How drag and drop of files is done?

Question

as in winamp or vlc player, how to do a file drag and drop? i mean i want to know what sort of coding goes into application? i want to know for c++

Answer 1

With com:

Create a class that public extends IDropTarget

Register your class for drops. Do this in WM_CREATE

RegisterDragDrop(hwnd,static_cast(pointer_to_your_class));

In your class you need to override a couple of functions since they are pure virtual:

virtual HRESULT STDMETHODCALLTYPE DragEnter( 
        /* [unique][in] */ __RPC__in_opt IDataObject *pDataObj,
        /* [in] */ DWORD grfKeyState,
        /* [in] */ POINTL pt,
        /* [out][in] */ __RPC__inout DWORD *pdwEffect) = 0;

virtual HRESULT STDMETHODCALLTYPE DragOver( 
    /* [in] */ DWORD grfKeyState,
    /* [in] */ POINTL pt,
    /* [out][in] */ __RPC__inout DWORD *pdwEffect) = 0;

virtual HRESULT STDMETHODCALLTYPE DragLeave( void) = 0;

virtual HRESULT STDMETHODCALLTYPE Drop( 
    /* [unique][in] */ __RPC__in_opt IDataObject *pDataObj,
    /* [in] */ DWORD grfKeyState,
    /* [in] */ POINTL pt,
    /* [out][in] */ __RPC__inout DWORD *pdwEffect) = 0;

Each of those functions will get called when those events occur, i.e. when someone pass the mouse in your window with a file DragEnter on your class will get called.

You will also need to implement a couple more functions that IDropTarget extends, check out IUnknown in your MSDN.

Then you need to query IDataObject param to get the data:

FORMATETC fdrop = {CF_HDROP, 0, DVASPECT_CONTENT, -1, TYMED_HGLOBAL};

if (SUCCEEDED(pDataObj->QueryGetData(&fdrop)) ){
    STGMEDIUM stgMedium = {0};
    stgMedium.tymed = TYMED_HGLOBAL;
    HRESULT hr = pDataObj->GetData(&fdrop, &stgMedium);
    if (SUCCEEDED(hr))
    {
        HGLOBAL gmem = stgMedium.hGlobal;
        HDROP hdrop = (HDROP)GlobalLock(gmem);
        UINT numOfFiles =  DragQueryFile( (HDROP) hdrop,
                            0xFFFFFFFF,
                           NULL,
                            0
                        );

        TCHAR buffer[MAX_PATH];
        for( int i=0;i

Cheers!