Using Trackbar Controls

Using Trackbar Controls

This section provides implementation details and examples for trackbar controls.

Creating a Trackbar
Processing Trackbar Notification Messages
Limiting Slider Movement
Using Buddy Windows

Creating a Trackbar

The following example shows how to create a trackbar with the TBS_AUTOTICKS and TBS_ENABLESELRANGE styles. When the trackbar is created, both its range and its selection range are initialized. The page size is also set at this time.

// CreateTrackbar - creates and initializes a trackbar. 
// 
// Global variable 
//     g_hinst - instance handle 
HWND WINAPI CreateTrackbar( 
    HWND hwndDlg,  // handle of dialog box (parent window) 
    UINT iMin,     // minimum value in trackbar range 
    UINT iMax,     // maximum value in trackbar range 
    UINT iSelMin,  // minimum value in trackbar selection 
    UINT iSelMax)  // maximum value in trackbar selection 
{ 

    InitCommonControls(); // loads common control's DLL 

    hwndTrack = CreateWindowEx( 
        0,                             // no extended styles 
        TRACKBAR_CLASS,                // class name 
        "Trackbar Control",            // title (caption) 
        WS_CHILD | WS_VISIBLE | 
        TBS_AUTOTICKS | TBS_ENABLESELRANGE,  // style 
        10, 10,                        // position 
        200, 30,                       // size 
        hwndDlg,                       // parent window 
        ID_TRACKBAR,             // control identifier 
        g_hinst,                       // instance 
        NULL                           // no WM_CREATE parameter 
        ); 

    SendMessage(hwndTrack, TBM_SETRANGE, 
        (WPARAM) TRUE,                   // redraw flag 
        (LPARAM) MAKELONG(iMin, iMax));  // min. & max. positions 
    SendMessage(hwndTrack, TBM_SETPAGESIZE, 
        0, (LPARAM) 4);                  // new page size 

    SendMessage(hwndTrack, TBM_SETSEL, 
        (WPARAM) FALSE,                  // redraw flag 
        (LPARAM) MAKELONG(iSelMin, iSelMax); 
    SendMessage(hwndTrack, TBM_SETPOS, 
        (WPARAM) TRUE,                   // redraw flag 
        (LPARAM) iSelMin); 

    SetFocus(hwndTrack); 

    return hwndTrack; 
}

Processing Trackbar Notification Messages

The following example is a function that is called whenever a WM_HSCROLL message is received by the dialog box containing the trackbar. The trackbar has the TBS_ENABLESELRANGE style. The position of the slider is compared against the selection range, and the slider is moved to the starting or ending position of the selection range, when necessary.

A dialog containing a trackbar with the TBS_VERT style could use this function when it receives a WM_VSCROLL message.

// TBNotifications - handles trackbar notifications received 
// in the wParam parameter of WM_HSCROLL. This function simply 
// ensures that the slider remains within the selection range. 

VOID WINAPI TBNotifications( 
    WPARAM wParam,  // wParam of WM_HSCROLL message 
    HWND hwndTrack, // handle of trackbar window 
    UINT iSelMin,   // minimum value of trackbar selection 
    UINT iSelMax)   // maximum value of trackbar selection 
    { 
    DWORD dwPos;    // current position of slider 

    switch (LOWORD(wParam)) { 
        case TB_ENDTRACK: 
            dwPos = SendMessage(hwndTrack, TBM_GETPOS, 0, 0); 
            if (dwPos > iSelMax) 
                SendMessage(hwndTrack, TBM_SETPOS, 
                    (WPARAM) TRUE,       // redraw flag 
                    (LPARAM) iSelMax); 
            else if (dwPos < iSelMin) 
                SendMessage(hwndTrack, TBM_SETPOS, 
                    (WPARAM) TRUE,       // redraw flag 
                    (LPARAM) iSelMin); 
            break; 

        default: 
            break; 

        } 
}

Limiting Slider Movement

As described in About Trackbar Controls, it is possible to set off part of the trackbar range as a selection range. One purpose of a selection range might be to limit movement of the slider, making some parts of the full range off limits.

The following example code, from the window procedure of a dialog box, limits movement of the slider by resetting its position whenever it is moved outside the selection range.

case WM_HSCROLL:
    {
        HWND hTrackbar = GetDlgItem(hDlg, IDC_SLIDER1);
        if (hTrackbar == (HWND)lParam)
        {
            int newPos = SendMessage(hTrackbar, TBM_GETPOS, 0, 0);
            int selStart = SendMessage(hTrackbar, TBM_GETSELSTART, 0, 0);
            int selEnd = SendMessage(hTrackbar, TBM_GETSELEND, 0, 0);
            if (newPos > selEnd)
            {
                SendMessage(hTrackbar, TBM_SETPOS, (WPARAM)TRUE, (LPARAM)selEnd);
            }
            else if (newPos < selStart)
            {
                SendMessage(hTrackbar, TBM_SETPOS, (WPARAM)TRUE, (LPARAM)selStart);
            }
        }
        break;
    }

Using Buddy Windows

By setting other controls as buddy windows for a trackbar, you can automatically position those controls at the ends of the trackbar as labels. The following illustration shows a horizontal and a vertical trackbar, both with static controls as buddy windows.

Trackbars with labels in buddy windows.

The following example function creates the buddy windows shown in the illustration.

// IDC_SLIDER1 and IDC_SLIDER2 are trackbars created in
// the resource editor.

void LabelTrackbarsWithBuddies(HWND hDlg)
{
    const int staticWidth = 50;
    const int staticHeight = 20;

    // Get horizontal trackbar.
    HWND hwndTrackbar = GetDlgItem(hDlg, IDC_SLIDER1);

    // Create buddies.
    HWND hwndBuddy = CreateWindowEx(0, L"STATIC", L"Left", 
        SS_RIGHT | WS_CHILD | WS_VISIBLE, 
        0, 0, 
        staticWidth, staticHeight, 
        hDlg, NULL, g_hInst, NULL);
    SendMessage(hwndTrackbar, TBM_SETBUDDY, 
        (WPARAM)TRUE, (LPARAM)hwndBuddy);

    hwndBuddy = CreateWindowEx(0, L"STATIC", L"Right", 
        SS_LEFT | WS_CHILD | WS_VISIBLE, 
        0, 0, 
        staticWidth, staticHeight, 
        hDlg, NULL, g_hInst, NULL);
    SendMessage(hwndTrackbar, TBM_SETBUDDY, 
        (WPARAM)FALSE, (LPARAM)hwndBuddy);

    // Get vertical trackbar.
    hwndTrackbar = GetDlgItem(hDlg, IDC_SLIDER2);

    // Create buddies.
    hwndBuddy = CreateWindowEx(0, L"STATIC", L"Top", 
        SS_CENTER | WS_CHILD | WS_VISIBLE,
        0, 0, 
        staticWidth, staticHeight, 
        hDlg, NULL, g_hInst, NULL);
    SendMessage(hwndTrackbar, TBM_SETBUDDY,
        (WPARAM)TRUE, (LPARAM)hwndBuddy);

    hwndBuddy = CreateWindowEx(0, L"STATIC", L"Bottom", 
        SS_CENTER | WS_CHILD | WS_VISIBLE,
        0, 0,
        staticWidth, staticHeight, 
        hDlg, NULL, g_hInst, NULL);
    SendMessage(hwndTrackbar, TBM_SETBUDDY,
        (WPARAM)FALSE, (LPARAM)hwndBuddy);
}