] FindConnectionPoint is 4th entry in vtable

sCode = sCode + CHR(0x83) + CHR(0xf8) + CHR(0x00) && cmp eax, 0 && if hr = SUCCESS

* jne FailedFindCPC

sCode = sCode + CHR(0x75)+CHR(0x00) && jne nBytes && nBytes calc'd below. je= 0x74, Jump if Equal, jne = 0x75

INSERT INTO jumps values ("FailedFindCPC","R",LEN(sCode)) && refer to a label to jump to at this pos

*now QI the fox object for the sink interface

*hr = oSrc->QueryInterface(IID_RecordSetEvents,&pRSEvents)

sCode = sCode + CHR(0x8d) + CHR(0x45)+CHR(0xe8) && lea eax, [ebp-18h] && addr to put pRSEvents COM ptr

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(cIid),"4rs") && mov eax, str

sCode = sCode + CHR(0x50) && push eax

*We must get the IDispatch COM interface for the VFP obj

IF ISNULL(this.oCOMVFP)

fGotit=.f.

FOR i = 1 TO _vfp.Objects.count

TRY

this.oComVFP=_vfp.Objects(i)

fGotit=LOWER(this.oComVFP.name) ="myclass"

CATCH

ENDTRY

IF fGotit

EXIT

ENDIF

ENDFOR

ENDIF

sCode = sCode + CHR(0xb8) + BINTOC(SYS(3095,this.oComVFP),"4rs") && mov eax, oSink: the THIS pointer

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x00) && call [eax+0h] && call indirect the function at 0h in the vTable, which is QI

sCode = sCode + CHR(0x83) + CHR(0xf8) + CHR(0x00) && cmp eax, 0 && if hr = SUCCESS

* jne FailedSinkIntface

sCode = sCode + CHR(0x75)+CHR(0x00) && jne nBytes && nBytes calc'd below. je= 0x74, Jump if Equal, jne = 0x75

INSERT INTO jumps values ("FailedSinkIntface","R",LEN(sCode)) && refer to a label to jump to at this pos

*hr = pcp->Advise(pSink, &dwCookie) && Advise if 5th entry in vtable

IF NOT fUnbind

dwCookieAddr=this.MakeStr(REPLICATE(CHR(0),4)) && place to put the cookie as a string

sCode = sCode + CHR(0x8d) + CHR(0x05) + BINTOC(dwCookieAddr,"4rs") && lea eax,dwCookieAddr ;addr to put dwCookie

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xe8) && mov eax, [ebp-18h] && the oSink

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xec) && mov eax, [ebp-14h] ; pcp

sCode = sCode + CHR(0x50) && push eax && push the THIS ptr

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x14) && call [eax+ 5*4h] && advise is 5th entry in vtable

ELSE && we're unbinding

*hr = pcp->UnAdvise(dwCookie)

sCode = sCode + CHR(0xb8) + BINTOC(this.dwCookie,"4rs") && mov eax, dwCookieAddr

sCode = sCode + CHR(0x50) && push eax

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xec) && mov eax, [ebp-14h] ;pcp

sCode = sCode + CHR(0x50) && push eax && push the THIS ptr

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x18) && call [eax+ 6*4h] && unadvise is 6th entry in vtable

ENDIF

sCode = sCode + CHR(0x83) + CHR(0xf8) + CHR(0x00) && cmp eax, 0 && if hr != SUCCESS

* je GotAdviseUnadvise ; now we jump if we succeed

sCode = sCode + CHR(0x74)+CHR(0x00) && je nBytes && nBytes calc'd below. je= 0x74, Jump if Equal, jne = 0x75

INSERT INTO jumps values ("GotAdviseUnadvise","R",LEN(sCode)) && refer to a label to jump to at this pos

*now save hr and gen err message

sCode = sCode + CHR(0xa3) + BINTOC(this.hr,"4rs") && mov this.hr,eax

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(jumps.cLabel),"4rs") && mov eax, str

sCode = sCode + CHR(0xa3) + BINTOC(this.cError,"4rs") && mov this.cError,eax

INSERT INTO jumps values ("GotAdviseUnadvise","D",LEN(sCode)) && define a label at this pos

sCode = sCode + CHR(0xEb) + CHR(0) && jmp around else clause

INSERT INTO jumps values ("GotSinkIntface","R",LEN(sCode)) && refer to a label to jump to at this pos

