load and save metadata to a QMD file

python/core/qgsmaplayer.sip

@@ -55,6 +55,12 @@ This is the base class for all map layer types (vector, raster).
       PluginLayer
     };
 
+    enum PropertyType
+    {
+      Style,
+      Metadata,
+    };
+
     QgsMapLayer( QgsMapLayer::LayerType type = VectorLayer, const QString &name = QString(), const QString &source = QString() );
 %Docstring
 Constructor for QgsMapLayer
@@ -80,6 +86,15 @@ is still unique.
     QgsMapLayer::LayerType type() const;
 %Docstring
 Returns the type of the layer.
+%End
+
+    static QString extensionPropertyType( PropertyType type );
+%Docstring
+Returns the extension of a Property.
+
+:return: The extension
+
+.. versionadded:: 3.0
 %End
 
     QString id() const;
@@ -545,6 +560,118 @@ Sets layer's spatial reference system
 %Docstring
 A convenience function to capitalize and format a layer ``name``.
 
+.. versionadded:: 3.0
+%End
+
+    virtual QString metadataUri() const;
+%Docstring
+Retrieve the metadata URI for this layer
+(either as a .qmd file on disk or as a
+record in the users style table in their personal qgis.db)
+
+:return: a QString with the metadata file name
+
+.. versionadded:: 3.0
+%End
+
+    void exportNamedMetadata( QDomDocument &doc, QString &errorMsg ) const;
+%Docstring
+Export the current metadata of this layer as named metadata in a QDomDocument
+
+:param doc: the target QDomDocument
+:param errorMsg: this QString will be initialized on error
+
+.. versionadded:: 3.0
+%End
+
+    virtual QString saveDefaultMetadata( bool &resultFlag /Out/ );
+%Docstring
+Save the current metadata of this layer as the default metadata
+(either as a .qmd file on disk or as a
+record in the users style table in their personal qgis.db)
+
+:param resultFlag: a reference to a flag that will be set to false if
+we did not manage to save the default metadata.
+
+:return: a QString with any status messages
+
+.. versionadded:: 3.0
+%End
+
+    QString saveNamedMetadata( const QString &uri, bool &resultFlag );
+%Docstring
+Save the current metadata of this layer as a named metadata
+(either as a .qmd file on disk or as a
+record in the users style table in their personal qgis.db)
+
+:param uri: the file name or other URI for the
+metadata file. First an attempt will be made to see if this
+is a file and save to that, if that fails the qgis.db metadata
+table will be used to create a metadata entry who's
+key matches the URI.
+:param resultFlag: a reference to a flag that will be set to false if
+we did not manage to save the default metadata.
+
+:return: a QString with any status messages
+
+.. versionadded:: 3.0
+%End
+
+    virtual QString loadNamedMetadata( const QString &uri, bool &resultFlag /Out/ );
+%Docstring
+Retrieve a named metadata for this layer if one
+exists (either as a .qmd file on disk or as a
+record in the users style table in their personal qgis.db)
+
+:param uri: - the file name or other URI for the
+metadata file. First an attempt will be made to see if this
+is a file and load that, if that fails the qgis.db metadata
+table will be consulted to see if there is a metadata who's
+key matches the URI.
+:param resultFlag: a reference to a flag that will be set to false if
+we did not manage to load the default metadata.
+
+:return: a QString with any status messages
+
+.. versionadded:: 3.0
+%End
+
+    QString loadDefaultMetadata( bool &resultFlag );
+%Docstring
+Retrieve the default metadata for this layer if one
+exists (either as a .qmd file on disk or as a
+record in the users metadata table in their personal qgis.db)
+
+:param resultFlag: a reference to a flag that will be set to false if
+we did not manage to load the default metadata.
+
+:return: a QString with any status messages
+
+.. versionadded:: 3.0
+%End
+
+    bool loadNamedMetadataFromDatabase( const QString &db, const QString &uri, QString &qmd );
+%Docstring
+Retrieve a named metadata for this layer from a sqlite database.
+
+:param db: path to sqlite database
+:param uri: uri for table
+:param qmd: will be set to QMD xml metadata content from database
+
+:return: true if style was successfully loaded
+
+.. versionadded:: 3.0
+%End
+
+    bool importNamedMetadata( QDomDocument &document, QString &errorMessage );
+%Docstring
+Import the metadata of this layer from a QDomDocument
+
+:param document: source QDomDocument
+:param errorMessage: this QString will be initialized on error
+
+:return: true on success
+
 .. versionadded:: 3.0
 %End
 

python/gui/qgsmetadatawidget.sip

@@ -46,6 +46,11 @@ If the CRS is updated.
     void acceptMetadata();
 %Docstring
 Saves the metadata to the layer.
+%End
+
+    virtual void setMetadata( const QgsLayerMetadata &metadata );
+%Docstring
+Sets the layer's ``metadata`` store.
 %End
 
     static QMap<QString, QString> parseLanguages();

src/app/qgisapp.cpp

@@ -4321,6 +4321,7 @@ bool QgisApp::addVectorLayers( const QStringList &layerQStringList, const QStrin
   {
     bool ok;
     l->loadDefaultStyle( ok );
+    l->loadDefaultMetadata( ok );
   }
   activateDeactivateLayerRelatedActions( activeLayer() );
 
@@ -4747,6 +4748,7 @@ void QgisApp::askUserForOGRSublayers( QgsVectorLayer *layer )
     {
       bool ok;
       l->loadDefaultStyle( ok );
+      l->loadDefaultMetadata( ok );
       if ( addToGroup )
         group->addLayer( l );
     }
@@ -4839,6 +4841,7 @@ void QgisApp::addDatabaseLayers( QStringList const &layerPathList, QString const
   {
     bool ok;
     l->loadDefaultStyle( ok );
+    l->loadDefaultMetadata( ok );
   }
 
   // draw the map
@@ -10249,6 +10252,7 @@ QgsVectorLayer *QgisApp::addVectorLayer( const QString &vectorLayerPath, const Q
       QgsProject::instance()->addMapLayers( myList );
       bool ok;
       layer->loadDefaultStyle( ok );
+      layer->loadDefaultMetadata( ok );
     }
   }
   else

src/app/qgsrasterlayerproperties.cpp

@@ -104,16 +104,26 @@ QgsRasterLayerProperties::QgsRasterLayerProperties( QgsMapLayer *lyr, QgsMapCanv
 
   connect( buttonBox, &QDialogButtonBox::helpRequested, this, &QgsRasterLayerProperties::showHelp );
 
