LibGUI+WindowServer: Start fleshing out drag&drop functionality

This patch enables basic drag&drop between applications.
You initiate a drag by creating a GDragOperation object and calling
exec() on it. This creates a nested event loop in the calling program
that only returns once the drag operation has ended.

On the receiving side, you get a call to GWidget::drop_event() with
a GDropEvent containing information about the dropped data.

The only data passed right now is a piece of text that's also used
to visually indicate that a drag is happening (by showing the text in
a little box that follows the mouse cursor around.)

There are things to fix here, but we're off to a nice start. :^)

Libraries/LibGUI/GDragOperation.cpp

@@ -0,0 +1,53 @@
+#include <LibGUI/GDragOperation.h>
+#include <LibGUI/GWindowServerConnection.h>
+
+static GDragOperation* s_current_drag_operation;
+
+GDragOperation::GDragOperation(CObject* parent)
+    : CObject(parent)
+{
+}
+
+GDragOperation::~GDragOperation()
+{
+}
+
+GDragOperation::Outcome GDragOperation::exec()
+{
+    ASSERT(!s_current_drag_operation);
+    ASSERT(!m_event_loop);
+
+    auto response = GWindowServerConnection::the().send_sync<WindowServer::StartDrag>(m_text, -1, Size());
+    if (!response->started()) {
+        m_outcome = Outcome::Cancelled;
+        return m_outcome;
+    }
+
+    s_current_drag_operation = this;
+    m_event_loop = make<CEventLoop>();
+    auto result = m_event_loop->exec();
+    m_event_loop = nullptr;
+    dbgprintf("%s: event loop returned with result %d\n", class_name(), result);
+    remove_from_parent();
+    s_current_drag_operation = nullptr;
+    return m_outcome;
+}
+
+void GDragOperation::done(Outcome outcome)
+{
+    ASSERT(m_outcome == Outcome::None);
+    m_outcome = outcome;
+    m_event_loop->quit(0);
+}
+
+void GDragOperation::notify_accepted(Badge<GWindowServerConnection>)
+{
+    ASSERT(s_current_drag_operation);
+    s_current_drag_operation->done(Outcome::Accepted);
+}
+
+void GDragOperation::notify_cancelled(Badge<GWindowServerConnection>)
+{
+    ASSERT(s_current_drag_operation);
+    s_current_drag_operation->done(Outcome::Cancelled);
+}

Libraries/LibGUI/GDragOperation.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <LibCore/CEventLoop.h>
+#include <LibCore/CObject.h>
+
+class GWindowServerConnection;
+
+class GDragOperation : public CObject {
+    C_OBJECT(GDragOperation)
+public:
+    enum class Outcome {
+        None,
+        Accepted,
+        Cancelled,
+    };
+
+    virtual ~GDragOperation() override;
+
+    void set_text(const String& text) { m_text = text; }
+
+    Outcome exec();
+    Outcome outcome() const { return m_outcome; }
+
+    static void notify_accepted(Badge<GWindowServerConnection>);
+    static void notify_cancelled(Badge<GWindowServerConnection>);
+
+protected:
+    explicit GDragOperation(CObject* parent = nullptr);
+
+private:
+    void done(Outcome);
+
+    OwnPtr<CEventLoop> m_event_loop;
+    Outcome m_outcome { Outcome::None };
+    String m_text;
+};

Libraries/LibGUI/GEvent.h

@@ -2,9 +2,9 @@
 
 #include <Kernel/KeyCode.h>
 #include <LibCore/CEvent.h>
-#include <LibGUI/GWindowType.h>
 #include <LibDraw/Point.h>
 #include <LibDraw/Rect.h>
+#include <LibGUI/GWindowType.h>
 
 class CObject;
 
@@ -34,6 +34,7 @@ public:
         WindowCloseRequest,
         ContextMenu,
         EnabledChange,
+        Drop,
 
