Windows Journal playback hook (WH_JOURNALPLAYBACK) ignores EVENTMSG HWND parameter - c++

Question

I am working a program to simulate keyboard and mouse clicks programmatically. It need to send the clicks to a target window handle (ex: notepad edit control). I am getting the handle of notepad window and generating generating WM_KEYDOWN, WM_SYSKEYDOWN, WM_KEYUP, WM_SYSKEYUP messages for that window. The events are stored in queue, and later on played using a WH_JOURNALPLAYBACK hook.
For the code snippet below, the target hwnd in the playback proc though set correctly, messages never reach to the target handle. If I bring the notepad to foreground, it does receive the messages.
I am not sure why WH_JOURNALPLAYBACK ignores the handle parameter. I would have liked to generate a series of automation messages for various handles and played it back so that even without bringing the window into focus we can send keyboard and mouse events.
Please let me know
if messages to various target handles can be sent using a journal
playback hook
why in code below hwnd is ignored
..
#include <queue>
#include <iostream>
#include <windows.h>
using std::cout;
using std::endl;
using std::error;
struct Event
{
UINT msg;
UINT wparam;
UINT lparam;
HWND hwnd;
Event(UINT m, UINT wp, UINT lp, HWND h)
:msg(m),
wparam(wp),
lparam(lp),
hwnd(h)
{}
};
HHOOK jhook= NULL;
std::queue<Event> events;
bool gotoNextMsg = false;
LRESULT CALLBACK JournalPlaybackProc(int code, WPARAM wParam, LPARAM lParam)
{
switch( code )
{
case HC_SKIP:
cout<<"skip: "<<endl;
if(!events.empty())
{
events.pop();
}
break;
case HC_GETNEXT:
{
cout<<"next: "<<events.size()<<endl;
gotoNextMsg = true;
EVENTMSG * evm = (EVENTMSG*) lParam;
Event e = events.front();
switch(e.msg)
{
case WM_KEYDOWN:
cout<<"WM_KEYDOWN"<<endl;
break;
case WM_KEYUP:
cout<<"WM_KEYUP"<<endl;
break;
case WM_SYSKEYDOWN:
cout<<"WM_SYSKEYDOWN"<<endl;
break;
case WM_SYSKEYUP:
cout<<"WM_SYSKEYUP"<<endl;
break;
}
cout<<"handle: "<<e.hwnd<<endl;
cout<<"handle1:"<<evm->hwnd<<endl;
evm->message = e.msg;
evm->paramL = e.wparam;
evm->paramH = e.lparam;
evm->hwnd = e.hwnd;
evm->time = ::GetTickCount();
}
break;
default:
if( code < 0 )
::CallNextHookEx(jhook, code, wParam, lParam);
break;
}
if(events.empty())
{
cout<<"uinstalled"<<endl;
::UnhookWindowsHookEx(jhook);
::PostMessage(NULL, WM_USER+100, 0, 0);
}
return 0;
}

Answer 1

A journal hook injects events into the system message queue. For keyboard and mouse messages, the system dispatches them to the current focused window, same as if the user had input them manually. The HWND you specify in the event is not used, it gets replaced during dispatching.
And if you consider that a recorded journal can be played multiple times, and its data can persist across application instances and even reboots, and that HWNDs can be reused for different things over time, it should make sense why a journal playback cannot make use of an event's HWND even if the system message queue were not involved.
So, you cannot use WH_JOURNALPLAYBACK to target a specific window that is not in the foreground. You would have to send the recorded messages yourself. But be aware of some caveats that Raymond Chen has blogged about:
You can't simulate keyboard input with PostMessage
Simulating input via WM_CHAR messages may fake out the recipient but it won't fake out the input system