-  QPushButton *b = new QPushButton( tr( "Style" ) );
+  mBtnStyle = new QPushButton( tr( "Style" ) );
-  QMenu *m = new QMenu( this );
+  QMenu *menuStyle = new QMenu( this );
-  m->addAction( tr( "Load Style..." ), this, SLOT( loadStyle_clicked() ) );
+  menuStyle->addAction( tr( "Load Style..." ), this, SLOT( loadStyle_clicked() ) );
-  m->addAction( tr( "Save Style..." ), this, SLOT( saveStyleAs_clicked() ) );
+  menuStyle->addAction( tr( "Save Style..." ), this, SLOT( saveStyleAs_clicked() ) );
-  m->addSeparator();
+  menuStyle->addSeparator();
-  m->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultStyle_clicked() ) );
+  menuStyle->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultStyle_clicked() ) );
-  m->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultStyle_clicked() ) );
+  menuStyle->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultStyle_clicked() ) );
-  b->setMenu( m );
+  mBtnStyle->setMenu( menuStyle );
-  connect( m, &QMenu::aboutToShow, this, &QgsRasterLayerProperties::aboutToShowStyleMenu );
+  connect( menuStyle, &QMenu::aboutToShow, this, &QgsRasterLayerProperties::aboutToShowStyleMenu );
-  buttonBox->addButton( b, QDialogButtonBox::ResetRole );
+  buttonBox->addButton( mBtnStyle, QDialogButtonBox::ResetRole );
+
+  mBtnMetadata = new QPushButton( tr( "Metadata" ), this );
+  QMenu *menuMetadata = new QMenu( this );
+  mActionLoadMetadata = menuMetadata->addAction( tr( "Load Metadata" ), this, SLOT( loadMetadata() ) );
+  mActionSaveMetadataAs = menuMetadata->addAction( tr( "Save Metadata" ), this, SLOT( saveMetadataAs() ) );
+  menuMetadata->addSeparator();
+  menuMetadata->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultMetadata() ) );
+  menuMetadata->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultMetadata() ) );
+  mBtnMetadata->setMenu( menuMetadata );
+  buttonBox->addButton( mBtnMetadata, QDialogButtonBox::ResetRole );
 
   connect( lyr->styleManager(), &QgsMapLayerStyleManager::currentStyleChanged, this, &QgsRasterLayerProperties::syncToLayer );
 
@@ -436,6 +446,7 @@ QgsRasterLayerProperties::QgsRasterLayerProperties( QgsMapLayer *lyr, QgsMapCanv
 
   QString title = QString( tr( "Layer Properties - %1" ) ).arg( lyr->name() );
   restoreOptionsBaseUi( title );
+  optionsStackedWidget_CurrentChanged( mOptionsStackedWidget->currentIndex() );
 } // QgsRasterLayerProperties ctor
 
 
@@ -1488,6 +1499,10 @@ void QgsRasterLayerProperties::optionsStackedWidget_CurrentChanged( int index )
 {
   QgsOptionsDialogBase::optionsStackedWidget_CurrentChanged( index );
 
+  bool isMetadataPanel = ( index == mOptStackedWidget->indexOf( mOptsPage_Metadata ) );
+  mBtnStyle->setVisible( ! isMetadataPanel );
+  mBtnMetadata->setVisible( isMetadataPanel );
+
   if ( !mHistogramWidget )
     return;
 
@@ -1856,6 +1871,102 @@ void QgsRasterLayerProperties::saveStyleAs_clicked()
     QMessageBox::information( this, tr( "Saved Style" ), message );
 }
 
+//
+//
+// Next four methods for saving and restoring QMD metadata
+//
+//
+
+void QgsRasterLayerProperties::loadMetadata()
+{
+  QgsSettings myQSettings;  // where we keep last used filter in persistent state
+  QString myLastUsedDir = myQSettings.value( QStringLiteral( "style/lastStyleDir" ), QDir::homePath() ).toString();
+
+  QString myFileName = QFileDialog::getOpenFileName( this, tr( "Load layer metadata from metadata file" ), myLastUsedDir,
+                       tr( "QGIS Layer Metadata File" ) + " (*.qmd)" );
+  if ( myFileName.isNull() )
+  {
+    return;
+  }
+
+  QString myMessage;
+  bool defaultLoadedFlag = false;
+  myMessage = mRasterLayer->loadNamedMetadata( myFileName, defaultLoadedFlag );
+
+  //reset if the default style was loaded OK only
+  if ( defaultLoadedFlag )
+  {
+    mMetadataWidget->setMetadata( mRasterLayer->metadata() );
+  }
+  else
+  {
+    //let the user know what went wrong
+    QMessageBox::warning( this, tr( "Load Metadata" ), myMessage );
+  }
+
+  QFileInfo myFI( myFileName );
+  QString myPath = myFI.path();
+  myQSettings.setValue( QStringLiteral( "style/lastStyleDir" ), myPath );
+
+  activateWindow(); // set focus back to properties dialog
+}
+
+void QgsRasterLayerProperties::saveMetadataAs()
+{
+  QgsSettings myQSettings;  // where we keep last used filter in persistent state
+  QString myLastUsedDir = myQSettings.value( QStringLiteral( "style/lastStyleDir" ), QDir::homePath() ).toString();
+
+  QString myOutputFileName = QFileDialog::getSaveFileName( this, tr( "Save layer metadata as QMD" ),
+                             myLastUsedDir, tr( "QMD File" ) + " (*.qmd)" );
+  if ( myOutputFileName.isNull() ) //dialog canceled
+  {
+    return;
+  }
+
+  mMetadataWidget->acceptMetadata();
+
+  //ensure the user never omitted the extension from the file name
+  if ( !myOutputFileName.endsWith( QgsMapLayer::extensionPropertyType( QgsMapLayer::Metadata ), Qt::CaseInsensitive ) )
+  {
+    myOutputFileName += QgsMapLayer::extensionPropertyType( QgsMapLayer::Metadata );
+  }
+
+  bool defaultLoadedFlag = false;
+  QString message = mRasterLayer->saveNamedMetadata( myOutputFileName, defaultLoadedFlag );
+  if ( defaultLoadedFlag )
+    myQSettings.setValue( QStringLiteral( "style/lastStyleDir" ), QFileInfo( myOutputFileName ).absolutePath() );
+  else
+    QMessageBox::information( this, tr( "Saved Metadata" ), message );
+}
+
+void QgsRasterLayerProperties::saveDefaultMetadata()
+{
+  mMetadataWidget->acceptMetadata();
+
+  bool defaultSavedFlag = false;
+  QString errorMsg = mRasterLayer->saveDefaultMetadata( defaultSavedFlag );
+  if ( !defaultSavedFlag )
+  {
+    QMessageBox::warning( this, tr( "Default Metadata" ), errorMsg );
+  }
+}
+
+void QgsRasterLayerProperties::loadDefaultMetadata()
+{
+  bool defaultLoadedFlag = false;
+  QString myMessage = mRasterLayer->loadNamedMetadata( mRasterLayer->metadataUri(), defaultLoadedFlag );
+  //reset if the default metadata was loaded OK only
+  if ( defaultLoadedFlag )
+  {
+    mMetadataWidget->setMetadata( mRasterLayer->metadata() );
+  }
+  else
+  {
+    QMessageBox::information( this, tr( "Default Metadata" ), myMessage );
+  }
+}
+
+
 void QgsRasterLayerProperties::toggleBuildPyramidsButton()
 {
   if ( lbxPyramidResolutions->selectedItems().empty() )

src/app/qgsrasterlayerproperties.h

@@ -103,6 +103,16 @@ class APP_EXPORT QgsRasterLayerProperties : public QgsOptionsDialogBase, private
     void loadStyle_clicked();
     //! Save a style when appriate button is pressed.
     void saveStyleAs_clicked();
+
+    //! Load a saved metadata file.
+    void loadMetadata();
+    //! Save a metadata.
+    void saveMetadataAs();
+    //! Save the default metadata.
+    void saveDefaultMetadata();
+    //! Load the default metadata.
+    void loadDefaultMetadata();
+
     //! Help button
     void showHelp();
 