         __Begin_WM_Events,
         WM_WindowRemoved,
@@ -278,3 +279,20 @@ private:
     unsigned m_modifiers { 0 };
     int m_wheel_delta { 0 };
 };
+
+class GDropEvent final : public GEvent {
+public:
+    GDropEvent(const Point& position, const String& text)
+        : GEvent(GEvent::Drop)
+        , m_position(position)
+        , m_text(text)
+    {
+    }
+
+    const Point& position() const { return m_position; }
+    const String& text() const { return m_text; }
+
+private:
+    Point m_position;
+    String m_text;
+};

Libraries/LibGUI/GItemView.cpp

@@ -1,4 +1,6 @@
+#include <AK/StringBuilder.h>
 #include <Kernel/KeyCode.h>
+#include <LibGUI/GDragOperation.h>
 #include <LibGUI/GItemView.h>
 #include <LibGUI/GModel.h>
 #include <LibGUI/GPainter.h>
@@ -83,6 +85,7 @@ void GItemView::mousedown_event(GMouseEvent& event)
     int item_index = item_at_event_position(event.position());
 
     if (event.button() == GMouseButton::Left) {
+        m_left_mousedown_position = event.position();
         if (item_index == -1) {
             selection().clear();
         } else {
@@ -97,6 +100,45 @@ void GItemView::mousedown_event(GMouseEvent& event)
     GAbstractView::mousedown_event(event);
 }
 
+void GItemView::mousemove_event(GMouseEvent& event)
+{
+    if (!model())
+        return GAbstractView::mousemove_event(event);
+
+    if (event.buttons() & GMouseButton::Left && !selection().is_empty()) {
+        auto diff = event.position() - m_left_mousedown_position;
+        auto distance_travelled_squared = diff.x() * diff.x() + diff.y() * diff.y();
+        constexpr int drag_distance_threshold = 5;
+        if (distance_travelled_squared > (drag_distance_threshold)) {
+            dbg() << "Initiate drag!";
+            auto drag_operation = GDragOperation::construct();
+
+            StringBuilder builder;
+            selection().for_each_index([&](auto& index) {
+                auto data = model()->data(index);
+                builder.append(data.to_string());
+                builder.append(" ");
+            });
+
+            drag_operation->set_text(builder.to_string());
+            auto outcome = drag_operation->exec();
+            switch (outcome) {
+            case GDragOperation::Outcome::Accepted:
+                dbg() << "Drag was accepted!";
+                break;
+            case GDragOperation::Outcome::Cancelled:
+                dbg() << "Drag was cancelled!";
+                break;
+            default:
+                ASSERT_NOT_REACHED();
+                break;
+            }
+        }
+    }
+
+    GAbstractView::mousemove_event(event);
+}
+
 void GItemView::context_menu_event(GContextMenuEvent& event)
 {
     if (!model())

Libraries/LibGUI/GItemView.h

@@ -29,6 +29,7 @@ private:
     virtual void paint_event(GPaintEvent&) override;
     virtual void resize_event(GResizeEvent&) override;
     virtual void mousedown_event(GMouseEvent&) override;
+    virtual void mousemove_event(GMouseEvent&) override;
     virtual void keydown_event(GKeyEvent&) override;
     virtual void doubleclick_event(GMouseEvent&) override;
     virtual void context_menu_event(GContextMenuEvent&) override;
@@ -43,5 +44,7 @@ private:
     int m_visual_column_count { 0 };
     int m_visual_row_count { 0 };
 
+    Point m_left_mousedown_position;
+
     Size m_effective_item_size { 80, 80 };
 };

Libraries/LibGUI/GWidget.cpp

@@ -156,6 +156,8 @@ void GWidget::event(CEvent& event)
         return handle_mouseup_event(static_cast<GMouseEvent&>(event));
     case GEvent::MouseWheel:
         return mousewheel_event(static_cast<GMouseEvent&>(event));
+    case GEvent::Drop:
+        return drop_event(static_cast<GDropEvent&>(event));
     case GEvent::Enter:
         return handle_enter_event(event);
     case GEvent::Leave:
@@ -350,6 +352,11 @@ void GWidget::change_event(GEvent&)
 {
 }
 
