Breaking a while loop in a called Function - c++

Question

On clicking the start button the function RunB() executes the while loop . I need to break the while loop execution midway using the Stop Button .
switch (msg)
{
case WM_CREATE:
CreateWindow(TEXT("button"), TEXT("Browser"),
WS_VISIBLE | WS_CHILD,
30, 100, 80, 25,
hwnd, (HMENU)1, NULL, NULL);
CreateWindow(TEXT("button"), TEXT("Stop"),
WS_VISIBLE | WS_CHILD,
30, 200, 80, 25,
hwnd, (HMENU)2, NULL, NULL);
break;
case WM_COMMAND:
if (LOWORD(wParam) == 1) {
Obj.RunB();
}
break;
if (LOWORD(wParam) == 2) {
//Code to break the while loop
}
break;
Code for the Called Function RunB:
void RunB(){
while(n<15){
//Some statements here:
}
How do I break the while loop using the Stop Button ?

Answer 1

The event driven action (i.e. pressing a button), shouldn't engage a looping or business logic.... Otherwise the whole windows will be halt until it gets the synchronous response.

Answer 2

If you find you have some modal action that is going to take a long time to complete and you absolutely can't break it up, then you need to periodically process messages from within your modal action.
For example:
while(n<15){
DoAPieceOfWork(n);
FlushPendingMessages();
}
And FlushPendingMessages would be implemented something like:
void FlushPendingMessages(){
MSG msg;
while(PeekMessage(&msg,0,0,0,PM_REMOVE)){
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
This will keep your application mostly responsive. Normaly you would also display a modeless dialog at the same time, so that people know your application is busy and cannot be interacted with.
To be more correct, before calling TranslateMessage/DispatchMessage you must also check for WM_QUIT and repost it to terminate your app, and you can also explicitly check for WM_APP+x messages- using code something like this:
#define WM_APP_QUITLOOP (WM_APP+1)
// use this from anywhere to tell your processing loop to quit
PostMessage(NULL,WM_APP_QUITLOOP,0,0L);
BOOL FlushPendingMessages(){
MSG msg;
while(PeekMessage(&msg,0,0,0,PM_REMOVE)){
switch(msg.message){
case WM_APP_QUITLOOP:
return FALSE;
case WM_QUIT:
PostQuitMessage(msg.wParam);
return FALSE;
default:
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return TRUE;
}
Your processing loop would need to check the return value from FlushPendingMessages and abort if its FALSE of course.
Edit: as per #IInspectable's comment.