@@ -131,6 +141,11 @@ class APP_EXPORT QgsRasterLayerProperties : public QgsOptionsDialogBase, private
     void refreshLegend( const QString &layerID, bool expandItem );
 
   private:
+    QPushButton *mBtnStyle = nullptr;
+    QPushButton *mBtnMetadata = nullptr;
+    QAction *mActionLoadMetadata = nullptr;
+    QAction *mActionSaveMetadataAs = nullptr;
+
     //! \brief  A constant that signals property not used
     const QString TRSTRING_NOT_SET;
 

src/app/qgsvectorlayerproperties.cpp

@@ -111,16 +111,26 @@ QgsVectorLayerProperties::QgsVectorLayerProperties(
   // and connecting QDialogButtonBox's accepted/rejected signals to dialog's accept/reject slots
   initOptionsBase( false );
 
-  QPushButton *b = new QPushButton( tr( "Style" ) );
+  mBtnStyle = new QPushButton( tr( "Style" ), this );
-  QMenu *m = new QMenu( this );
+  QMenu *menuStyle = new QMenu( this );
-  mActionLoadStyle = m->addAction( tr( "Load Style" ), this, SLOT( loadStyle_clicked() ) );
+  mActionLoadStyle = menuStyle->addAction( tr( "Load Style" ), this, SLOT( loadStyle_clicked() ) );
-  mActionSaveStyleAs = m->addAction( tr( "Save Style" ), this, SLOT( saveStyleAs_clicked() ) );
+  mActionSaveStyleAs = menuStyle->addAction( tr( "Save Style" ), this, SLOT( saveStyleAs_clicked() ) );
-  m->addSeparator();
+  menuStyle->addSeparator();
-  m->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultStyle_clicked() ) );
+  menuStyle->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultStyle_clicked() ) );
-  m->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultStyle_clicked() ) );
+  menuStyle->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultStyle_clicked() ) );
-  b->setMenu( m );
+  mBtnStyle->setMenu( menuStyle );
-  connect( m, &QMenu::aboutToShow, this, &QgsVectorLayerProperties::aboutToShowStyleMenu );
+  connect( menuStyle, &QMenu::aboutToShow, this, &QgsVectorLayerProperties::aboutToShowStyleMenu );
-  buttonBox->addButton( b, QDialogButtonBox::ResetRole );
+  buttonBox->addButton( mBtnStyle, QDialogButtonBox::ResetRole );
+
+  mBtnMetadata = new QPushButton( tr( "Metadata" ), this );
+  QMenu *menuMetadata = new QMenu( this );
+  mActionLoadMetadata = menuMetadata->addAction( tr( "Load Metadata" ), this, SLOT( loadMetadata() ) );
+  mActionSaveMetadataAs = menuMetadata->addAction( tr( "Save Metadata" ), this, SLOT( saveMetadataAs() ) );
+  menuMetadata->addSeparator();
+  menuMetadata->addAction( tr( "Save as Default" ), this, SLOT( saveDefaultMetadata() ) );
+  menuMetadata->addAction( tr( "Restore Default" ), this, SLOT( loadDefaultMetadata() ) );
+  mBtnMetadata->setMenu( menuMetadata );
+  buttonBox->addButton( mBtnMetadata, QDialogButtonBox::ResetRole );
 
   connect( lyr->styleManager(), &QgsMapLayerStyleManager::currentStyleChanged, this, &QgsVectorLayerProperties::syncToLayer );
 
@@ -227,6 +237,15 @@ QgsVectorLayerProperties::QgsVectorLayerProperties(
   delete mOptsPage_3DView;  // removes both the "3d view" list item and its page
 #endif
 
+  // Metadata tab, before the syncToLayer
+  QVBoxLayout *metadataLayout = new QVBoxLayout( metadataFrame );
+  metadataLayout->setMargin( 0 );
+  mMetadataWidget = new QgsMetadataWidget( this, mLayer );
+  mMetadataWidget->layout()->setContentsMargins( -1, 0, -1, 0 );
+  mMetadataWidget->setMapCanvas( QgisApp::instance()->mapCanvas() );
+  metadataLayout->addWidget( mMetadataWidget );
+  metadataFrame->setLayout( metadataLayout );
+
   syncToLayer();
 
   if ( mLayer->dataProvider() )//enable spatial index button group if supported by provider
@@ -282,15 +301,6 @@ QgsVectorLayerProperties::QgsVectorLayerProperties(
   diagLayout->addWidget( diagramPropertiesDialog );
   mDiagramFrame->setLayout( diagLayout );
 
-  // Metadata tab
-  QVBoxLayout *metadataLayout = new QVBoxLayout( metadataFrame );
-  metadataLayout->setMargin( 0 );
-  mMetadataWidget = new QgsMetadataWidget( this, mLayer );
-  mMetadataWidget->layout()->setContentsMargins( -1, 0, -1, 0 );
-  mMetadataWidget->setMapCanvas( QgisApp::instance()->mapCanvas() );
-  metadataLayout->addWidget( mMetadataWidget );
-  metadataFrame->setLayout( metadataLayout );
-
   // Legend tab
   mLegendConfigEmbeddedWidget->setLayer( mLayer );
 
@@ -401,6 +411,8 @@ QgsVectorLayerProperties::QgsVectorLayerProperties(
   connect( mAuxiliaryStorageFieldsAddBtn, &QPushButton::clicked, this, &QgsVectorLayerProperties::onAuxiliaryLayerAddField );
 
   updateAuxiliaryStoragePage();
+
+  optionsStackedWidget_CurrentChanged( mOptStackedWidget->currentIndex() );
 }
 
 void QgsVectorLayerProperties::toggleEditing()
@@ -546,6 +558,8 @@ void QgsVectorLayerProperties::syncToLayer()
   mVector3DWidget->setLayer( mLayer );
 #endif
 
+  mMetadataWidget->setMetadata( mLayer->metadata() );
+
 } // syncToLayer()
 
 void QgsVectorLayerProperties::apply()
@@ -982,6 +996,108 @@ void QgsVectorLayerProperties::saveStyleAs_clicked()
   saveStyleAs( QML );
 }
 
