Introduce drag & drop events to widgets

src/ui/base.h

@@ -29,8 +29,9 @@ namespace ui {
     DOUBLE_BUFFERED  = 0x00002000, // The widget is painted in a back-buffer and then flipped to the main display
     TRANSPARENT      = 0x00004000, // The widget has transparent parts that needs the background painted before
     CTRL_RIGHT_CLICK = 0x00008000, // The widget should transform Ctrl+click to right-click on OS X.
+    ALLOW_DROP       = 0x40000000, // The widget can participate as a drop target in a drag & drop operation.
     IGNORE_MOUSE     = 0x80000000, // Don't process mouse messages for this widget (useful for labels, boxes, grids, etc.)
-    PROPERTIES_MASK  = 0x8000ffff,
+    PROPERTIES_MASK  = 0xC000ffff,
 
     HORIZONTAL       = 0x00010000,
     VERTICAL         = 0x00020000,
@@ -43,7 +44,7 @@ namespace ui {
     HOMOGENEOUS      = 0x01000000,
     WORDWRAP         = 0x02000000,
     CHARWRAP         = 0x04000000,
-    ALIGN_MASK       = 0x7fff0000,
+    ALIGN_MASK       = 0x3fff0000,
   };
 
 } // namespace ui

src/ui/drag_event.h

@@ -0,0 +1,48 @@
+// Aseprite UI Library
+// Copyright (C) 2024  Igara Studio S.A.
+//
+// This file is released under the terms of the MIT license.
+// Read LICENSE.txt for more information.
+
+#ifndef UI_DRAG_EVENT_H_INCLUDED
+#define UI_DRAG_EVENT_H_INCLUDED
+#pragma once
+
+#include "os/dnd.h"
+#include "ui/event.h"
+#include "ui/widget.h"
+
+namespace ui {
+
+  class DragEvent : public Event {
+  public:
+    DragEvent(Component* source, ui::Widget* target, os::DragEvent& ev)
+      : Event(source)
+      , m_position(ev.position() - target->bounds().origin())
+      , m_ev(ev) {}
+
+    bool handled() const { return m_handled; }
+    void handled(bool value) { m_handled = value; }
+
+    // Operations allowed by the source of the drag & drop operation. Can be a
+    // bitwise combination of values.
+    os::DropOperation allowedOperations() const { return m_ev.supportedOperations(); }
+
+    // Operation supported by the target of the drag & drop operation. Cannot
+    // be a bitwise combination of values.
+    os::DropOperation supportsOperation() const { return m_ev.dropResult(); }
+    // Set the operation supported by the target of the drag & drop operation.
+    // Cannot be a bitwise combination of values.
+    void supportsOperation(os::DropOperation operation) { m_ev.dropResult(operation); }
+
+    const gfx::Point& position() const { return m_position; }
+
+  private:
+    bool m_handled = false;
+    gfx::Point m_position;
+    os::DragEvent& m_ev;
+  };
+
+} // namespace ui
+
+#endif  // UI_DRAG_EVENT_H_INCLUDED

src/ui/manager.cpp

@@ -28,6 +28,8 @@
 #include "os/system.h"
 #include "os/window.h"
 #include "os/window_spec.h"
+#include "ui/base.h"
+#include "ui/drag_event.h"
 #include "ui/intern.h"
 #include "ui/ui.h"
 
@@ -205,8 +207,10 @@ Manager::Manager(const os::WindowRef& nativeWindow)
   , m_mouseButton(kButtonNone)
 {
   // The native window can be nullptr when running tests
-  if (nativeWindow)
+  if (nativeWindow) {
     nativeWindow->setUserData(&m_display);
+    nativeWindow->setDragTarget(this);
+  }
 
 #ifdef DEBUG_UI_THREADS
   ASSERT(manager_thread == std::thread::id());
@@ -2037,6 +2041,76 @@ bool Manager::sendMessageToWidget(Message* msg, Widget* widget)
   return used;
 }
 
+void Manager::dragEnter(os::DragEvent& ev)
+{
+  Widget* widget = pick(ev.position());
+  if (widget && widget->hasFlags(ALLOW_DROP)) {
+    m_dragOverWidget = widget;
+    DragEvent uiev(this, widget, ev);
+    widget->onDragEnter(uiev);
+    ev.dropResult(uiev.supportsOperation());
+  }
+}
+
+void Manager::dragLeave(os::DragEvent& ev)
+{
+  Widget* widget = m_dragOverWidget;
+  if (widget) {
+    DragEvent uiev(this, widget, ev);
+    widget->onDragLeave(uiev);
+    m_dragOverWidget = nullptr;
+  }
+}
+
+void Manager::drag(os::DragEvent& ev)
+{
+  Widget* widget = pick(ev.position());
+
+  if (m_dragOverWidget && m_dragOverWidget != widget) {
+    DragEvent uiev(this, m_dragOverWidget, ev);
+    m_dragOverWidget->onDragLeave(uiev);
+    m_dragOverWidget = nullptr;
+  }
+
+  if (widget && widget->hasFlags(ALLOW_DROP)) {
+    DragEvent uiev(this, widget, ev);
+    if (m_dragOverWidget != widget) {
+      m_dragOverWidget = widget;
+      widget->onDragEnter(uiev);
+    }
+    widget->onDrag(uiev);
+    ev.dropResult(uiev.supportsOperation());
+  }
+}
+
+void Manager::drop(os::DragEvent& ev)
+{
+  m_dragOverWidget = nullptr;
+  Widget* widget = pick(ev.position());
+  if (widget && widget->hasFlags(ALLOW_DROP)) {
+    DragEvent uiev(this, widget, ev);
+    widget->onDrop(uiev);
+    if (uiev.handled()) {
+      ev.acceptDrop(true);
+      return;
+    }
+  }
+
+  // There were no widget that accepted the drop, then see if we can treat it
+  // like a DropFiles event.
+  if (ev.dataProvider()->contains(os::DragDataItemType::Paths)) {
+    ev.acceptDrop(true);
+    // We must queue an os::Event to wakeup the underlying system queue on
+    // masOS. If we had used the enqueueMessage() method instead, it could
+    // happen that the program might look unresponsive because it is waiting
+    // for an OS event.
+    os::Event dropFilesEv;
+    dropFilesEv.setType(os::Event::DropFiles);
+    dropFilesEv.setFiles(ev.dataProvider()->getPaths());
+    os::System::instance()->eventQueue()->queueEvent(dropFilesEv);
+  }
+}
+
 // It's like Widget::onInvalidateRegion() but optimized for the
 // Manager (as we know that all children in a Manager will be windows,
 // we can use this knowledge to avoid some calculations).