*else { // FailedSinkIntface

INSERT INTO jumps values ("FailedSinkIntface","D",LEN(sCode)) && define a label at this pos

*now save hr and gen err message

sCode = sCode + CHR(0xa3) + BINTOC(this.hr,"4rs") && mov this.hr,eax

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(jumps.cLabel),"4rs") && mov eax, str

sCode = sCode + CHR(0xa3) + BINTOC(this.cError,"4rs") && mov this.cError,eax

*}

INSERT INTO jumps values ("GotSinkIntface","D",LEN(sCode)) && define a label at this pos

*pCP->Release()

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xec) && mov eax, [ebp-14h]

sCode = sCode + CHR(0x50) && push eax && push the THIS ptr

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x8) && call [eax+8h]

sCode = sCode + CHR(0xEb) + CHR(0) && jmp around else clause

INSERT INTO jumps values ("GotFindCPC","R",LEN(sCode)) && refer to a label to jump to at this pos

*else { // FailedFindCPC

INSERT INTO jumps values ("FailedFindCPC","D",LEN(sCode)) && define a label at this pos

*now save hr and gen err message

sCode = sCode + CHR(0xa3) + BINTOC(this.hr,"4rs") && mov this.hr,eax

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(jumps.cLabel),"4rs") && mov eax, str

sCode = sCode + CHR(0xa3) + BINTOC(this.cError,"4rs") && mov this.cError,eax

*}

INSERT INTO jumps values ("GotFindCPC","D",LEN(sCode)) && define a label at this pos

*pCPC->Release()

sCode = sCode + CHR(0x8b) + CHR(0x45) + CHR(0xf0) && mov eax, [ebp-10h]

sCode = sCode + CHR(0x50) && push eax && push the THIS ptr

sCode = sCode + CHR(0x8b) + CHR(0) && mov eax, [eax] && get the vTable

sCode = sCode + CHR(0xff) + CHR(0x50) + CHR(0x8) && call [eax+8h]

sCode = sCode + CHR(0xEb) + CHR(0) && jmp around else clause

INSERT INTO jumps values ("GotQICPC","R",LEN(sCode)) && refer to a label to jump to at this pos

*else { //FailedQICPC

INSERT INTO jumps values ("FailedQICPC","D",LEN(sCode)) && define a label at this pos

*now save hr and gen err message

sCode = sCode + CHR(0xa3) + BINTOC(this.hr,"4rs") && mov this.hr,eax

sCode = sCode + CHR(0xb8) + BINTOC(this.MakeStr(jumps.cLabel),"4rs") && mov eax, str

sCode = sCode + CHR(0xa3) + BINTOC(this.cError,"4rs") && mov this.cError,eax

*}

INSERT INTO jumps values ("GotQICPC","D",LEN(sCode)) && define a label at this pos

sCode = sCode + CHR(0x33) + CHR(0xc0) && xor eax,eax && make return value 0 so won't enum any more windows

sCode = sCode + CHR(0x8b) + CHR(0xe5) && mov esp, ebp

sCode = sCode + CHR(0x5d) && pop ebp

sCode = sCode + CHR(0xc2)+CHR(0x08)+CHR(0x00) && ret 8 && EnumChildProc has 2 parms: pop 2 args=8 bytes

USE jumps AGAIN IN 0 ORDER 1 ALIAS jumpdefs

SCAN FOR cFlag="R" && look for all references

=SEEK(jumps.cLabel+"D","jumpdefs")

sCode=LEFT(sCode,jumps.sCodePos-1)+CHR(jumpdefs.sCodePos - jumps.sCodePos) + SUBSTR(sCode,jumps.sCodePos+1) && now fix up the jump location to jump to the definition

ENDSCAN

AdrCode=this.memAlloc(LEN(sCode),sCode) && allocate memory for the code

EnumChildWindows(_screen.HWnd,AdrCode,0) && EnumChildWindows needs a callback function. We'll give it our code.Added benefit: Win32 Exception handling of Declare dll