+void QgsVectorLayerProperties::loadMetadata()
+{
+  QgsSettings myQSettings;  // where we keep last used filter in persistent state
+  QString myLastUsedDir = myQSettings.value( QStringLiteral( "style/lastStyleDir" ), QDir::homePath() ).toString();
+
+  QString myFileName = QFileDialog::getOpenFileName( this, tr( "Load layer metadata from metadata file" ), myLastUsedDir,
+                       tr( "QGIS Layer Metadata File" ) + " (*.qmd)" );
+  if ( myFileName.isNull() )
+  {
+    return;
+  }
+
+  QString myMessage;
+  bool defaultLoadedFlag = false;
+  myMessage = mLayer->loadNamedMetadata( myFileName, defaultLoadedFlag );
+
+  //reset if the default style was loaded OK only
+  if ( defaultLoadedFlag )
+  {
+    mMetadataWidget->setMetadata( mLayer->metadata() );
+  }
+  else
+  {
+    //let the user know what went wrong
+    QMessageBox::warning( this, tr( "Load Metadata" ), myMessage );
+  }
+
+  QFileInfo myFI( myFileName );
+  QString myPath = myFI.path();
+  myQSettings.setValue( QStringLiteral( "style/lastStyleDir" ), myPath );
+
+  activateWindow(); // set focus back to properties dialog
+}
+
+void QgsVectorLayerProperties::saveMetadataAs()
+{
+  QgsSettings myQSettings;  // where we keep last used filter in persistent state
+  QString myLastUsedDir = myQSettings.value( QStringLiteral( "style/lastStyleDir" ), QDir::homePath() ).toString();
+
+  QString myOutputFileName = QFileDialog::getSaveFileName( this, tr( "Save layer metadata as QMD" ),
+                             myLastUsedDir, tr( "QMD File" ) + " (*.qmd)" );
+  if ( myOutputFileName.isNull() ) //dialog canceled
+  {
+    return;
+  }
+
+  mMetadataWidget->acceptMetadata();
+
+  //ensure the user never omitted the extension from the file name
+  if ( !myOutputFileName.endsWith( QgsMapLayer::extensionPropertyType( QgsMapLayer::Metadata ), Qt::CaseInsensitive ) )
+  {
+    myOutputFileName += QgsMapLayer::extensionPropertyType( QgsMapLayer::Metadata );
+  }
+
+  QString myMessage;
+  bool defaultLoadedFlag = false;
+  myMessage = mLayer->saveNamedMetadata( myOutputFileName, defaultLoadedFlag );
+
+  //reset if the default style was loaded OK only
+  if ( defaultLoadedFlag )
+  {
+    syncToLayer();
+  }
+  else
+  {
+    //let the user know what went wrong
+    QMessageBox::information( this, tr( "Saved Metadata" ), myMessage );
+  }
+
+  QFileInfo myFI( myOutputFileName );
+  QString myPath = myFI.path();
+  // Persist last used dir
+  myQSettings.setValue( QStringLiteral( "style/lastStyleDir" ), myPath );
+}
+
+void QgsVectorLayerProperties::saveDefaultMetadata()
+{
+  mMetadataWidget->acceptMetadata();
+
+  bool defaultSavedFlag = false;
+  QString errorMsg = mLayer->saveDefaultMetadata( defaultSavedFlag );
+  if ( !defaultSavedFlag )
+  {
+    QMessageBox::warning( this, tr( "Default Metadata" ), errorMsg );
+  }
+}
+
+void QgsVectorLayerProperties::loadDefaultMetadata()
+{
+  bool defaultLoadedFlag = false;
+  QString myMessage = mLayer->loadNamedMetadata( mLayer->metadataUri(), defaultLoadedFlag );
+  //reset if the default metadata was loaded OK only
+  if ( defaultLoadedFlag )
+  {
+    mMetadataWidget->setMetadata( mLayer->metadata() );
+  }
+  else
+  {
+    QMessageBox::information( this, tr( "Default Metadata" ), myMessage );
+  }
+}
+
 void QgsVectorLayerProperties::saveStyleAsMenuTriggered( QAction *action )
 {
   QMenu *menu = qobject_cast<QMenu *>( sender() );
@@ -1045,7 +1161,7 @@ void QgsVectorLayerProperties::saveStyleAs( StyleType styleType )
     else
     {
       format = tr( "QGIS Layer Style File" ) + " (*.qml)";
-      extension = QStringLiteral( ".qml" );
+      extension = QgsMapLayer::extensionPropertyType( QgsMapLayer::Style );
     }
 
     QString myOutputFileName = QFileDialog::getSaveFileName( this, tr( "Save layer properties as style file" ),
@@ -1480,6 +1596,10 @@ void QgsVectorLayerProperties::optionsStackedWidget_CurrentChanged( int index )
 {
   QgsOptionsDialogBase::optionsStackedWidget_CurrentChanged( index );
 
+  bool isMetadataPanel = ( index == mOptStackedWidget->indexOf( mOptsPage_Metadata ) );
+  mBtnStyle->setVisible( ! isMetadataPanel );
+  mBtnMetadata->setVisible( isMetadataPanel );
+
   if ( index != mOptStackedWidget->indexOf( mOptsPage_Information ) || mMetadataFilled )
     return;
 

src/app/qgsvectorlayerproperties.h

@@ -118,6 +118,10 @@ class APP_EXPORT QgsVectorLayerProperties : public QgsOptionsDialogBase, private
     void saveDefaultStyle_clicked();
     void loadStyle_clicked();
     void saveStyleAs_clicked();
+    void loadMetadata();
+    void saveMetadataAs();
+    void saveDefaultMetadata();
+    void loadDefaultMetadata();
     void optionsStackedWidget_CurrentChanged( int index ) override;
     void pbnUpdateExtents_clicked();
 
@@ -167,6 +171,12 @@ class APP_EXPORT QgsVectorLayerProperties : public QgsOptionsDialogBase, private
 
   private:
 
+    enum PropertyType
+    {
+      Style = 0,
+      Metadata,
+    };
+
     void saveStyleAs( StyleType styleType );
 
     //! When provider supports, it will list all the styles relative the layer in a dialog
@@ -182,6 +192,11 @@ class APP_EXPORT QgsVectorLayerProperties : public QgsOptionsDialogBase, private
 
     QString mOriginalSubsetSQL;
 
+    QPushButton *mBtnStyle = nullptr;
+    QPushButton *mBtnMetadata = nullptr;
+    QAction *mActionLoadMetadata = nullptr;
+    QAction *mActionSaveMetadataAs = nullptr;
+
     QMenu *mSaveAsMenu = nullptr;
     QMenu *mLoadStyleMenu = nullptr;
 

src/core/qgsmaplayer.cpp

@@ -54,6 +54,19 @@
 #include "qgssettings.h" // TODO: get rid of it [MD]
 #include "qgsstringutils.h"
 
+QString QgsMapLayer::extensionPropertyType( QgsMapLayer::PropertyType type )
+{
+  switch ( type )
+  {
+    case Metadata:
+      return QStringLiteral( ".qmd" );
+
+    case Style:
+      return QStringLiteral( ".qml" );
+  }
+  return QStringLiteral();
+}
+
 QgsMapLayer::QgsMapLayer( QgsMapLayer::LayerType type,
                           const QString &lyrname,
                           const QString &source )
@@ -1009,7 +1022,7 @@ QString QgsMapLayer::formatLayerName( const QString &name )
   return layerName;
 }
 
