[FEATURE] API to allow drag'n'drop of custom browser items

QgsDataItem implementations may provide hasDragEnabled(), mimeUri()
and QgsCustomDropHandler implementation to deal with drop of custom items.

doc/api_break.dox

@@ -450,6 +450,12 @@ be returned instead of a null pointer if no transformation is required.</li>
 plugins calling this method will need to be updated.</li>
 </ul>
 
+\subsection qgis_api_break_3_0_QgsMimeDataUtils QgsMimeDataUtils
+
+<ul>
+<li>Constructor taking QgsLayerItem argument has been removed. Use QgsDataItem::mimeUri() instead.</li>
+</ul>
+
 \subsection qgis_api_break_3_0_QgsOSMElement QgsOSMElement
 
 <ul>

python/core/qgsdataitem.sip

@@ -135,6 +135,21 @@ class QgsDataItem : QObject
      */
     virtual bool handleDrop( const QMimeData * /*data*/, Qt::DropAction /*action*/ );
 
+    /** Returns true if the item may be dragged.
+     * Default implementation returns false.
+     * A draggable item has to implement mimeUri() that will be used to pass data.
+     * @see mimeUri()
+     * @note added in 3.0
+     */
+    virtual bool hasDragEnabled() const;
+
+    /** Return mime URI for the data item.
+     * Items that return valid URI will be returned in mime data when dragging a selection from browser model.
+     * @see hasDragEnabled()
+     * @note added in 3.0
+     */
+    virtual QgsMimeDataUtils::Uri mimeUri() const;
+
     enum Capability
     {
       NoCapabilities,
@@ -262,26 +277,30 @@ class QgsLayerItem : QgsDataItem
 
     virtual bool equal( const QgsDataItem *other );
 
+    virtual bool hasDragEnabled() const;
+
+    virtual QgsMimeDataUtils::Uri mimeUri() const;
+
     // --- New virtual methods for layer item derived classes ---
 
     // Returns QgsMapLayer::LayerType
-    QgsMapLayer::LayerType mapLayerType();
+    QgsMapLayer::LayerType mapLayerType() const;
 
     // Returns layer uri or empty string if layer cannot be created
-    QString uri();
+    QString uri() const;
 
     // Returns provider key
-    QString providerKey();
+    QString providerKey() const;
 
     /** Returns the supported CRS
      *  @note Added in 2.8
      */
-    QStringList supportedCrs();
+    QStringList supportedCrs() const;
 
     /** Returns the supported formats
      *  @note Added in 2.8
      */
-    QStringList supportedFormats();
+    QStringList supportedFormats() const;
 
     /** Returns comments of the layer
      * @note added in 2.12
@@ -389,6 +408,7 @@ class QgsProjectItem : QgsDataItem
     QgsProjectItem( QgsDataItem* parent, const QString& name, const QString& path );
     ~QgsProjectItem();
 
+    virtual bool hasDragEnabled() const;
 };
 
 /**

python/core/qgsmimedatautils.sip

@@ -7,14 +7,27 @@ class QgsMimeDataUtils
 
     struct Uri
     {
-      Uri( QgsLayerItem* layer );
-      Uri( QString& encData );
+      //! Constructs invalid URI
+      Uri();
+      //! Constructs URI from encoded data
+      explicit Uri( QString& encData );
 
+      //! Returns whether the object contains valid data
+      //! @note added in 3.0
+      bool isValid() const;
+
+      //! Returns encoded representation of the object
       QString data() const;
 
