[Feature] Data dependencies between layers

This allows to declare data dependencies between layers. A data
dependency occurs when a data modification in a layer, not by direct
user manipulation may modify data of other layers.
This is the case for instance when geometry of a layer is updated by a
database trigger after modification of another layer's geometry.

images/images.qrc

@@ -665,6 +665,7 @@
         <file>themes/default/mActionAddAfsLayer.svg</file>
         <file>themes/default/mIconFormSelect.svg</file>
         <file>themes/default/mActionMultiEdit.svg</file>
+        <file>themes/default/dependencies.svg</file>
     </qresource>
     <qresource prefix="/images/tips">
         <file alias="symbol_levels.png">qgis_tips/symbol_levels.png</file>

python/core/qgsmaplayer.sip

@@ -676,6 +676,23 @@ class QgsMapLayer : QObject
      */
     void emitStyleChanged();
 
+    /**
+     * Sets the list of layers that may modify data/geometries of this layer when modified.
+     * @see dataDependencies
+     *
+     * @param layersIds IDs of the layers that this layer depends on
+     * @returns false if a dependency cycle has been detected (the change dependency set is not changed in that case)
+     */
+    virtual bool setDataDependencies( const QSet<QString>& layersIds );
+
+    /**
+     * Gets the list of layers that may modify data/geometries of this layer when modified.
+     * @see setDataDependencies
+     *
+     * @returns IDs of the layers that this layer depends on
+     */
+    virtual QSet<QString> dataDependencies() const;
+
   signals:
 
     /** Emit a signal with status (e.g. to be caught by QgisApp and display a msg on status bar) */
@@ -766,4 +783,7 @@ class QgsMapLayer : QObject
     void appendError( const QgsErrorMessage &error );
     /** Set error message */
     void setError( const QgsError &error );
+
+    //! Checks if new change dependency candidates introduce a cycle
+    bool hasDataDependencyCycle( const QSet<QString>& layersIds ) const;
 };

python/core/qgsvectorlayer.sip

@@ -422,10 +422,30 @@ 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.
+     * Gets the list of layer ids on which this layer depends, as returned by the provider.
+     * This in particular determines the order of layer loading.
      */
     virtual QSet<QString> layerDependencies() const;
 
+    /**
+     * Sets the list of layers that may modify data/geometries of this layer when modified.
+     * This is meant mainly to declare database triggers between layers.
+     * When one of these layers is modified (feature added/deleted or geometry changed),
+     * dataChanged() will be emitted, allowing users of this layer to refresh / update it.
+     *
+     * @param layersIds IDs of the layers that this layer depends on
+     * @returns false if a dependency cycle has been detected (the change dependency set is not changed in that case)
+     */
+    bool setDataDependencies( const QSet<QString>& layersIds );
+
+    /**
+     * Gets the list of layers that may modify data/geometries of this layer when modified.
+     * @see setDataDependencies
+     *
+     * @returns IDs of the layers that this layer depends on
+     */
+    QSet<QString> dataDependencies() const;
+
     /**
      * Add a new field which is calculated by the expression specified
      *

src/app/qgsvectorlayerproperties.cpp

@@ -52,6 +52,7 @@
 #include "qgsdatasourceuri.h"
 #include "qgsrenderer.h"
 #include "qgsexpressioncontext.h"
+#include "layertree/qgslayertreelayer.h"
 
 #include <QMessageBox>
 #include <QDir>
@@ -291,6 +292,23 @@ QgsVectorLayerProperties::QgsVectorLayerProperties(
 
   QString title = QString( tr( "Layer Properties - %1" ) ).arg( mLayer->name() );
   restoreOptionsBaseUi( title );
+
+  mLayersDependenciesTreeGroup.reset( QgsProject::instance()->layerTreeRoot()->clone() );
+  QgsLayerTreeLayer* layer = mLayersDependenciesTreeGroup->findLayer( mLayer->id() );
+  layer->parent()->takeChild( layer );
+  mLayersDependenciesTreeModel.reset( new QgsLayerTreeModel( mLayersDependenciesTreeGroup.data() ) );
+  // use visibility as selection
+  mLayersDependenciesTreeModel->setFlag( QgsLayerTreeModel::AllowNodeChangeVisibility );
+
+  mLayersDependenciesTreeGroup->setVisible( Qt::Unchecked );
+
+  QSet<QString> dependencySources = mLayer->dataDependencies();
+  Q_FOREACH ( QgsLayerTreeLayer* layer, mLayersDependenciesTreeGroup->findLayers() )
+  {
+    layer->setVisible( dependencySources.contains( layer->layerId() ) ? Qt::Checked : Qt::Unchecked );
+  }
+
+  mLayersDependenciesTreeView->setModel( mLayersDependenciesTreeModel.data() );
 } // QgsVectorLayerProperties ctor
 