-QString QgsMapLayer::styleURI() const
+QString QgsMapLayer::baseURI( PropertyType type ) const
 {
   QString myURI = publicSource();
 
@@ -1051,7 +1064,7 @@ QString QgsMapLayer::styleURI() const
       myURI.chop( 4 );
     myFileInfo.setFile( myURI );
     // get the file name for our .qml style file
-    key = myFileInfo.path() + QDir::separator() + myFileInfo.completeBaseName() + ".qml";
+    key = myFileInfo.path() + QDir::separator() + myFileInfo.completeBaseName() + QgsMapLayer::extensionPropertyType( type );
   }
   else
   {
@@ -1061,12 +1074,42 @@ QString QgsMapLayer::styleURI() const
   return key;
 }
 
+QString QgsMapLayer::metadataUri() const
+{
+  return baseURI( PropertyType::Metadata );
+}
+
+QString QgsMapLayer::saveDefaultMetadata( bool &resultFlag )
+{
+  return saveNamedMetadata( metadataUri(), resultFlag );
+}
+
+QString QgsMapLayer::loadDefaultMetadata( bool &resultFlag )
+{
+  return loadNamedMetadata( metadataUri(), resultFlag );
+}
+
+QString QgsMapLayer::styleURI() const
+{
+  return baseURI( PropertyType::Style );
+}
+
 QString QgsMapLayer::loadDefaultStyle( bool &resultFlag )
 {
   return loadNamedStyle( styleURI(), resultFlag );
 }
 
+bool QgsMapLayer::loadNamedMetadataFromDatabase( const QString &db, const QString &uri, QString &qmd )
+{
+  return loadNamedPropertyFromDatabase( db, uri, qmd, PropertyType::Metadata );
+}
+
 bool QgsMapLayer::loadNamedStyleFromDatabase( const QString &db, const QString &uri, QString &qml )
+{
+  return loadNamedPropertyFromDatabase( db, uri, qml, PropertyType::Style );
+}
+
+bool QgsMapLayer::loadNamedPropertyFromDatabase( const QString &db, const QString &uri, QString &xml, QgsMapLayer::PropertyType type )
 {
   QgsDebugMsgLevel( QString( "db = %1 uri = %2" ).arg( db, uri ), 4 );
 
@@ -1078,7 +1121,7 @@ bool QgsMapLayer::loadNamedStyleFromDatabase( const QString &db, const QString &
 
   int myResult;
 
-  QgsDebugMsgLevel( QString( "Trying to load style for \"%1\" from \"%2\"" ).arg( uri, db ), 4 );
+  QgsDebugMsgLevel( QString( "Trying to load style or metadata for \"%1\" from \"%2\"" ).arg( uri, db ), 4 );
 
   if ( db.isEmpty() || !QFile( db ).exists() )
     return false;
@@ -1089,7 +1132,18 @@ bool QgsMapLayer::loadNamedStyleFromDatabase( const QString &db, const QString &
     return false;
   }
 
-  QString mySql = QStringLiteral( "select qml from tbl_styles where style=?" );
+  QString mySql;
+  switch ( type )
+  {
+    case Metadata:
+      mySql = QStringLiteral( "select qmd from tbl_metadata where metadata=?" );
+      break;
+
+    case Style:
+      mySql = QStringLiteral( "select qml from tbl_styles where style=?" );
+      break;
+  }
+
   statement = database.prepare( mySql, myResult );
   if ( myResult == SQLITE_OK )
   {
@@ -1098,7 +1152,7 @@ bool QgsMapLayer::loadNamedStyleFromDatabase( const QString &db, const QString &
     if ( sqlite3_bind_text( statement.get(), 1, param.data(), param.length(), SQLITE_STATIC ) == SQLITE_OK &&
          sqlite3_step( statement.get() ) == SQLITE_ROW )
     {
-      qml = QString::fromUtf8( reinterpret_cast< const char * >( sqlite3_column_text( statement.get(), 0 ) ) );
+      xml = QString::fromUtf8( reinterpret_cast< const char * >( sqlite3_column_text( statement.get(), 0 ) ) );
       resultFlag = true;
     }
   }
@@ -1107,9 +1161,15 @@ bool QgsMapLayer::loadNamedStyleFromDatabase( const QString &db, const QString &
 
 
 QString QgsMapLayer::loadNamedStyle( const QString &uri, bool &resultFlag )
+{
+  return loadNamedProperty( uri, PropertyType::Style, resultFlag );
+}
+
+QString QgsMapLayer::loadNamedProperty( const QString &uri, QgsMapLayer::PropertyType type, bool &resultFlag )
 {
   QgsDebugMsgLevel( QString( "uri = %1 myURI = %2" ).arg( uri, publicSource() ), 4 );
 
+  QgsDebugMsg( "loadNamedProperty" );
   resultFlag = false;
 
   QDomDocument myDocument( QStringLiteral( "qgis" ) );
@@ -1121,6 +1181,7 @@ QString QgsMapLayer::loadNamedStyle( const QString &uri, bool &resultFlag )
   QFile myFile( uri );
   if ( myFile.open( QFile::ReadOnly ) )
   {
+    QgsDebugMsg( QString( "file found %1" ).arg( uri ) );
     // read file
     resultFlag = myDocument.setContent( &myFile, &myErrorMessage, &line, &column );
     if ( !resultFlag )
@@ -1132,20 +1193,47 @@ QString QgsMapLayer::loadNamedStyle( const QString &uri, bool &resultFlag )
     QFileInfo project( QgsProject::instance()->fileName() );
     QgsDebugMsgLevel( QString( "project fileName: %1" ).arg( project.absoluteFilePath() ), 4 );
 