+      //! Type of URI. Recognized types: "vector" / "raster" / "plugin" / "custom"
       QString layerType;
+      //! For "vector" / "raster" type: provider id.
+      //! For "plugin" type: plugin layer type name.
+      //! For "custom" type: key of its QgsCustomDropHandler
       QString providerKey;
+      //! Human readable name to be used e.g. in layer tree
       QString name;
+      //! Identifier of the data source recognized by its providerKey
       QString uri;
       QStringList supportedCrs;
       QStringList supportedFormats;

python/gui/gui.sip

@@ -57,6 +57,7 @@
 %Include qgscompoundcolorwidget.sip
 %Include qgsconfigureshortcutsdialog.sip
 %Include qgscredentialdialog.sip
+%Include qgscustomdrophandler.sip
 %Include qgsdatadefinedbutton.sip
 %Include qgsdetaileditemdata.sip
 %Include qgsdetaileditemdelegate.sip

python/gui/qgisinterface.sip

@@ -292,6 +292,18 @@ class QgisInterface : QObject
     */
     virtual void unregisterMapLayerConfigWidgetFactory( QgsMapLayerConfigWidgetFactory* factory ) = 0;
 
+    /** Register a new custom drop handler.
+     * @note added in QGIS 3.0
+     * @note Ownership of the factory is not transferred, and the factory must
+     *       be unregistered when plugin is unloaded.
+     * @see unregisterCustomDropHandler() */
+    virtual void registerCustomDropHandler( QgsCustomDropHandler* handler ) = 0;
+
+    /** Unregister a previously registered custom drop handler.
+     * @note added in QGIS 3.0
+     * @see registerCustomDropHandler() */
+    virtual void unregisterCustomDropHandler( QgsCustomDropHandler* handler ) = 0;
+
     // @todo is this deprecated in favour of QgsContextHelp?
     /** Open a url in the users browser. By default the QGIS doc directory is used
      * as the base for the URL. To open a URL that is not relative to the installed

python/gui/qgscustomdrophandler.sip

@@ -0,0 +1,21 @@
+/** \ingroup gui
+ * Abstract base class that may be implemented to handle new types of data to be dropped in QGIS.
+ * Implementations will be used when a QgsMimeDataUtils::Uri has layerType equal to "custom",
+ * and the providerKey is equal to key() returned by the implementation.
+ * @note added in QGIS 3.0
+ */
+class QgsCustomDropHandler
+{
+%TypeHeaderCode
+#include <qgscustomdrophandler.h>
+%End
+
+  public:
+    virtual ~QgsCustomDropHandler();
+
+    //! Type of custom URI recognized by the handler
+    virtual QString key() const = 0;
+
+    //! Method called from QGIS after a drop event with custom URI known by the handler
+    virtual void handleDrop( const QgsMimeDataUtils::Uri& uri ) const = 0;
+};

src/app/qgisapp.cpp

@@ -128,6 +128,7 @@
 #include "qgscoordinateutils.h"
 #include "qgscredentialdialog.h"
 #include "qgscursors.h"
+#include "qgscustomdrophandler.h"
 #include "qgscustomization.h"
 #include "qgscustomlayerorderwidget.h"
 #include "qgscustomprojectiondialog.h"
@@ -1341,29 +1342,58 @@ void QgisApp::dropEvent( QDropEvent *event )
   if ( QgsMimeDataUtils::isUriList( event->mimeData() ) )
   {
     QgsMimeDataUtils::UriList lst = QgsMimeDataUtils::decodeUriList( event->mimeData() );
-    Q_FOREACH ( const QgsMimeDataUtils::Uri& u, lst )
-    {
-      QString uri = crsAndFormatAdjustedLayerUri( u.uri, u.supportedCrs, u.supportedFormats );
+    handleDropUriList( lst );
+  }
+  mMapCanvas->freeze( false );
+  mMapCanvas->refresh();
+  event->acceptProposedAction();
+}
 
