Push button and combo box WinAPI - c++

Question

I am trying to do some windows programming. I created a combo box and a push button. My objective is, when the user selects an item in the combo box, the button is activated and needs to be clicked to proceed. I have done something like this so far:
This is how i create the combo box:
Func.h
#ifndef FUNCS_H
#define FUNCS_H
// Winapi headers....
#include <windows.h>
#include <windowsx.h>
#include <shlobj.h>
//created lib
#include "Resource.h"
// Standard C/C++ headers...
#include <iostream>
#include <string>
#include <dirent.h> // directory manipulation....
#include <cstring>
#include <fstream>
using std::cout;
using std::endl;
using std::string;
using std::ofstream;
HWND hselectOK;
HWND hComboBox1;
void createControls(HWND hwnd) // Create our combo box
{
HWND hselectFeature;
HINSTANCE hInstance = GetModuleHandle(NULL);
// Create our List box
hComboBox1 = CreateWindow(WC_COMBOBOX,"", CBS_DROPDOWNLIST | CBS_HASSTRINGS | WS_CHILD |WS_OVERLAPPED |WS_VISIBLE , 7, 20, 300, 100, hwnd, NULL, hInstance, NULL);
SendMessage(hComboBox1, CB_ADDSTRING, 0,(LPARAM)"Histogram of Oriented Gradients (HOG)");
SendMessage(hComboBox1, CB_ADDSTRING, 0,(LPARAM)"Scale Invariant Feature Transform (SIFT)");
SendMessage(hComboBox1, CB_ADDSTRING, 0,(LPARAM)"Speeded Up Robust Features (SURF)");
// SendMessage(hComboBox1, CB_SETCURSEL, (WPARAM)2,(LPARAM)0); //CB_SETCURSEL
// Create our push bottons
hselectFeature = CreateWindow("STATIC", "Select Feature", SS_LEFT | WS_CHILD, 320,20, 100, 21,hwnd, (HMENU)1, hInstance, NULL);
ShowWindow(hselectFeature,1);
hselectOK = CreateWindow("BUTTON", "Ok", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON | WS_DISABLED, 320,45, 100, 21,hwnd, NULL, hInstance, NULL);
}
#endif // FUNCS_H
WinProc.h
#ifndef WINPROC_H
#define WINPROC_H
// Winapi Headers
#include <CommDlg.h>
#include <winuser.h>
// OpenCV headers
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
// Standard C/C++ headers
#include <iostream>
#include <string>
// Created headers;
#include "Funcs.h"
#include "Resource.h"
using std::cout;
using std::endl;
using std::string;
using namespace cv;
int classNumber;
LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
string dirPath;
int comboIndex;
switch (message) /* handle the messages */
{
case WM_CREATE:
createControls(hwnd);
break;
case WM_COMMAND:
{
switch(HIWORD(wParam))
{
case CBN_SELCHANGE: // When user select item in combo box, enable the button.
{
EnableWindow(hselectOK, TRUE); // enable the button
}
break;
case BN_CLICKED: // When user has chosen a list, the button is used to proceed with further task associated to the selected item.
{
char listName[200];
comboIndex = SendMessage(hComboBox1, (UINT) CB_GETCURSEL, 0, 0);
SendMessage(hComboBox1, (UINT)CB_GETLBTEXT, (WPARAM)comboIndex, (LPARAM)listName);
if(comboIndex == 0)
{
MessageBox(hwnd,listName, "You chose", MB_OK);
// Want to Do some function here.
}
else if(comboIndex == 1)
{
MessageBox(hwnd,listName, "You chose", MB_OK);
// Want to Do some function here.
}
else if(comboIndex == 2)
{
MessageBox(hwnd,listName, "You chose", MB_OK);
// Want to Do some function here.
}
}
break;
}
switch LOWORD(wParam)
{
case IDM_IMG_PATH:
{
dirPath = browseFolder(hwnd);
DialogBox(GetModuleHandle(NULL),MAKEINTRESOURCE(IDD_CLASS),hwnd, ClassDlgProcedure);
createClassLabelFile(hwnd, dirPath, classNumber);
return 0;
}
case IDM_EXIT:
{
PostMessage(hwnd, WM_CLOSE,0 , 0);
}
break;
}
}
break;
case WM_DESTROY:
PostQuitMessage(0); /* send a WM_QUIT to the message queue */
break;
default: /* for messages that we don't deal with */
return DefWindowProc (hwnd, message, wParam, lParam);
}
return 0;
}
#endif // WINPROC_H
Those part are only some of the relevant parts of the code.
The problem is, when I exit the program using the case IDM_EXIT, the message box under the case BN_CLICKED always appears and then the program closes. I am expecting that when we close the program, no such message box would appear, but this is not the case. What I'm saying is, the message box appears twice, once when you select click the button and when you want to close the program. Why is that happening. Any Ideas or suggestions ?

Answer 1

The problem is that you are assuming that WM_COMMAND is a message that is unique to your ComboBox, Which is not.
Take a look at this old but gold blog post.
Quoting from the author,
The high-order word of the wParam parameter "specifies the
notification code if the message is from a control". What does
"control" mean here? Remember that you have to take things in context.
The WM_COMMAND message is being presented in the context of Win32 in
general, and in the context of the window manager in particular.
Windows such as edit boxes, push buttons, and list boxes are commonly
called "controls", as are all the window classes in the "common
controls library". In the world of the window manager, a "control" is
a window whose purpose is to provide some degree of interactivity
(which, in the case of the static control, might be no interactivity
at all) in the service of its parent window. The fact that the
WM_COMMAND is used primarily in the context of dialog boxes further
emphasizes the point that the term "control" here is just a synonym
for "child window".
To summarize, any button click inside your application window will translate as a WM_COMMAND with a BN_CLICKED wParam. This includes the close button of the window.
To handle specific clicks from your combo box you have two options. The easy way is to filter the hWnd of the control that sent the message, you should already know the window handle of your combo box and it shouldn't be a problem to compare against it.
The other option is to define your own messages and check against those in your WndProc handler. The web is full of examples of how to define your own application/control specific messages.

Answer 2

When you click your 'Exit' menu, windows also send a 'BN_CLICKED' message to the 'WindowProcedure' function and that is why the message box appears. You should use an ID for the button rather than making the 'hmenu' parameter 'NULL' like this:
hselectOK = CreateWindow("BUTTON", "Ok", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON | WS_DISABLED, 320,45, 100, 21,hwnd,
(HMENU)305, // Here is the ID of your button ( You may use your own ID )
hInstance, NULL);
And you have to add some ID checking code in your 'case BN_CLICKED' like that:
case BN_CLICKED: // When user has chosen a list, the button is used to proceed with further task associated to the selected item.
{
// You must do this check otherwise the message box will appear again when you click the 'Exit' menu
if ( LOWORD(wParam) == 305 ) // '305' is the ID which I have used as the button ID in above code
{
// Now add your click event code here
}
}
break;