IF NOT fUnBind

this.dwCookie=CTOBIN(SYS(2600,dwCookieAddr,4),"4rs")

ENDIF

IF CTOBIN(SYS(2600,this.cError,4),"4rs")!=0

?"Error Location=",SYS(2600,CTOBIN(SYS(2600,this.cError,4),"4rs"),20),TRANSFORM(CTOBIN(SYS(2600,this.hr,4),"4rs"),"@0x")

ENDIF

USE IN jumpdefs

USE IN jumps

PROCEDURE MemAlloc(nSize as Integer, cStr as String) as Integer

LOCAL nAddr

nAddr = HeapAlloc(this.hProcHeap, 0, nSize) && allocate memory

ASSERT nAddr != 0 MESSAGE "Out of memory"

INSERT INTO memAllocs VALUES (nAddr,"H") && track them for freeing later

SYS(2600,nAddr, LEN(cStr),cStr) && copy the string into the mem

RETURN nAddr

PROCEDURE CallDllFunction(strExport as String, strDllName as String) as String

*Create a string of machine code that calls a function in a DLL. Parms should already be pushed

LOCAL nAddr as Integer, hModule as Integer

hModule = LoadLibrary(strDllName)

INSERT INTO memAllocs VALUES (hModule,"L") && track loads for freeing later

nAddr=GetProcAddress(hModule,strExport)

ASSERT nAddr != 0 MESSAGE "Error: Export not found "+ strExport+" "+ strDllName

RETURN CHR(0xb8)+BINTOC(nAddr,"4rs") + CHR(0xff) + CHR(0xd0) && mov eax, addr; call eax

PROCEDURE MakeStr(str as String, fConvertToUnicode as Logical, fMakeBstr as Logical) as Integer

* converts a string into a memory allocation and returns a pointer

LOCAL nRetval as Integer

IF fConvertToUnicode

str=STRCONV(str+CHR(0),5)

ELSE

str = str + CHR(0) && null terminate

ENDIF

IF fMakeBstr

nRetval= SysAllocString(str)

ASSERT nRetval != 0 MESSAGE "Out of memory"

INSERT INTO memAllocs VALUES (nRetval,"B") && track them for freeing later

ELSE

nRetval= this.MemAlloc(LEN(str),str)

ENDIF

RETURN nRetval

PROCEDURE Destroy

LOCAL i

SELECT memAllocs

SCAN

DO CASE

CASE AllocType="B" && BSTR

SysFreeString(memPtr)

CASE AllocType="H" && Heap

HeapFree(this.hProcHeap,0,memPtr)

CASE AllocType="L" && LoadLibrary

FreeLibrary(memPtr)

ENDCASE

ENDSCAN

ENDDEFINE

DEFINE CLASS myclass AS custom

* IMPLEMENTS RecordsetEvents IN "c:\Program Files\Common Files\System\Ado\msado15.dll"

IMPLEMENTS RecordsetEvents IN "adodb.recordset"

PROCEDURE Recordsetevents_WillChangeField(cFields AS Number @, Fields AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_FieldChangeComplete(cFields AS Number @, Fields AS VARIANT @, pError AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_WillChangeRecord(adReason AS VARIANT @, cRecords AS Number @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_RecordChangeComplete(adReason AS VARIANT @, cRecords AS Number @, pError AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_WillChangeRecordset(adReason AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

?adreason,adstatus,precordset.recordcount

PROCEDURE Recordsetevents_RecordsetChangeComplete(adReason AS VARIANT @, pError AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_WillMove(adReason AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_MoveComplete(adReason AS VARIANT @, pError AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_EndOfRecordset(fMoreData AS LOGICAL @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_FetchProgress(Progress AS Number @, MaxProgress AS Number @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

PROCEDURE Recordsetevents_FetchComplete(pError AS VARIANT @, adStatus AS VARIANT @, pRecordset AS VARIANT @) AS VARIANT

? " "+program() + ' ' + TRANSFORM(DATETIME())

ENDDEFINE

*edited to add Craig's suggestion