-      if ( u.layerType == "vector" )
-      {
-        addVectorLayer( uri, u.name, u.providerKey );
-      }
-      else if ( u.layerType == "raster" )
-      {
-        addRasterLayer( uri, u.name, u.providerKey );
-      }
-      else if ( u.layerType == "plugin" )
+
+void QgisApp::registerCustomDropHandler( QgsCustomDropHandler* handler )
+{
+  if ( !mCustomDropHandlers.contains( handler ) )
+    mCustomDropHandlers << handler;
+}
+
+void QgisApp::unregisterCustomDropHandler( QgsCustomDropHandler* handler )
+{
+  mCustomDropHandlers.removeOne( handler );
+}
+
+void QgisApp::handleDropUriList( const QgsMimeDataUtils::UriList& lst )
+{
+  Q_FOREACH ( const QgsMimeDataUtils::Uri& u, lst )
+  {
+    QString uri = crsAndFormatAdjustedLayerUri( u.uri, u.supportedCrs, u.supportedFormats );
+
+    if ( u.layerType == "vector" )
+    {
+      addVectorLayer( uri, u.name, u.providerKey );
+    }
+    else if ( u.layerType == "raster" )
+    {
+      addRasterLayer( uri, u.name, u.providerKey );
+    }
+    else if ( u.layerType == "plugin" )
+    {
+      addPluginLayer( uri, u.name, u.providerKey );
+    }
+    else if ( u.layerType == "custom" )
+    {
+      Q_FOREACH ( QgsCustomDropHandler* handler, mCustomDropHandlers )
       {
-        addPluginLayer( uri, u.name, u.providerKey );
+        if ( handler->key() == u.providerKey )
+        {
+          handler->handleDrop( u );
+          break;
+        }
       }
     }
   }
-  mMapCanvas->freeze( false );
-  mMapCanvas->refresh();
-  event->acceptProposedAction();
 }
 
+
 bool QgisApp::event( QEvent * event )
 {
   bool done = false;

src/app/qgisapp.h

@@ -44,6 +44,7 @@ class QgsClipboard;
 class QgsComposer;
 class QgsComposerManager;
 class QgsComposerView;
+class QgsCustomDropHandler;
 class QgsStatusBarCoordinatesWidget;
 class QgsStatusBarMagnifierWidget;
 class QgsStatusBarScaleWidget;
@@ -120,6 +121,7 @@ class QgsDiagramProperties;
 #include "qgsfeature.h"
 #include "qgspoint.h"
 #include "qgsmessagebar.h"
+#include "qgsmimedatautils.h"
 #include "qgswelcomepageitemsmodel.h"
 #include "qgsraster.h"
 
@@ -513,6 +515,15 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
     /** Unregister a previously registered tab in the layer properties dialog */
     void unregisterMapLayerPropertiesFactory( QgsMapLayerConfigWidgetFactory* factory );
 
+    /** Register a new custom drop handler. */
+    void registerCustomDropHandler( QgsCustomDropHandler* handler );
+
+    /** Unregister a previously registered custom drop handler. */
+    void unregisterCustomDropHandler( QgsCustomDropHandler* handler );
+
+    /** Process the list of URIs that have been dropped in QGIS */
+    void handleDropUriList( const QgsMimeDataUtils::UriList& lst );
+
   public slots:
     void layerTreeViewDoubleClicked( const QModelIndex& index );
     //! Make sure the insertion point for new layers is up-to-date with the current item in layer tree view
@@ -1772,6 +1783,8 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
 
     QList<QgsMapLayerConfigWidgetFactory*> mMapLayerPanelFactories;
 
+    QList<QgsCustomDropHandler*> mCustomDropHandlers;
+
     QDateTime mProjectLastModified;
 
     QgsWelcomePage* mWelcomePage;

src/app/qgisappinterface.cpp

@@ -485,6 +485,16 @@ void QgisAppInterface::unregisterMapLayerConfigWidgetFactory( QgsMapLayerConfigW
   qgis->unregisterMapLayerPropertiesFactory( factory );
 }
 