 
@@ -558,6 +576,18 @@ void QgsVectorLayerProperties::apply()
   QgsExpressionContextUtils::setLayerVariables( mLayer, mVariableEditor->variablesInActiveScope() );
   updateVariableEditor();
 
+  // save layer dependencies
+  QSet<QString> deps;
+  Q_FOREACH ( const QgsLayerTreeLayer* layer, mLayersDependenciesTreeGroup->findLayers() )
+  {
+    if ( layer->isVisible() )
+      deps << layer->layerId();
+  }
+  if ( ! mLayer->setDataDependencies( deps ) )
+  {
+    QMessageBox::warning( nullptr, tr( "Dependency cycle" ), tr( "This configuration introduces a cycle in data dependencies and will be ignored" ) );
+  }
+
   // update symbology
   emit refreshLegend( mLayer->id() );
 

src/app/qgsvectorlayerproperties.h

@@ -25,6 +25,8 @@
 #include "qgscontexthelp.h"
 #include "qgsmaplayerstylemanager.h"
 #include "qgsvectorlayer.h"
+#include "layertree/qgslayertreemodel.h"
+#include "layertree/qgslayertreegroup.h"
 
 class QgsMapLayer;
 
@@ -193,6 +195,9 @@ class APP_EXPORT QgsVectorLayerProperties : public QgsOptionsDialogBase, private
 
     QgsExpressionContext createExpressionContext() const override;
 
+    QScopedPointer<QgsLayerTreeGroup> mLayersDependenciesTreeGroup;
+    QScopedPointer<QgsLayerTreeModel> mLayersDependenciesTreeModel;
+
   private slots:
     void openPanel( QgsPanelWidget* panel );
 };

src/core/qgslayerdefinition.cpp

@@ -111,6 +111,22 @@ bool QgsLayerDefinition::loadLayerDefinition( QDomDocument doc, QgsLayerTreeGrou
         joinNode.toElement().setAttribute( "joinLayerId", newid );
       }
     }
+
+    // change IDs of dependencies
+    QDomNodeList dataDeps = doc.elementsByTagName( "dataDependencies" );
+    for ( int i = 0; i < dataDeps.size(); i++ )
+    {
+      QDomNodeList layers = dataDeps.at( i ).childNodes();
+      for ( int j = 0; j < layers.size(); j++ )
+      {
+        QDomElement elt = layers.at( j ).toElement();
+        if ( elt.attribute( "id" ) == oldid )
+        {
+          elt.setAttribute( "id", newid );
+        }
+      }
+    }
+
   }
 
   QDomElement layerTreeElem = doc.documentElement().firstChildElement( "layer-tree-group" );

src/core/qgsmaplayer.cpp

@@ -1680,3 +1680,62 @@ void QgsMapLayer::setExtent( const QgsRectangle &r )
 {
   mExtent = r;
 }
+
+static QList<const QgsMapLayer*> _depOutEdges( const QgsMapLayer* vl, const QgsMapLayer* that, const QSet<QString>& layersIds )
+{
+  QList<const QgsMapLayer*> lst;
+  if ( vl == that )
+  {
+    Q_FOREACH ( QString layerId, layersIds )
+    {
+      if ( const QgsMapLayer* l = QgsMapLayerRegistry::instance()->mapLayer( layerId ) )
+        lst << l;
+    }
+  }
+  else
+  {
+    Q_FOREACH ( QString layerId, vl->dataDependencies() )
+    {
+      if ( const QgsMapLayer* l = QgsMapLayerRegistry::instance()->mapLayer( layerId ) )
+        lst << l;
+    }
+  }
+  return lst;
+}
+
+static bool _depHasCycleDFS( const QgsMapLayer* n, QHash<const QgsMapLayer*, int>& mark, const QgsMapLayer* that, const QSet<QString>& layersIds )
+{
+  if ( mark.value( n ) == 1 ) // temporary
+    return true;
+  if ( mark.value( n ) == 0 ) // not visited
+  {
+    mark[n] = 1; // temporary
+    Q_FOREACH ( const QgsMapLayer* m, _depOutEdges( n, that, layersIds ) )
+    {
+      if ( _depHasCycleDFS( m, mark, that, layersIds ) )
+        return true;
+    }
+    mark[n] = 2; // permanent
+  }
+  return false;
+}
+
+bool QgsMapLayer::hasDataDependencyCycle( const QSet<QString>& layersIds ) const
+{
+  QHash<const QgsMapLayer*, int> marks;
+  return _depHasCycleDFS( this, marks, this, layersIds );
+}
+
+bool QgsMapLayer::setDataDependencies( const QSet<QString>& layersIds )
+{
+  if ( hasDataDependencyCycle( layersIds ) )
+    return false;
+
+  mDataDependencies = layersIds;
+  return true;
+}
+
+QSet<QString> QgsMapLayer::dataDependencies() const
+{
+  return mDataDependencies;
+}