How does Windows changes keyboard layout? - windows

Question

The hotkeys for doing so are:
Alt+Shift - Windows 7 and Win button + Space in Win 8 and 10.
Programmatically I can do that using WM_INPUTLANGCHANGEREQUEST, but it is not the way Windows does that. I am trying to figure out how Windows changes the layout. Using spy++ I figured out that Windows sends WM_INPUTLANGCHANGE message which is changing layout, so I tried it myself:
SendMessage(myHWND, WM_INPUTLANGCHANGE, 0xCC, 0x4190419);
I have keyboard hook bind, when press f1 do the sendmessage to currently active window's active control.
but it didn't work, though the messages in spy++ are thes same:
first one using "Windows hotkey"/"Taskbar" to change layout, it works. Second my message, it did not work. Am I missing something, why message works for "Windows" but not for me.
The WM_INPUTLANGCHANGEREQUEST works, but it freezes some certain apps, and I would like to figure out the way Windows does the layout changing to avoid that.
-- update.
In DxO Photolab 3 it freezes when using WM_INPUTLANGCHANGE in "Export to Disk" Dialog:
When you change layout using "Windows" Method(Keyboard Hotkey/Taskbar):
Works normally, no freezing.
Posting the WM_INPUTLANGCHANGE:
Received the WM_INPUTLANGCHANGEREQUEST and froze:
Also similar freezing I've seen in Skype, MS Office, Adobe After Effects.

Answer 1

From WM_INPUTLANGCHANGEREQUEST,
When the DefWindowProc function receives the
WM_INPUTLANGCHANGEREQUEST message, it activates the new input locale
and notifies the application of the change by sending the
WM_INPUTLANGCHANGE message.
We can view the details through spy++.
Only after the application receives the WM_INPUTLANGCHANGEREQUEST message, it will activate the new input locale, and notifies the application of the change by sending the WM_INPUTLANGCHANGE message.
A simple test:
According to my understanding, what actually works is the WM_INPUTLANGCHANGEREQUEST message, but I have not found an alternative API to complete its work.
For the freezing problem of some certain apps you encountered, I found some similar cases.
Refer #Barmak Shemirani's answer,
Apparently WM_INPUTLANGCHANGEREQUEST fails if the target itself is a
dialog based application (I don't know why!) To solve the problem you
can post WM_INPUTLANGCHANGEREQUEST message to dialog's descendants (in
addition to WM_INPUTLANGCHANGEREQUEST message to the dialog itself)
Updated:
My test code:
#include <Windows.h>
int main()
{
HWND hwnd = (HWND)0x00070EBA; // hwnd of skype
while (1)
{
Sleep(1000);
PostMessage(hwnd, WM_INPUTLANGCHANGEREQUEST, 0, 0);
}
return 0;
}
Result: