Allow QgsVectorDataProviders to create QgsFeatureRenderers

Implements https://github.com/qgis/QGIS-Enhancement-Proposals/issues/111

Adds support to QgsVectorDataProvider to create vector layer renderers
using provider-specific backend information.

python/core/qgsvectordataprovider.sip.in

@@ -51,6 +51,7 @@ of feature and attribute information from a spatial datasource.
       ReadLayerMetadata,
       WriteLayerMetadata,
       CancelSupport,
+      CreateRenderer,
     };
 
     typedef QFlags<QgsVectorDataProvider::Capability> Capabilities;
@@ -477,6 +478,23 @@ Must be implemented by providers that support saving and loading styles to db re
 %Docstring
 It returns false by default.
 Must be implemented by providers that support delete styles from db returning true
+%End
+
+    virtual QgsFeatureRenderer *createRenderer( const QVariantMap &configuration = QVariantMap() ) const;
+%Docstring
+Creates a new vector layer feature renderer, using provider backend specific information.
+
+The ``configuration`` map can be used to pass provider-specific configuration maps to the provider to
+allow customisation of the returned renderer. Support and format of ``configuration`` varies by provider.
+
+When called with an empty ``configuration`` map the provider's default renderer will be returned.
+
+This method returns a new renderer and the caller takes ownership of the returned object.
+
+Only providers which report the CreateRenderer capability will return a feature renderer. Other
+providers will return None.
+
+.. versionadded:: 3.2
 %End
 
     static QVariant convertValue( QVariant::Type type, const QString &value );

python/core/qgsvectorlayer.sip.in

@@ -936,6 +936,9 @@ data source
 .. versionadded:: 2.10
 %End
 
+    virtual QString loadDefaultStyle( bool &resultFlag /Out/ );
+
+
     QgsVectorLayerFeatureCounter *countSymbolFeatures();
 %Docstring
 Count features for symbols.

src/core/qgsvectordataprovider.cpp

@@ -681,6 +681,11 @@ bool QgsVectorDataProvider::isDeleteStyleFromDatabaseSupported() const
   return false;
 }
 
+QgsFeatureRenderer *QgsVectorDataProvider::createRenderer( const QVariantMap & ) const SIP_FACTORY
+{
+  return nullptr;
+}
+
 void QgsVectorDataProvider::pushError( const QString &msg ) const
 {
   QgsDebugMsg( msg );

src/core/qgsvectordataprovider.h

@@ -40,6 +40,7 @@ typedef QHash<int, QString> QgsAttrPalIndexNameHash;
 class QgsFeatureIterator;
 class QgsTransaction;
 class QgsFeedback;
+class QgsFeatureRenderer;
 
 #include "qgsfeaturerequest.h"
 
@@ -91,6 +92,7 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider, public QgsFeat
       ReadLayerMetadata = 1 << 21, //!< Provider can read layer metadata from data store. Since QGIS 3.0. See QgsDataProvider::layerMetadata()
       WriteLayerMetadata = 1 << 22, //!< Provider can write layer metadata to the data store. Since QGIS 3.0. See QgsDataProvider::writeLayerMetadata()
       CancelSupport = 1 << 23, //!< Supports interruption of pending queries from a separated thread. Since QGIS 3.2
+      CreateRenderer = 1 << 24, //!< Provider can create feature renderers using backend-specific formatting information. Since QGIS 3.2. See QgsVectorDataProvider::createRenderer().
     };
 
     Q_DECLARE_FLAGS( Capabilities, Capability )
@@ -479,6 +481,23 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider, public QgsFeat
      */
     virtual bool isDeleteStyleFromDatabaseSupported() const;
 
+    /**
+     * Creates a new vector layer feature renderer, using provider backend specific information.
+     *
+     * The \a configuration map can be used to pass provider-specific configuration maps to the provider to
+     * allow customisation of the returned renderer. Support and format of \a configuration varies by provider.
+     *
+     * When called with an empty \a configuration map the provider's default renderer will be returned.
+     *
+     * This method returns a new renderer and the caller takes ownership of the returned object.
+     *
+     * Only providers which report the CreateRenderer capability will return a feature renderer. Other
+     * providers will return nullptr.
+     *
+     * \since QGIS 3.2
+     */
+    virtual QgsFeatureRenderer *createRenderer( const QVariantMap &configuration = QVariantMap() ) const;
+
     static QVariant convertValue( QVariant::Type type, const QString &value );
 
     /**

src/core/qgsvectorlayer.cpp

@@ -1465,10 +1465,22 @@ void QgsVectorLayer::setDataSource( const QString &dataSource, const QString &ba
   // reset style if loading default style, style is missing, or geometry type has changed
   if ( !renderer() || !legend() || geomType != geometryType() || loadDefaultStyleFlag )
   {
-    // check if there is a default style / propertysheet defined
-    // for this layer and if so apply it
     bool defaultLoadedFlag = false;
-    if ( loadDefaultStyleFlag )
+
+    if ( loadDefaultStyleFlag && isSpatial() && mDataProvider->capabilities() & QgsVectorDataProvider::CreateRenderer )
+    {
+      // first try to create a renderer directly from the data provider
+      std::unique_ptr< QgsFeatureRenderer > defaultRenderer( mDataProvider->createRenderer() );
+      if ( defaultRenderer )
+      {
+        defaultLoadedFlag = true;
+        setRenderer( defaultRenderer.release() );
+      }
+    }
+
+    // else check if there is a default style / propertysheet defined
+    // for this layer and if so apply it
+    if ( !defaultLoadedFlag && loadDefaultStyleFlag )
     {
       loadDefaultStyle( defaultLoadedFlag );
     }
@@ -1486,6 +1498,23 @@ void QgsVectorLayer::setDataSource( const QString &dataSource, const QString &ba
   emit repaintRequested();
 }
 
+QString QgsVectorLayer::loadDefaultStyle( bool &resultFlag )
+{
+  if ( isSpatial() && mDataProvider->capabilities() & QgsVectorDataProvider::CreateRenderer )
+  {
+    // first try to create a renderer directly from the data provider
+    std::unique_ptr< QgsFeatureRenderer > defaultRenderer( mDataProvider->createRenderer() );
+    if ( defaultRenderer )
+    {
+      resultFlag = true;
+      setRenderer( defaultRenderer.release() );
+      return QString();
+    }
+  }
+
+  return QgsMapLayer::loadDefaultStyle( resultFlag );
+}
+
 
 bool QgsVectorLayer::setDataProvider( QString const &provider )
 {

src/core/qgsvectorlayer.h

@@ -930,6 +930,8 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
      */
     void setDataSource( const QString &dataSource, const QString &baseName, const QString &provider, bool loadDefaultStyleFlag = false );
 
+    QString loadDefaultStyle( bool &resultFlag SIP_OUT ) override;
+
     /**
      * Count features for symbols.
      * The method will return the feature counter task. You will need to