Allow to declare layer dependencies on a vector layer

python/core/qgsvectordataprovider.sip

@@ -322,6 +322,11 @@ class QgsVectorDataProvider : QgsDataProvider
 
     virtual void forceReload();
 
+    /**
+     * Get the list of layer ids on which this layer depends. This in particular determines the order of layer loading.
+     */
+    virtual QSet<QString> layerDependencies() const;
+
   protected:
     void clearMinMaxCache();
     void fillMinMaxCache();

python/core/qgsvectorlayer.sip

@@ -441,6 +441,11 @@ class QgsVectorLayer : QgsMapLayer
 
     const QList<QgsVectorJoinInfo> vectorJoins() const;
 
+    /**
+     * Get the list of layer ids on which this layer depends. This in particular determines the order of layer loading.
+     */
+    virtual QSet<QString> layerDependencies() const;
+
     /**
      * Add a new field which is calculated by the expression specified
      *

src/core/qgsproject.cpp

@@ -672,12 +672,82 @@ QPair< bool, QList<QDomNode> > QgsProject::_getMapLayers( QDomDocument const &do
 
   emit layerLoaded( 0, nl.count() );
 
+  // Determine a loading order of layers based on a graph of dependencies
+  QMap< QString, QVector< QString > > dependencies;
+  QVector<QString> sortedLayers;
+  QMap<QString, int> layerIdIdx;
+  QList<QString> layersToSort;
+
+  for ( int i = 0; i < nl.count(); i++ )
+  {
+    QVector<QString> deps;
+    QDomNode node = nl.item( i );
+    QDomElement element = node.toElement();
+
+    QString id = node.namedItem( "id" ).toElement().text();
+
+    // dependencies for this layer
+    QDomElement layerDependenciesElem = node.firstChildElement( "layerDependencies" );
+    if ( !layerDependenciesElem.isNull() )
+    {
+      QDomNodeList dependencyList = layerDependenciesElem.elementsByTagName( "layer" );
+      for ( int j = 0; j < dependencyList.size(); ++j )
+      {
+        QDomElement depElem = dependencyList.at( j ).toElement();
+        deps << depElem.attribute( "id" );
+      }
+    }
+    dependencies[id] = deps;
+
+    if ( deps.empty() )
+      sortedLayers << id;
+    else
+      layersToSort << id;
+    layerIdIdx[id] = i;
+  }
+
+  bool hasCycle = false;
+  while ( !layersToSort.empty() && !hasCycle )
+  {
+    QList<QString>::iterator it = layersToSort.begin();
+    while ( it != layersToSort.end() )
+    {
+      hasCycle = true;
+      bool resolved = true;
+      foreach ( QString dep, dependencies[*it] )
+      {
+        if ( !sortedLayers.contains( dep ) )
+        {
+          resolved = false;
+          break;
+        }
+      }
+      if ( resolved ) // dependencies for this layer are resolved
+      {
+        sortedLayers << *it;
+        it = layersToSort.erase( it ); // erase and go to the next
+        hasCycle = false;
+      }
+      else
+      {
+        it++;
+      }
+    }
+  }
+
+  if ( hasCycle )
+  {
+    // should not happen, since layers with cyclic dependencies may only be created by
+    // manually modifying the project file
+    return qMakePair( false, QList<QDomNode>() );
+  }
+
   // Collect vector layers with joins.
   // They need to refresh join caches and symbology infos after all layers are loaded
   QList< QPair< QgsVectorLayer*, QDomElement > > vLayerList;
-
-  for ( int i = 0; i < nl.count(); i++ )
+  foreach ( QString id, sortedLayers )
   {
+    int i = layerIdIdx[id];
     QDomNode node = nl.item( i );
     QDomElement element = node.toElement();
 

src/core/qgsvectordataprovider.cpp

@@ -548,4 +548,9 @@ void QgsVectorDataProvider::pushError( const QString& msg )
   mErrors << msg;
 }
 
+QSet<QString> QgsVectorDataProvider::layerDependencies() const
+{
+  return QSet<QString>();
+}
+
 QStringList QgsVectorDataProvider::smEncodings;

src/core/qgsvectordataprovider.h

@@ -383,6 +383,11 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider
       emit dataChanged();
     }
 
+    /**
+     * Get the list of layer ids on which this layer depends. This in particular determines the order of layer loading.
+     */
+    virtual QSet<QString> layerDependencies() const;
+
   protected:
     void clearMinMaxCache();
     void fillMinMaxCache();

src/core/qgsvectorlayer.cpp

@@ -1620,6 +1620,16 @@ bool QgsVectorLayer::writeXml( QDomNode & layer_node,
   //save joins
   mJoinBuffer->writeXml( layer_node, document );
 
+  // dependencies
+  QDomElement dependenciesElement = document.createElement( "layerDependencies" );
+  foreach ( QString layerId, layerDependencies() )
+  {
+    QDomElement depElem = document.createElement( "layer" );
+    depElem.setAttribute( "id", layerId );
+    dependenciesElement.appendChild( depElem );
+  }
+  layer_node.appendChild( dependenciesElement );
+
   // save expression fields
   mExpressionFieldBuffer->writeXml( layer_node, document );
 
@@ -3911,3 +3921,12 @@ bool QgsAttributeEditorRelation::init( QgsRelationManager* relationManager )
   mRelation = relationManager->relation( mRelationId );
   return mRelation.isValid();
 }
+
+QSet<QString> QgsVectorLayer::layerDependencies() const
+{
+  if ( mDataProvider )
+  {
+    return mDataProvider->layerDependencies();
+  }
+  return QSet<QString>();
+}

src/core/qgsvectorlayer.h

@@ -548,6 +548,11 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer
 
     const QList<QgsVectorJoinInfo> vectorJoins() const;
 
+    /**
+     * Get the list of layer ids on which this layer depends. This in particular determines the order of layer loading.
+     */
+    virtual QSet<QString> layerDependencies() const;
+
     /**
      * Add a new field which is calculated by the expression specified
      *