+void QgisAppInterface::registerCustomDropHandler( QgsCustomDropHandler *handler )
+{
+  qgis->registerCustomDropHandler( handler );
+}
+
+void QgisAppInterface::unregisterCustomDropHandler( QgsCustomDropHandler *handler )
+{
+  qgis->unregisterCustomDropHandler( handler );
+}
+
 //! Menus
 QMenu *QgisAppInterface::projectMenu() { return qgis->projectMenu(); }
 QMenu *QgisAppInterface::editMenu() { return qgis->editMenu(); }

src/app/qgisappinterface.h

@@ -301,6 +301,18 @@ class APP_EXPORT QgisAppInterface : public QgisInterface
     */
     virtual void unregisterMapLayerConfigWidgetFactory( QgsMapLayerConfigWidgetFactory* factory ) override;
 
+    /** Register a new custom drop handler.
+     * @note added in QGIS 3.0
+     * @note Ownership of the factory is not transferred, and the factory must
+     *       be unregistered when plugin is unloaded.
+     * @see unregisterCustomDropHandler() */
+    virtual void registerCustomDropHandler( QgsCustomDropHandler* handler ) override;
+
+    /** Unregister a previously registered custom drop handler.
+     * @note added in QGIS 3.0
+     * @see registerCustomDropHandler() */
+    virtual void unregisterCustomDropHandler( QgsCustomDropHandler* handler ) override;
+
     /** Accessors for inserting items into menus and toolbars.
      * An item can be inserted before any existing action.
      */

src/app/qgsbrowserdockwidget.cpp

@@ -516,26 +516,9 @@ void QgsBrowserDockWidget::addLayer( QgsLayerItem *layerItem )
   if ( !layerItem )
     return;
 
-  QString uri = QgisApp::instance()->crsAndFormatAdjustedLayerUri( layerItem->uri(), layerItem->supportedCrs(), layerItem->supportedFormats() );
-  if ( uri.isEmpty() )
-    return;
-
-  QgsMapLayer::LayerType type = layerItem->mapLayerType();
-  QString providerKey = layerItem->providerKey();
-
-  QgsDebugMsg( providerKey + " : " + uri );
-  if ( type == QgsMapLayer::VectorLayer )
-  {
-    QgisApp::instance()->addVectorLayer( uri, layerItem->layerName(), providerKey );
-  }
-  if ( type == QgsMapLayer::RasterLayer )
-  {
-    QgisApp::instance()->addRasterLayer( uri, layerItem->layerName(), providerKey );
-  }
-  if ( type == QgsMapLayer::PluginLayer )
-  {
-    QgisApp::instance()->addPluginLayer( uri, layerItem->layerName(), providerKey );
-  }
+  QgsMimeDataUtils::UriList list;
+  list << layerItem->mimeUri();
+  QgisApp::instance()->handleDropUriList( list );
 }
 
 void QgsBrowserDockWidget::addLayerAtIndex( const QModelIndex& index )

src/core/qgsbrowsermodel.cpp

@@ -186,10 +186,9 @@ Qt::ItemFlags QgsBrowserModel::flags( const QModelIndex & index ) const
   Qt::ItemFlags flags = Qt::ItemIsEnabled | Qt::ItemIsSelectable;
 
   QgsDataItem* ptr = reinterpret_cast< QgsDataItem* >( index.internalPointer() );
-  if ( ptr->type() == QgsDataItem::Layer || ptr->type() == QgsDataItem::Project )
-  {
+  if ( ptr->hasDragEnabled() )
     flags |= Qt::ItemIsDragEnabled;
-  }
+
   if ( ptr->acceptDrop() )
     flags |= Qt::ItemIsDropEnabled;
   return flags;
@@ -461,9 +460,9 @@ QMimeData * QgsBrowserModel::mimeData( const QModelIndexList &indexes ) const
         return mimeData;
       }
 
-      if ( ptr->type() != QgsDataItem::Layer ) continue;
-      QgsLayerItem *layer = static_cast< QgsLayerItem* >( ptr );
-      lst.append( QgsMimeDataUtils::Uri( layer ) );
+      QgsMimeDataUtils::Uri uri = ptr->mimeUri();
+      if ( uri.isValid() )
+        lst.append( uri );
     }
   }
   return QgsMimeDataUtils::encodeUriList( lst );