I have a tree view of resource types that consists of:
Sprites
Backgrounds
Tilesets
Sounds
Objects
Markers
Maps

Using QStandardItemModel's default drag and drop, it allows items to be dropped anywhere meaning that I could mix resource types by dragging a sprite resource into the backgrounds item. To prevent this I tried reimplementing mimeTypes(), mimeData(), and dropMimeData(). When mimeData is reimplemented, for some reason drops are not being accepted even if setAcceptDrops is set to true, but if I comment it out, drops are magically accepted again but are unreadable. What could be the problem?

ResourceModel.h
Qt Code: 
  1. class ResourceModel : public QStandardItemModel
  2. {
  3.     Q_OBJECT
  4. public:
  5.     explicit ResourceModel( QObject *parent, Document *doc, bool createTree = true);
  6.  
  7.     QModelIndex AddResource( Resource *res, QModelIndex *pParent = NULL);
  8.     void AddGroup( QModelIndex parent);
  9.  
  10.     friend class Document;
  11.     friend class Resource;
  12.     friend class Sprite;
  13.     friend class Background;
  14.     friend class Tileset;
  15.     friend class Sounds;
  16.     friend class Object;
  17.     friend class Map;
  18.     friend class CollisionMask;
  19.     friend class Marker;
  20.  
  21. protected:
  22.     Qt::ItemFlags flags(const QModelIndex &index) const;
  23.  
  24.     Qt::DropActions supportedDropActions() const
  25.     {
  26.         return Qt::MoveAction;
  27.     }
  28.  
  29.     QStringList mimeTypes();
  30.     QMimeData *mimeData(const QModelIndexList &indexes) const;
  31.     bool dropMimeData(const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent);
  32.  
  33. private:
  34.     Document *document;
  36.     QStandardItem *Sprites;
  37.     QStandardItem *Backgrounds;
  38.     QStandardItem *Tilesets;
  39.     QStandardItem *Sounds;
  40.     QStandardItem *Objects;
  41.     QStandardItem *CollisionMasks;
  42.     QStandardItem *Markers;
  44.  
  45.     int sprCount, bgCount, tlsCount, sndCount, objCount, collisionMaskCount, mapCount, markerCount;
  46. };
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ResourceModel.cpp
Qt Code: 
  1. #include "resourcemodel.h"
  2. #include "document.h"
  3.  
  4. ResourceModel::ResourceModel(QObject *parent, Document *doc, bool createTree) :
  5.     QStandardItemModel(parent), document( doc)
  6. {
  7.     if (createTree)
  8.     {
  9.         root = invisibleRootItem();
  10.         Sprites = new QStandardItem( "Sprites");
  11.         Backgrounds = new QStandardItem( "Backgrounds");
  12.         Tilesets = new QStandardItem("Tilesets");
  13.         Sounds = new QStandardItem( "Sounds");
  14.         Objects = new QStandardItem( "Objects");
  15.         CollisionMasks = new QStandardItem( "Collision Masks");
  16.         Markers = new QStandardItem( "Markers");
  17.         Maps = new QStandardItem( "Maps");
  18.  
  19.         Sprites->setEditable( false);
  20.         Sprites->setData( QVariant(Resource_Sprite), Qt::UserRole+1);
  21.         Sprites->setData( QVariant( -1), Qt::UserRole+2);
  22.         Sprites->setData( QVariant( true), Qt::UserRole+3);
  23.         Sprites->setData( QVariant( true), Qt::UserRole+4);
  24.  
  25.         Backgrounds->setEditable( false);
  26.         Backgrounds->setData( QVariant(Resource_Background), Qt::UserRole+1);
  27.         Backgrounds->setData( QVariant( -1), Qt::UserRole+2);
  28.         Backgrounds->setData( QVariant( true), Qt::UserRole+3);
  29.         Backgrounds->setData( QVariant( true), Qt::UserRole+4);
  30.  
  31.         Tilesets->setEditable( false);
  32.         Tilesets->setData( QVariant(Resource_Tileset), Qt::UserRole+1);
  33.         Tilesets->setData( QVariant( -1), Qt::UserRole+2);
  34.         Tilesets->setData( QVariant( true), Qt::UserRole+3);
  35.         Tilesets->setData( QVariant( true), Qt::UserRole+4);
  36.  
  37.         Sounds->setEditable( false);
  38.         Sounds->setData( QVariant(Resource_Sound), Qt::UserRole+1);
  39.         Sounds->setData( QVariant( -1), Qt::UserRole+2);
  40.         Sounds->setData( QVariant( true), Qt::UserRole+3);
  41.         Sounds->setData( QVariant( true), Qt::UserRole+4);
  42.  