src/ui/manager.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "gfx/region.h"
+#include "os/dnd.h"
 #include "ui/display.h"
 #include "ui/keys.h"
 #include "ui/message_type.h"
@@ -28,7 +29,8 @@ namespace ui {
   class Timer;
   class Window;
 
-  class Manager : public Widget {
+  class Manager : public Widget
+                , public os::DragTarget {
   public:
     static Manager* getDefault() { return m_defaultManager; }
     static bool widgetAssociatedToManager(Widget* widget);
@@ -161,6 +163,11 @@ namespace ui {
     int pumpQueue();
     bool sendMessageToWidget(Message* msg, Widget* widget);
 
+    void dragEnter(os::DragEvent& ev) override;
+    void dragLeave(os::DragEvent& ev) override;
+    void drag(os::DragEvent& ev) override;
+    void drop(os::DragEvent& ev) override;
+
     static Widget* findLowestCommonAncestor(Widget* a, Widget* b);
     static bool someParentIsFocusStop(Widget* widget);
     static Widget* findMagneticWidget(Widget* widget);
@@ -189,6 +196,9 @@ namespace ui {
 
     // Last pressed mouse button.
     MouseButton m_mouseButton;
+
+    // Widget over which the drag is being hovered in a drag & drop operation.
+    Widget* m_dragOverWidget = nullptr;
   };
 
 } // namespace ui

src/ui/ui.h

@@ -21,6 +21,7 @@
 #include "ui/cursor.h"
 #include "ui/cursor_type.h"
 #include "ui/display.h"
+#include "ui/drag_event.h"
 #include "ui/entry.h"
 #include "ui/event.h"
 #include "ui/fit_bounds.h"

src/ui/widget.cpp

@@ -22,6 +22,7 @@
 #include "text/font.h"
 #include "text/font_mgr.h"
 #include "ui/app_state.h"
+#include "ui/drag_event.h"
 #include "ui/init_theme_event.h"
 #include "ui/intern.h"
 #include "ui/layout_io.h"
@@ -1800,6 +1801,38 @@ text::ShaperFeatures Widget::onGetTextShaperFeatures() const
   return text::ShaperFeatures();
 }
 
+void Widget::onDragEnter(DragEvent& e)
+{
+#ifdef _DEBUG
+  LOG(VERBOSE, "UI: [id=%s, type=%d]: onDragEnter(), position: (%d, %d)\n",
+      id().c_str(), type(), e.position().x, e.position().y);
+#endif
+}
+
+void Widget::onDragLeave(DragEvent& e)
+{
+#ifdef _DEBUG
+  LOG(VERBOSE, "UI: [id=%s, type=%d]: onDragLeave(), position: (%d, %d)\n",
+      id().c_str(), type(), e.position().x, e.position().y);
+#endif
+}
+
+void Widget::onDrag(DragEvent& e)
+{
+#ifdef _DEBUG
+  LOG(VERBOSE, "UI: [id=%s, type=%d]: onDrag(), position: (%d, %d)\n",
+      id().c_str(), type(), e.position().x, e.position().y);
+#endif
+}
+
+void Widget::onDrop(DragEvent& e)
+{
+#ifdef _DEBUG
+  LOG(VERBOSE, "UI: [id=%s, type=%d]: onDrop(), position: (%d, %d)\n",
+      id().c_str(), type(), e.position().x, e.position().y);
+#endif
+}
+
 void Widget::offsetWidgets(int dx, int dy)
 {
   if (dx == 0 && dy == 0)

src/ui/widget.h

@@ -44,6 +44,7 @@ namespace ui {
   class Style;
   class Theme;
   class Window;
+  class DragEvent;
 
   class Widget : public Component {
   public:
@@ -445,6 +446,11 @@ namespace ui {
     virtual text::TextBlobRef onMakeTextBlob() const;
     virtual text::ShaperFeatures onGetTextShaperFeatures() const;
 
+    virtual void onDragEnter(DragEvent& e);
+    virtual void onDragLeave(DragEvent& e);
+    virtual void onDrag(DragEvent& e);
+    virtual void onDrop(DragEvent& e);
+
   private:
     void removeChild(const WidgetsList::iterator& it);
     void paint(Graphics* graphics,
@@ -483,6 +489,8 @@ namespace ui {
 
     gfx::Border m_border;       // Border separation with the parent
     int m_childSpacing;         // Separation between children
+
+    friend Manager;
   };
 
   WidgetType register_widget_type();