Drag and Drop on Window

This script show how to use shell32.dll in order to have use “drag and drop” to move file inside you application.
I launched the script and obtained:
Drag some files and drop them on this window...

then I dropped a file in the window and obtained:
You dropped 1 files.
at coordinate 63x313
1. /C/Documents and Settings/max/Desktop/Good day.pdf
Finishing drag operation
Unregistering window from DragAcceptFiles queue

Here is the source code:

REBOL [
    Title:   "System Port - Drag Accept Demo"
    File:   %sys-port-drag-accept.r
    Author: "Gregg Irwin"
    EMail:   greggirwin@acm.org
    Version: 0.0.1
    Date:     25-sep-2003
    Purpose: {
        Demo using system port to catch WM_DROPFILE messages for drag-n-drop
        support.
    }
]
; Stripped version of win-shell object for demo purposes.
win-shell: make object! [
    win-lib:   load/library %shell32.dll
    null-buff: func [
        {Returns a null-filled string buffer of the specified length.}
        len [integer!]
    ][head insert/dup make string! len #”^@” len ]
    drag-accept-files: make routine! [
        hWnd     [integer!]
        fAccept [integer!]
        return: [integer!]
    ] win-lib “DragAcceptFiles”
    drag-finish: make routine! [
        hDrop   [integer!]
        return: [integer!]
    ] win-lib “DragFinish”
    point: make struct! [
        x [integer!]
        y [integer!]
    ] none
    drag-query-point: make routine! compose/deep [
        {Returns nonzero if the drop occurred in the client area of the window,
        or zero if the drop did not occur in the client area of the window.}

        hDrop   [integer!]
        lpPoint [struct! [(first point)]]
        return: [integer!]
    ] win-lib “DragQueryPoint”
    drag-query-file: make routine! [
        hWnd     [integer!]
        iFile   [integer!]
        lpszFile[string!]
        cb       [integer!]
        return: [integer!]
    ] win-lib “DragQueryFile”
    drag-query-filename-size: make routine! [
        hWnd     [integer!]
        iFile   [integer!]
        lpszFile[integer!]
        cb       [integer!]
        return: [integer!]
    ] win-lib “DragQueryFile”
    num-files-dropped?: func [hdrop [integer!]] [drag-query-file hdrop -1 “” 0 ]
    ; When they give us a filename index, we’ll subtract one for them,
    ; because Windows has the list as zero based, but I’d much rather let
    ; everything be one based on the REBOL side.
    dropped-filename-size?: func [
        hdrop [integer!] index [integer!]
    ][drag-query-filename-size hdrop index - 1 0 0 ]
    dropped-filename?: func [
        hdrop [integer!] index [integer!] /local result len
    ][
        result: null-buff add 1 dropped-filename-size? hdrop index
        len: drag-query-file hdrop index - 1 result length? result
        copy/part result len
    ]
]
my-hwnd?: does [second get-modes system/ports/system [window]]
WM_DESTROY:   2   ; &H2
WM_DROPFILES: 563 ; &H233

enable-system-trap: does [
    ; Trap OS interrupts
    if not system/ports/system [
        if none? attempt [system/ports/system: open [scheme: ’system]][print “NOTE: Missing System Port” exit ]
    ]
    if find get-modes system/ports/system ’system-modes ‘winmsg [
        set-modes system/ports/system [winmsg: WM_DROPFILES]
    ]
    append system/ports/wait-list system/ports/system
]
check-system-trap: func [port /local msg pt] [
    if not port? port [return none]
    if any [port/scheme <> ’system   port <> system/ports/system][return port]
    if not system/ports/system [return none]
    while [msg: pick system/ports/system 1] [
        ;print [”msg:” mold msg]
        if msg/1 = ‘winmsg [
            if msg/2   = WM_DROPFILES [
                print [
                    “You dropped “ win-shell/num-files-dropped? msg/3 “files.”
                ]
                pt: make struct! win-shell/point [0 0]
                win-shell/drag-query-point msg/3 pt
                print [” at coordinate “ to pair! reduce [pt/x pt/y]]
                repeat i win-shell/num-files-dropped? msg/3 [
                    file: to-rebol-file win-shell/dropped-filename? msg/3 i
                    print [
                        tab join i “.” tab either dir? file [dirize file][file]
                    ]
                ]
                print “Finishing drag operation”
                win-shell/drag-finish msg/3
                print “Unregistering window from DragAcceptFiles queue”
                win-shell/drag-accept-files my-hwnd? to integer! false
                ; (do something with files here)
                halt
            ]
        ]
    ]
    none
]
print “Drag some files and drop them on this window…^/”
win-shell/drag-accept-files my-hwnd? to integer! true
enable-system-trap
forever [
    wake-port: wait []
    if wake-port [check-system-trap wake-port]
]

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