-    QString qml;
+    QString xml;
-    if ( loadNamedStyleFromDatabase( QDir( QgsApplication::qgisSettingsDirPath() ).absoluteFilePath( QStringLiteral( "qgis.qmldb" ) ), uri, qml ) ||
+    switch ( type )
-         ( project.exists() && loadNamedStyleFromDatabase( project.absoluteDir().absoluteFilePath( project.baseName() + ".qmldb" ), uri, qml ) ) ||
-         loadNamedStyleFromDatabase( QDir( QgsApplication::pkgDataPath() ).absoluteFilePath( QStringLiteral( "resources/qgis.qmldb" ) ), uri, qml ) )
     {
-      resultFlag = myDocument.setContent( qml, &myErrorMessage, &line, &column );
+      case QgsMapLayer::Style:
-      if ( !resultFlag )
       {
-        myErrorMessage = tr( "%1 at line %2 column %3" ).arg( myErrorMessage ).arg( line ).arg( column );
+        if ( loadNamedStyleFromDatabase( QDir( QgsApplication::qgisSettingsDirPath() ).absoluteFilePath( QStringLiteral( "qgis.qmldb" ) ), uri, xml ) ||
+             ( project.exists() && loadNamedStyleFromDatabase( project.absoluteDir().absoluteFilePath( project.baseName() + ".qmldb" ), uri, xml ) ) ||
+             loadNamedStyleFromDatabase( QDir( QgsApplication::pkgDataPath() ).absoluteFilePath( QStringLiteral( "resources/qgis.qmldb" ) ), uri, xml ) )
+        {
+          resultFlag = myDocument.setContent( xml, &myErrorMessage, &line, &column );
+          if ( !resultFlag )
+          {
+            myErrorMessage = tr( "%1 at line %2 column %3" ).arg( myErrorMessage ).arg( line ).arg( column );
+          }
+        }
+        else
+        {
+          myErrorMessage = tr( "Style not found in database" );
+          resultFlag = false;
+        }
+        break;
+      }
+      case QgsMapLayer::Metadata:
+      {
+        if ( loadNamedMetadataFromDatabase( QDir( QgsApplication::qgisSettingsDirPath() ).absoluteFilePath( QStringLiteral( "qgis.qmldb" ) ), uri, xml ) ||
+             ( project.exists() && loadNamedMetadataFromDatabase( project.absoluteDir().absoluteFilePath( project.baseName() + ".qmldb" ), uri, xml ) ) ||
+             loadNamedMetadataFromDatabase( QDir( QgsApplication::pkgDataPath() ).absoluteFilePath( QStringLiteral( "resources/qgis.qmldb" ) ), uri, xml ) )
+        {
+          resultFlag = myDocument.setContent( xml, &myErrorMessage, &line, &column );
+          if ( !resultFlag )
+          {
+            myErrorMessage = tr( "%1 at line %2 column %3" ).arg( myErrorMessage ).arg( line ).arg( column );
+          }
+        }
+        else
+        {
+          myErrorMessage = tr( "Metadata not found in database" );
+          resultFlag = false;
+        }
+        break;
       }
-    }
-    else
-    {
-      myErrorMessage = tr( "Style not found in database" );
     }
   }
 
@@ -1154,13 +1242,33 @@ QString QgsMapLayer::loadNamedStyle( const QString &uri, bool &resultFlag )
     return myErrorMessage;
   }
 
-  resultFlag = importNamedStyle( myDocument, myErrorMessage );
+  switch ( type )
-  if ( !resultFlag )
+  {
-    myErrorMessage = tr( "Loading style file %1 failed because:\n%2" ).arg( uri, myErrorMessage );
+    case QgsMapLayer::Style:
-
+      resultFlag = importNamedStyle( myDocument, myErrorMessage );
+      if ( !resultFlag )
+        myErrorMessage = tr( "Loading style file %1 failed because:\n%2" ).arg( uri, myErrorMessage );
+      break;
+    case QgsMapLayer::Metadata:
+      resultFlag = importNamedMetadata( myDocument, myErrorMessage );
+      if ( !resultFlag )
+        myErrorMessage = tr( "Loading metadata file %1 failed because:\n%2" ).arg( uri, myErrorMessage );
+      break;
+  }
   return myErrorMessage;
 }
 
+bool QgsMapLayer::importNamedMetadata( QDomDocument &document, QString &errorMessage )
+{
+  QDomElement myRoot = document.firstChildElement( QStringLiteral( "qgis" ) );
+  if ( myRoot.isNull() )
+  {
+    errorMessage = tr( "Root <qgis> element could not be found" );
+    return false;
+  }
+
+  return mMetadata.readMetadataXml( myRoot );
+}
 
 bool QgsMapLayer::importNamedStyle( QDomDocument &myDocument, QString &myErrorMessage )
 {
@@ -1210,6 +1318,25 @@ bool QgsMapLayer::importNamedStyle( QDomDocument &myDocument, QString &myErrorMe
   return readSymbology( myRoot, myErrorMessage, QgsReadWriteContext() ); // TODO: support relative paths in QML?
 }
 
+void QgsMapLayer::exportNamedMetadata( QDomDocument &doc, QString &errorMsg ) const
+{
+  QDomImplementation DomImplementation;
+  QDomDocumentType documentType = DomImplementation.createDocumentType( QStringLiteral( "qgis" ), QStringLiteral( "http://mrcc.com/qgis.dtd" ), QStringLiteral( "SYSTEM" ) );
+  QDomDocument myDocument( documentType );
+
+  QDomElement myRootNode = myDocument.createElement( QStringLiteral( "qgis" ) );
+  myRootNode.setAttribute( QStringLiteral( "version" ), Qgis::QGIS_VERSION );
+  myDocument.appendChild( myRootNode );
+
+  if ( !mMetadata.writeMetadataXml( myRootNode, myDocument ) )
+  {
+    errorMsg = QObject::tr( "Could not save metadata" );
+    return;
+  }
+
+  doc = myDocument;
+}
+
 void QgsMapLayer::exportNamedStyle( QDomDocument &doc, QString &errorMsg ) const
 {
   QDomImplementation DomImplementation;
@@ -1256,13 +1383,19 @@ QString QgsMapLayer::saveDefaultStyle( bool &resultFlag )
   return saveNamedStyle( styleURI(), resultFlag );
 }
 
-QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
+QString QgsMapLayer::saveNamedMetadata( const QString &uri, bool &resultFlag )
 {
-  QString myErrorMessage;
+  return saveNamedProperty( uri, QgsMapLayer::Metadata, resultFlag );
-  QDomDocument myDocument;
+}
-  exportNamedStyle( myDocument, myErrorMessage );
 
-  // check if the uri is a file or ends with .qml,
+QString QgsMapLayer::loadNamedMetadata( const QString &uri, bool &resultFlag )
+{
+  return loadNamedProperty( uri, QgsMapLayer::Metadata, resultFlag );
+}
+
+QString QgsMapLayer::saveNamedProperty( const QString &uri, QgsMapLayer::PropertyType type, bool &resultFlag )
+{
+  // check if the uri is a file or ends with .qml/.qmd,
   // which indicates that it should become one
   // everything else goes to the database
   QString filename;
@@ -1290,8 +1423,21 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
     filename = uri;
   }
 
+  QString myErrorMessage;
+  QDomDocument myDocument;
+  switch ( type )
+  {
+    case Metadata:
+      exportNamedMetadata( myDocument, myErrorMessage );
+      break;
+
+    case Style:
+      exportNamedStyle( myDocument, myErrorMessage );
+      break;
+  }
+
   QFileInfo myFileInfo( filename );
-  if ( myFileInfo.exists() || filename.endsWith( QLatin1String( ".qml" ), Qt::CaseInsensitive ) )
+  if ( myFileInfo.exists() || filename.endsWith( QgsMapLayer::extensionPropertyType( type ), Qt::CaseInsensitive ) )
   {
     QFileInfo myDirInfo( myFileInfo.path() );  //excludes file name
     if ( !myDirInfo.isWritable() )
@@ -1299,8 +1445,8 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
       return tr( "The directory containing your dataset needs to be writable!" );
     }
 
