dnd_glfw – GLFW Drag-and-Drop Helper

dnd_glfw is a very small helper around GLFW that exposes a simple, C‑style drag‑and‑drop callback interface. It is designed for applications that already use GLFW for their main window (e.g. Dear ImGui with the GLFW/OpenGL backend) and want better control over file drops and drag‑hover overlays.

The implementation is intentionally minimal and C‑style C++: no heavy abstractions, only basic C++17 containers and RAII where needed.

Supported platforms

• Windows

  • Uses the GLFW window handle (HWND) and registers an OLE IDropTarget.
  • Provides real OS‑level events for file drags:
    • dragEnter, dragOver, dragLeave, dragCancel.
  • File paths are still obtained via GLFW’s existing drop callback semantics and forwarded to your drop handler.

• macOS

  • Uses Cocoa drag‑and‑drop via an NSView<NSDraggingDestination> overlay on the GLFW window’s content view.
  • Provides OS‑level dragEntered, draggingUpdated, draggingExited, and performDragOperation mapped to:
    • dragEnter, dragOver, dragLeave, dragCancel, drop.
  • File paths are read from the pasteboard and passed to your drop handler as UTF‑8 strings.

• Linux / X11 / Wayland

  • Relies on GLFW’s internal Xdnd / DnD handling.
  • Wraps glfwSetDropCallback and exposes file paths and cursor position via the drop callback.
  • OS‑level drag‑enter / drag‑hover / drag‑leave for external file drags are not available via GLFW’s public API, so dragEnter / dragOver / dragLeave / dragCancel are not fired for system file drags on Linux.

In practice this means:

• You get full overlay‑style drag feedback on Windows and macOS.
• On Linux you still get correct file drops with a consistent API, but no OS‑level “drag in progress” notifications.

API overview

Header: dnd_glfw/dnd_glfw.h

namespace dnd_glfw {

enum class PayloadKind {
    Unknown = 0,
    Files   = 1
};

struct DragEvent {
    double x;
    double y;
    PayloadKind kind;
};

struct DropEvent {
    double x;
    double y;
    PayloadKind kind;
    std::vector<std::string> paths; // UTF‑8 file paths
};

struct Callbacks {
    void (*dragEnter)(GLFWwindow*, const DragEvent&, void* userData);
    void (*dragOver)(GLFWwindow*, const DragEvent&, void* userData);
    void (*dragLeave)(GLFWwindow*, void* userData);
    void (*drop)(GLFWwindow*, const DropEvent&, void* userData);
    void (*dragCancel)(GLFWwindow*, void* userData);
};

bool init(GLFWwindow* window, const Callbacks& callbacks, void* userData);
void shutdown(GLFWwindow* window);

} // namespace dnd_glfw

All callbacks are optional; set a pointer to nullptr to ignore that event type.

Typical usage

1. Create your GLFW window as usual and make it current.
2. Register drag‑and‑drop callbacks once, after the window exists:

static void onDragEnter(GLFWwindow* window, const dnd_glfw::DragEvent& e, void* user)
{
    (void)window;
    AppState* app = static_cast<AppState*>(user);
    if (!app) return;

    if (e.kind == dnd_glfw::PayloadKind::Files) {
        app->dragOverlayActive = true;
    }
}

static void onDrop(GLFWwindow* window, const dnd_glfw::DropEvent& e, void* user)
{
    (void)window;
    AppState* app = static_cast<AppState*>(user);
    if (!app) return;

    std::lock_guard<std::mutex> lock(app->dropMutex);
    app->pendingDrops = e.paths;
    app->dragOverlayActive = false;
}

static void onDragLeave(GLFWwindow* window, void* user)
{
    (void)window;
    AppState* app = static_cast<AppState*>(user);
    if (!app) return;
    app->dragOverlayActive = false;
}

// ...

dnd_glfw::Callbacks cbs{};
cbs.dragEnter  = &onDragEnter;
cbs.dragOver   = nullptr;      // not needed for simple overlay
cbs.dragLeave  = &onDragLeave;
cbs.drop       = &onDrop;
cbs.dragCancel = &onDragLeave; // treat cancel as leave

dnd_glfw::init(window, cbs, &app);

3. Use your own flag to drive an overlay in ImGui (or another GUI):

if (app.dragOverlayActive) {
    // Draw a full‑window dim background and center text
}

4. On shutdown, before destroying the GLFW window:

dnd_glfw::shutdown(window);
glfwDestroyWindow(window);

Integration details

• GLFW drop callback chaining

  • On Windows and Linux, dnd_glfw::init installs an internal glfwSetDropCallback handler and remembers any previously registered callback.
  • When a drop occurs, the internal handler:
    • Builds a DropEvent (cursor position, PayloadKind::Files, file paths).
    • Calls your Callbacks::drop (if provided).
    • Forwards the event to the previous GLFW drop callback (if any).
  • On macOS, GLFW’s own DnD is handled internally; dnd_glfw uses Cocoa APIs instead and does not override glfwSetDropCallback.

• Windows / OLE

  • Uses glfwGetWin32Window(window) to get the HWND and registers an IDropTarget implementation.
  • Only CF_HDROP (file paths) is accepted; other payloads are ignored.
  • DragEnter / DragOver use client‑area coordinates (converted from screen) for the DragEvent::x/y fields.
  • The actual file paths are still delivered via GLFW’s drop events to keep behavior consistent with GLFW/X11 and Cocoa.

• macOS / Cocoa

  • Obtains the native NSWindow* for the GLFW window with glfwGetCocoaWindow.
  • Adds a transparent NSView subclass (DndGlfwDragView) above the content view and registers it for file URL types.
  • Uses the system pasteboard to extract file URLs and forwards them as UTF‑8 paths in DropEvent::paths.

• Linux

  • Depends on GLFW’s own X11/Wayland drag‑and‑drop implementation.
  • Only the drop callback is meaningful for OS‑level file drags; hover/enter/leave for external drags are not exposed by GLFW and therefore not emitted by dnd_glfw.

CMake integration

The top‑level CMakeLists.txt for this project adds the helper as a static library:

if(APPLE)
    add_library(dnd_glfw STATIC
        dnd_glfw_macos.mm
    )
else()
    add_library(dnd_glfw STATIC
        dnd_glfw_dummy.cpp
    )
endif()

target_include_directories(dnd_glfw
    PUBLIC
        ${CMAKE_CURRENT_SOURCE_DIR}
)

if(APPLE)
    target_link_libraries(dnd_glfw PUBLIC "-framework Cocoa")
endif()

To use it from another target (like the GUI):

add_subdirectory(dnd_glfw)

target_link_libraries(meshrepair_gui
    PRIVATE
        dnd_glfw
        # other libs: glfw, OpenGL, ImGui, etc.
)

if(WIN32)
    target_link_libraries(meshrepair_gui PRIVATE ole32)
endif()

The macOS Cocoa framework and Windows ole32 import library are wired automatically via these targets.

Notes and limitations

• Only a small subset of drag‑and‑drop is implemented: file paths dragged from the OS (Explorer/Finder/desktop/file manager) into your GLFW window.
• No attempt is made to support arbitrary MIME types or cross‑process data beyond file URLs / paths.
• On Linux, GLFW owns the Xdnd handling; dnd_glfw intentionally does not duplicate Xdnd parsing to avoid conflicting with GLFW’s internal event loop.
• The internal state is stored in a small global vector keyed by GLFWwindow*. It assumes a small number of windows (typically one) and is not optimized for hundreds of windows. This matches the intended usage for a single ImGui main window.