+void GWidget::drop_event(GDropEvent& event)
+{
+    dbg() << class_name() << "{" << this << "} DROP  position: " << event.position() << ", text: '" << event.text() << "'";
+}
+
 void GWidget::update()
 {
     if (rect().is_empty())
@@ -686,4 +693,3 @@ Vector<GWidget*> GWidget::child_widgets() const
     }
     return widgets;
 }
-

Libraries/LibGUI/GWidget.h

@@ -254,6 +254,7 @@ protected:
     virtual void leave_event(CEvent&);
     virtual void child_event(CChildEvent&) override;
     virtual void change_event(GEvent&);
+    virtual void drop_event(GDropEvent&);
 
 private:
     void handle_paint_event(GPaintEvent&);

Libraries/LibGUI/GWindow.cpp

@@ -154,6 +154,16 @@ void GWindow::set_override_cursor(GStandardCursor cursor)
 
 void GWindow::event(CEvent& event)
 {
+    if (event.type() == GEvent::Drop) {
+        auto& drop_event = static_cast<GDropEvent&>(event);
+        if (!m_main_widget)
+            return;
+        auto result = m_main_widget->hit_test(drop_event.position());
+        auto local_event = make<GDropEvent>(result.local_position, drop_event.text());
+        ASSERT(result.widget);
+        return result.widget->dispatch_event(*local_event, this);
+    }
+
     if (event.type() == GEvent::MouseUp || event.type() == GEvent::MouseDown || event.type() == GEvent::MouseDoubleClick || event.type() == GEvent::MouseMove || event.type() == GEvent::MouseWheel) {
         auto& mouse_event = static_cast<GMouseEvent&>(event);
         if (m_global_cursor_tracking_widget) {

Libraries/LibGUI/GWindowServerConnection.cpp

@@ -2,6 +2,7 @@
 #include <LibGUI/GApplication.h>
 #include <LibGUI/GClipboard.h>
 #include <LibGUI/GDesktop.h>
+#include <LibGUI/GDragOperation.h>
 #include <LibGUI/GEvent.h>
 #include <LibGUI/GMenu.h>
 #include <LibGUI/GWidget.h>
@@ -261,3 +262,19 @@ void GWindowServerConnection::handle(const WindowClient::AsyncSetWallpaperFinish
 {
     // This is handled manually by GDesktop::set_wallpaper().
 }
+
+void GWindowServerConnection::handle(const WindowClient::DragDropped& message)
+{
+    if (auto* window = GWindow::from_window_id(message.window_id()))
+        CEventLoop::current().post_event(*window, make<GDropEvent>(message.mouse_position(), message.text()));
+}
+
+void GWindowServerConnection::handle(const WindowClient::DragAccepted&)
+{
+    GDragOperation::notify_accepted({});
+}
+
+void GWindowServerConnection::handle(const WindowClient::DragCancelled&)
+{
+    GDragOperation::notify_cancelled({});
+}

Libraries/LibGUI/GWindowServerConnection.h

@@ -41,4 +41,7 @@ private:
     virtual void handle(const WindowClient::WM_WindowIconBitmapChanged&) override;
     virtual void handle(const WindowClient::WM_WindowRectChanged&) override;
     virtual void handle(const WindowClient::AsyncSetWallpaperFinished&) override;
+    virtual void handle(const WindowClient::DragDropped&) override;
+    virtual void handle(const WindowClient::DragAccepted&) override;
+    virtual void handle(const WindowClient::DragCancelled&) override;
 };

Libraries/LibGUI/Makefile

@@ -59,6 +59,7 @@ OBJS = \
     GLazyWidget.o \
     GCommand.o \
     GUndoStack.o \
+    GDragOperation.o \
     GWindow.o
 
 LIBRARY = libgui.a