-    // now construct the file name for our .qml style file
+    // now construct the file name for our .qml or .qmd file
-    QString myFileName = myFileInfo.path() + QDir::separator() + myFileInfo.completeBaseName() + ".qml";
+    QString myFileName = myFileInfo.path() + QDir::separator() + myFileInfo.completeBaseName() + QgsMapLayer::extensionPropertyType( type );
 
     QFile myFile( myFileName );
     if ( myFile.open( QFile::WriteOnly | QFile::Truncate ) )
@@ -1310,12 +1456,27 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
       myDocument.save( myFileStream, 2 );
       myFile.close();
       resultFlag = true;
-      return tr( "Created default style file as %1" ).arg( myFileName );
+      switch ( type )
+      {
+        case Metadata:
+          return tr( "Created default metadata file as %1" ).arg( myFileName );
+
+        case Style:
+          return tr( "Created default style file as %1" ).arg( myFileName );
+      }
+
     }
     else
     {
       resultFlag = false;
-      return tr( "ERROR: Failed to created default style file as %1. Check file permissions and retry." ).arg( myFileName );
+      switch ( type )
+      {
+        case Metadata:
+          return tr( "ERROR: Failed to created default metadata file as %1. Check file permissions and retry." ).arg( myFileName );
+
+        case Style:
+          return tr( "ERROR: Failed to created default style file as %1. Check file permissions and retry." ).arg( myFileName );
+      }
     }
   }
   else
@@ -1335,18 +1496,45 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
     QByteArray param0 = uri.toUtf8();
     QByteArray param1 = qml.toUtf8();
 
-    QString mySql = QStringLiteral( "create table if not exists tbl_styles(style varchar primary key,qml varchar)" );
+    QString mySql;
+    switch ( type )
+    {
+      case Metadata:
+        mySql = QStringLiteral( "create table if not exists tbl_metadata(metadata varchar primary key,qmd varchar)" );
+        break;
+
+      case Style:
+        mySql = QStringLiteral( "create table if not exists tbl_styles(style varchar primary key,qml varchar)" );
+        break;
+    }
+
     statement = database.prepare( mySql, myResult );
     if ( myResult == SQLITE_OK )
     {
       if ( sqlite3_step( statement.get() ) != SQLITE_DONE )
       {
         resultFlag = false;
-        return tr( "The style table could not be created." );
+        switch ( type )
+        {
+          case Metadata:
+            return tr( "The metadata table could not be created." );
+
+          case Style:
+            return tr( "The style table could not be created." );
+        }
       }
     }
 
-    mySql = QStringLiteral( "insert into tbl_styles(style,qml) values (?,?)" );
+    switch ( type )
+    {
+      case Metadata:
+        mySql = QStringLiteral( "insert into tbl_metadata(metadata,qmd) values (?,?)" );
+        break;
+
+      case Style:
+        mySql = QStringLiteral( "insert into tbl_styles(style,qml) values (?,?)" );
+        break;
+    }
     statement = database.prepare( mySql, myResult );
     if ( myResult == SQLITE_OK )
     {
@@ -1355,15 +1543,33 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
            sqlite3_step( statement.get() ) == SQLITE_DONE )
       {
         resultFlag = true;
-        myErrorMessage = tr( "The style %1 was saved to database" ).arg( uri );
+        switch ( type )
+        {
+          case Metadata:
+            myErrorMessage = tr( "The metadata %1 was saved to database" ).arg( uri );
+            break;
+
+          case Style:
+            myErrorMessage = tr( "The style %1 was saved to database" ).arg( uri );
+            break;
+        }
       }
     }
 
     if ( !resultFlag )
     {
-      QString mySql = QStringLiteral( "update tbl_styles set qml=? where style=?" );
+      QString mySql;
-      statement = database.prepare( mySql, myResult );
+      switch ( type )
+      {
+        case Metadata:
+          mySql = QStringLiteral( "update tbl_metadata set qmd=? where metadata=?" );
+          break;
 
+        case Style:
+          mySql = QStringLiteral( "update tbl_styles set qml=? where style=?" );
+          break;
+      }
+      statement = database.prepare( mySql, myResult );
       if ( myResult == SQLITE_OK )
       {
         if ( sqlite3_bind_text( statement.get(), 2, param0.data(), param0.length(), SQLITE_STATIC ) == SQLITE_OK &&
@@ -1371,18 +1577,45 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
              sqlite3_step( statement.get() ) == SQLITE_DONE )
         {
           resultFlag = true;
-          myErrorMessage = tr( "The style %1 was updated in the database." ).arg( uri );
+          switch ( type )
+          {
+            case Metadata:
+              myErrorMessage = tr( "The metadata %1 was updated in the database." ).arg( uri );
+              break;
+
+            case Style:
+              myErrorMessage = tr( "The style %1 was updated in the database." ).arg( uri );
+              break;
+          }
         }
         else
         {
           resultFlag = false;
-          myErrorMessage = tr( "The style %1 could not be updated in the database." ).arg( uri );
+          switch ( type )
+          {
+            case Metadata:
+              myErrorMessage = tr( "The metadata %1 could not be updated in the database." ).arg( uri );
+              break;
+
+            case Style:
+              myErrorMessage = tr( "The style %1 could not be updated in the database." ).arg( uri );
+              break;
+          }
         }
       }
       else
       {
         resultFlag = false;
-        myErrorMessage = tr( "The style %1 could not be inserted into database." ).arg( uri );
+        switch ( type )
+        {
+          case Metadata:
+            myErrorMessage = tr( "The metadata %1 could not be inserted into database." ).arg( uri );
+            break;
+
+          case Style:
+            myErrorMessage = tr( "The style %1 could not be inserted into database." ).arg( uri );
+            break;
+        }
       }
     }
   }
@@ -1390,6 +1623,11 @@ QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
   return myErrorMessage;
 }
 
+QString QgsMapLayer::saveNamedStyle( const QString &uri, bool &resultFlag )
+{
+  return saveNamedProperty( uri, QgsMapLayer::Style, resultFlag );
+}
+
 void QgsMapLayer::exportSldStyle( QDomDocument &doc, QString &errorMsg ) const
 {
   QDomDocument myDocument = QDomDocument();

src/core/qgsmaplayer.h

@@ -98,6 +98,16 @@ class CORE_EXPORT QgsMapLayer : public QObject
       PluginLayer
     };
 
+    /**
+     * Maplayer has a style and a metadata property
+     * \since QGIS 3.0
+     */
+    enum PropertyType
+    {
+      Style = 0,
+      Metadata,
+    };
+
     /**
      * Constructor for QgsMapLayer
      * \param type layer type
@@ -126,6 +136,13 @@ class CORE_EXPORT QgsMapLayer : public QObject
      */
     QgsMapLayer::LayerType type() const;
 
+    /**
+     * Returns the extension of a Property.
+     * \returns The extension
+     * \since QGIS 3.0
+     */
+    static QString extensionPropertyType( PropertyType type );
+
     //! Returns the layer's unique ID, which is used to access this layer from QgsProject.
     QString id() const;
 