  43.         Objects->setEditable( false);
  44.         Objects->setData( QVariant(Resource_Object), Qt::UserRole+1);
  45.         Objects->setData( QVariant( -1), Qt::UserRole+2);
  46.         Objects->setData( QVariant( true), Qt::UserRole+3);
  47.         Objects->setData( QVariant( true), Qt::UserRole+4);
  48.  
  49.         CollisionMasks->setEditable( false);
  50.         CollisionMasks->setData( QVariant(Resource_Collision_Mask), Qt::UserRole+1);
  51.         CollisionMasks->setData( QVariant( -1), Qt::UserRole+2);
  52.         CollisionMasks->setData( QVariant( true), Qt::UserRole+3);
  53.         CollisionMasks->setData( QVariant( true), Qt::UserRole+4);
  54.  
  55.         Markers->setEditable( false);
  56.         Markers->setData( QVariant(Resource_Marker), Qt::UserRole+1);
  57.         Markers->setData( QVariant( -1), Qt::UserRole+2);
  58.         Markers->setData( QVariant( true), Qt::UserRole+3);
  59.         Markers->setData( QVariant( true), Qt::UserRole+4);
  60.  
  61.         Maps->setEditable( false);
  62.         Maps->setData( QVariant(Resource_Map), Qt::UserRole+1);
  63.         Maps->setData( QVariant( -1), Qt::UserRole+2);
  64.         Maps->setData( QVariant( true), Qt::UserRole+3);
  65.         Maps->setData( QVariant( true), Qt::UserRole+4);
  66.  
  67.         root->appendRow( Sprites);
  68.         root->appendRow( Backgrounds);
  69.         root->appendRow( Tilesets);
  70.         root->appendRow( Sounds);
  71.         root->appendRow( Objects);
  72.         root->appendRow( CollisionMasks);
  73.         root->appendRow( Markers);
  74.         root->appendRow( Maps);
  75.     }
  76.  
  77.     sprCount = bgCount = tlsCount = sndCount = objCount = collisionMaskCount = markerCount = mapCount = 0;
  78. }
  79.  
  80. Qt::ItemFlags ResourceModel::flags(const QModelIndex &index) const
  81. {
  82.     Qt::ItemFlags flags = QStandardItemModel::flags( index);
  83.  
  84.     if (!index.isValid())
  85.         return flags;
  86.  
  87.     if (!index.data( Qt::UserRole+4).toBool())
  88.     {
  89.         flags |= Qt::ItemIsDragEnabled;
  90.     }
  91.  
  92.     if (index.data( Qt::UserRole+3).toBool())
  93.     {
  94.         flags |= Qt::ItemIsDropEnabled;
  95.     }
  96.  
  97.     return flags;
  98. }
  99.  
  100. QStringList ResourceModel::mimeTypes()
  101. {
  103.     list << "resource/index";
  104.     return list;
  105. }
  106.  
  107. QMimeData *ResourceModel::mimeData(const QModelIndexList &indexes) const
  108. {
  109.     QMimeData *mimeData = new QMimeData();
  110.     QByteArray encodedData;
  111.  
  112.     QDataStream stream(&encodedData, QIODevice::WriteOnly);
  113.  
  114.     foreach (const QModelIndex &index, indexes)
  115.     {
  116.         if (index.isValid())
  117.         {
  118.          stream << index.data( Qt::UserRole+1);
  119.          itemFromIndex( index)->write( stream);
  120.         }
  121.     }
  122.  
  123.     mimeData->setData("resource/index", encodedData);
  124.     return mimeData;
  125. }
  126.  
  127. bool ResourceModel::dropMimeData(const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent)
  128. {
  129.     QByteArray encodedData = data->data("resource/index");
  130.     QDataStream stream(&encodedData, QIODevice::ReadOnly);
  131.  
  132.     if (action == Qt::IgnoreAction)
  133.         return true;
  134.  
  135.     if (encodedData.isEmpty())
  136.         return false;
  137.  
  138.     int type;
  139.     stream >> type;
  140.  
  141.     if (!data->hasFormat("resource/index") && ( type != parent.data( Qt::UserRole+1).toInt()))
  142.         return false;
  143.  
  144.     if (column > 0)
  145.         return false;
  146.  
  147.     int beginRow;
  148.  
  149.     if (row != -1)
  150.         beginRow = row;
  151.     else if (parent.isValid())
  152.         beginRow = parent.row();
  153.     else
  154.         beginRow = rowCount(QModelIndex());
  155.  
  157.     item->read( stream);
  158.  
  159.     insertRow( beginRow, item);
  160.  
  161.     return true;
  162. }
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