How to Drag an Image

This topic demonstrates how to drag an image on the screen. The dragging functions move an image smoothly, in color, and without any flashing of the cursor. Both masked and unmasked images can be dragged.

What you need to know

Technologies

Prerequisites

  • C/C++
  • Windows User Interface Programming

Instructions

Step 1: Begin the drag operation.

Use the ImageList_BeginDrag function to begin a drag operation.

The user-defined function in the following C++ code example is intended to be called in response to a mouse button-down message, such as WM_LBUTTONDOWN. The function determines whether the user has clicked within the bounding rectangle of the image. If the user has clicked, the function captures the mouse input, erases the image from the client area, and calculates the position for the hot spot within the image. The function sets the hot spot to coincide with the hot spot of the mouse cursor. Then the function begins the drag operation by calling ImageList_BeginDrag.



// StartDragging - begins dragging a bitmap. 
// Returns TRUE if successful, or FALSE otherwise. 
// hwnd - handle to the window in which the bitmap is dragged. 
// ptCur - coordinates of the cursor. 
// himl - handle to the image list. 
// 
// Global variables 
//     g_rcImage - bounding rectangle of the image to drag. 
//     g_nImage - index of the image. 
//     g_ptHotSpot - location of the image's hot spot. 
//     g_cxBorder and g_cyBorder - width and height of border. 
//     g_cyCaption and g_cyMenu - height of title bar and menu bar. 
extern RECT g_rcImage; 
extern int g_nImage; 
extern POINT g_ptHotSpot; 
 
BOOL StartDragging(HWND hwnd, POINT ptCur, HIMAGELIST himl) 
{ 
    // Return if the cursor is not in the bounding rectangle of 
    // the image. 
    if (!PtInRect(&g_rcImage, ptCur)) 
        return FALSE; 
 
    // Capture the mouse input. 
    SetCapture(hwnd); 
 
    // Erase the image from the client area. 
    InvalidateRect(hwnd, &g_rcImage, TRUE); 
    UpdateWindow(hwnd); 
 
    // Calculate the location of the hot spot, and save it. 
    g_ptHotSpot.x = ptCur.x - g_rcImage.left; 
    g_ptHotSpot.y = ptCur.y - g_rcImage.top; 
 
    // Begin the drag operation. 
    if (!ImageList_BeginDrag(himl, g_nImage, 
            g_ptHotSpot.x, g_ptHotSpot.y)) 
        return FALSE; 
 
    // Set the initial location of the image, and make it visible. 
    // Because the ImageList_DragEnter function expects coordinates to 
    // be relative to the upper-left corner of the given window, the 
    // width of the border, title bar, and menu bar need to be taken 
    // into account. 
    
    ImageList_DragEnter(hwnd, ptCur.x + g_cxBorder, 
        ptCur.y + g_cyBorder + g_cyCaption + g_cyMenu); 
 
    g_fDragging = TRUE; 
 
    return TRUE; 
} 


Step 2: Move the image.

The ImageList_DragMove function moves the image to a new location.

The user-defined function in the following C++ code example is intended to be called in response to the WM_MOUSEMOVE message. It drags the image to a new location.



// MoveTheImage - drags an image to the specified coordinates. 
// Returns TRUE if successful, or FALSE otherwise. 
// ptCur - new coordinates for the image. 
BOOL MoveTheImage(POINT ptCur) 
{ 
    if (!ImageList_DragMove(ptCur.x, ptCur.y)) 
        return FALSE; 
 
    return TRUE; 
} 



Step 3: End the drag operation.

The user-defined function in the following C++ code example calls the ImageList_EndDrag function to end the drag operation. It then calls the ImageList_DragLeave function to unlock the window and hide the drag image, allowing the window to be updated.



// StopDragging - ends a drag operation and draws the image 
// at its final location. 
// Returns TRUE if successful, or FALSE otherwise. 
// hwnd - handle to the window in which the bitmap is dragged. 
// himl - handle to the image list. 
// ptCur - coordinates of the cursor. 
// 
// Global variable 
//     g_ptHotSpot - location of the image's hot spot. 
 
extern POINT g_ptHotSpot; 
 
BOOL StopDragging(HWND hwnd, HIMAGELIST himl, POINT ptCur) 
{ 
    ImageList_EndDrag(); 
    ImageList_DragLeave(hwnd); 
 
    g_fDragging = FALSE; 
 
    DrawTheImage(hwnd, himl, ptCur.x - g_ptHotSpot.x, 
        ptCur.y - g_ptHotSpot.y); 
 
    ReleaseCapture(); 
    return TRUE; 
} 



Related topics

Image Lists Reference
About Image Lists
Using Image Lists

 

 

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