@@ -525,6 +542,96 @@ class CORE_EXPORT QgsMapLayer : public QObject
      */
     static QString formatLayerName( const QString &name );
 
+    /**
+     * Retrieve the metadata URI for this layer
+     * (either as a .qmd file on disk or as a
+     * record in the users style table in their personal qgis.db)
+     * \returns a QString with the metadata file name
+     * \since QGIS 3.0
+     */
+    virtual QString metadataUri() const;
+
+    /**
+     * Export the current metadata of this layer as named metadata in a QDomDocument
+     * \param doc the target QDomDocument
+     * \param errorMsg this QString will be initialized on error
+     * \since QGIS 3.0
+     */
+    void exportNamedMetadata( QDomDocument &doc, QString &errorMsg ) const;
+
+    /**
+     * Save the current metadata of this layer as the default metadata
+     * (either as a .qmd file on disk or as a
+     * record in the users style table in their personal qgis.db)
+     * \param resultFlag a reference to a flag that will be set to false if
+     * we did not manage to save the default metadata.
+     * \returns a QString with any status messages
+     * \since QGIS 3.0
+     */
+    virtual QString saveDefaultMetadata( bool &resultFlag SIP_OUT );
+
+    /**
+     * Save the current metadata of this layer as a named metadata
+     * (either as a .qmd file on disk or as a
+     * record in the users style table in their personal qgis.db)
+     * \param uri the file name or other URI for the
+     * metadata file. First an attempt will be made to see if this
+     * is a file and save to that, if that fails the qgis.db metadata
+     * table will be used to create a metadata entry who's
+     * key matches the URI.
+     * \param resultFlag a reference to a flag that will be set to false if
+     * we did not manage to save the default metadata.
+     * \returns a QString with any status messages
+     * \since QGIS 3.0
+     */
+    QString saveNamedMetadata( const QString &uri, bool &resultFlag );
+
+    /**
+     * Retrieve a named metadata for this layer if one
+     * exists (either as a .qmd file on disk or as a
+     * record in the users style table in their personal qgis.db)
+     * \param uri - the file name or other URI for the
+     * metadata file. First an attempt will be made to see if this
+     * is a file and load that, if that fails the qgis.db metadata
+     * table will be consulted to see if there is a metadata who's
+     * key matches the URI.
+     * \param resultFlag a reference to a flag that will be set to false if
+     * we did not manage to load the default metadata.
+     * \returns a QString with any status messages
+     * \since QGIS 3.0
+     */
+    virtual QString loadNamedMetadata( const QString &uri, bool &resultFlag SIP_OUT );
+
+    /**
+     * Retrieve the default metadata for this layer if one
+     * exists (either as a .qmd file on disk or as a
+     * record in the users metadata table in their personal qgis.db)
+     * \param resultFlag a reference to a flag that will be set to false if
+     * we did not manage to load the default metadata.
+     * \returns a QString with any status messages
+     * \since QGIS 3.0
+     */
+    QString loadDefaultMetadata( bool &resultFlag );
+
+    /**
+     * Retrieve a named metadata for this layer from a sqlite database.
+     * \param db path to sqlite database
+     * \param uri uri for table
+     * \param qmd will be set to QMD xml metadata content from database
+     * \returns true if style was successfully loaded
+     * \since QGIS 3.0
+     */
+    bool loadNamedMetadataFromDatabase( const QString &db, const QString &uri, QString &qmd );
+
+    /**
+     * Import the metadata of this layer from a QDomDocument
+     * \param document source QDomDocument
+     * \param errorMessage this QString will be initialized on error
+     * \returns true on success
+     * \since QGIS 3.0
+     */
+    bool importNamedMetadata( QDomDocument &document, QString &errorMessage );
+
     /**
      * Retrieve the style URI for this layer
      * (either as a .qml file on disk or as a
@@ -1169,6 +1276,11 @@ class CORE_EXPORT QgsMapLayer : public QObject
 
   private:
 
+    virtual QString baseURI( PropertyType type ) const;
+    QString saveNamedProperty( const QString &uri, QgsMapLayer::PropertyType type, bool &resultFlag );
+    QString loadNamedProperty( const QString &uri, QgsMapLayer::PropertyType type, bool &resultFlag );
+    bool loadNamedPropertyFromDatabase( const QString &db, const QString &uri, QString &xml, QgsMapLayer::PropertyType type );
+
     /**
      * This method returns true by default but can be overwritten to specify
      * that a certain layer is writable.

src/gui/qgsmetadatawidget.cpp

@@ -820,6 +820,12 @@ void QgsMetadataWidget::acceptMetadata()
   mLayer->setMetadata( mMetadata );
 }
 
+void QgsMetadataWidget::setMetadata( const QgsLayerMetadata &metadata )
+{
+  mMetadata = metadata;
+  setPropertiesFromLayer();
+}
+
 void QgsMetadataWidget::syncFromCategoriesTabToKeywordsTab() const
 {
   if ( mCategoriesModel->rowCount() > 0 )

src/gui/qgsmetadatawidget.h

@@ -66,6 +66,11 @@ class GUI_EXPORT QgsMetadataWidget : public QWidget, private Ui::QgsMetadataWidg
      */
     void acceptMetadata();
 
+    /**
+     * Sets the layer's \a metadata store.
+     */
+    virtual void setMetadata( const QgsLayerMetadata &metadata );
+
     /**
      * Returns a list of languages available by default in the wizard.
      */

tests/src/python/test_qgsmaplayer.py

@@ -13,6 +13,7 @@ __copyright__ = 'Copyright 2017, The QGIS Project'
 __revision__ = '$Format:%H$'
 
 import qgis  # NOQA
+import tempfile
 
 from qgis.core import (QgsReadWriteContext,
                        QgsVectorLayer,
@@ -88,6 +89,22 @@ class TestQgsMapLayer(unittest.TestCase):
         self.assertTrue(layer2.hasAutoRefreshEnabled())
         self.assertEqual(layer2.autoRefreshInterval(), 56)
 
+    def testReadWriteMetadata(self):
+        layer = QgsVectorLayer("Point?field=fldtxt:string", "layer", "memory")
+        m = layer.metadata()
+        # Only abstract, more tests are done in test_qgslayermetadata.py
+        m.setAbstract('My abstract')
+        layer.setMetadata(m)
+        self.assertTrue(layer.metadata().abstract(), 'My abstract')
+        destination = tempfile.NamedTemporaryFile(suffix='.qmd').name
+        message, status = layer.saveNamedMetadata(destination)
+        self.assertTrue(status, message)
+
+        layer2 = QgsVectorLayer("Point?field=fldtxt:string", "layer", "memory")
+        message, status = layer2.loadNamedMetadata(destination)
+        self.assertTrue(status)
+        self.assertTrue(layer2.metadata().abstract(), 'My abstract')
+
 
 if __name__ == '__main__':
     unittest.main()