Make layer tree implementation independent from QgsProject::instance()

Another bit in the project refactoring work to get rid of dependencies
on QgsProject singleton.

Reading of layer trees from XML is now split into two phases:
1. read XML and keep layer IDs
2. resolve layer IDs to QgsMapLayer instances (using QgsProject)

There are convenience methods to do both phases in one go.

doc/api_break.dox

@@ -1194,13 +1194,20 @@ QgsLayerTreeGroup        {#qgis_api_break_3_0_QgsLayerTreeGroup}
 - isVisible() is moved to QgsLayerTreeNode
 - setVisible() is replaced by QgsLayerTreeNode::setItemVisibilityChecked()
 - protected methods updateVisibilityFromChildren() and updateChildVisibility() removed
+- readXml() and readChildrenFromXml() do not resolve layers from the layer IDs anymore. Call resolveReferences() or use readXml() override with QgsProject as the second argument.
 
 QgsLayerTreeLayer        {#qgis_api_break_3_0_QgsLayerTreeLayer}
 -----------------
 
 - setLayerName(), layerName() were renamed to setName(), name()
 - isVisible() is moved to QgsLayerTreeNode
 - setVisible() is replaced by QgsLayerTreeNode::setItemVisibilityChecked()
+- readXml() does not resolve layer from the layer ID anymore. Call resolveReferences() or use readXml() override with QgsProject as the second argument.
+
+QgsLayerTreeNode        {#qgis_api_break_3_0_QgsLayerTreeNode}
+----------------
+
+- readXml() does not resolve layers from the layer IDs anymore. Call resolveReferences() or use readXml() override with QgsProject as the second argument.
 
 
 QgsLayerTreeModel        {#qgis_api_break_3_0_QgsLayerTreeMode}

python/core/layertree/qgslayertreegroup.sip

@@ -58,11 +58,18 @@ class QgsLayerTreeGroup : QgsLayerTreeNode
     //! Find group node with specified name. Searches recursively the whole sub-tree.
     QgsLayerTreeGroup* findGroup( const QString& name );
 
-    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error)
+    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error).
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
     static QgsLayerTreeGroup* readXml( QDomElement& element ) /Factory/;
+    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error).
+    //! Also resolves textual references to layers from the project (calls resolveReferences() internally).
+    //! @note added in 3.0
+    static QgsLayerTreeGroup* readXml( QDomElement& element, const QgsProject* project ) /Factory/;
+
     //! Write group (tree) as XML element <layer-tree-group> and add it to the given parent element
     virtual void writeXml( QDomElement& parentElement );
     //! Read children from XML and append them to the group.
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
     void readChildrenFromXml( QDomElement& element );
 
     //! Return text representation of the tree. For debugging purposes only.
@@ -71,6 +78,10 @@ class QgsLayerTreeGroup : QgsLayerTreeNode
     //! Return a clone of the group. The children are cloned too.
     virtual QgsLayerTreeGroup* clone() const /Factory/;
 
+    //! Calls resolveReferences() on child tree nodes
+    //! @note added in 3.0
+    virtual void resolveReferences( const QgsProject* project );
+
     //! Return whether the group is mutually exclusive (only one child can be checked at a time)
     //! @note added in 2.12
     bool isMutuallyExclusive() const;

python/core/layertree/qgslayertreelayer.sip

@@ -1,14 +1,10 @@
 /**
  * Layer tree node points to a map layer.
  *
- * When using with existing QgsMapLayer instance, it is expected that the layer
- * has been registered in QgsProject earlier.
- *
  * The node can exist also without a valid instance of a layer (just ID). That
  * means the referenced layer does not need to be loaded in order to use it
- * in layer tree. In such case, the node will start listening to map layer
- * registry updates in expectation that the layer (identified by its ID) will
- * be loaded later.
+ * in layer tree. In such case, resolveReferences() method can be called
+ * once the layer is loaded.
  *
  * A map layer is supposed to be present in one layer tree just once. It is
  * however possible that temporarily a layer exists in one tree more than just
@@ -38,19 +34,23 @@ class QgsLayerTreeLayer : QgsLayerTreeNode
     //! @note added in 3.0
     void setName( const QString& n );
 
+    //! Read layer node from XML. Returns new instance.
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
     static QgsLayerTreeLayer* readXml( QDomElement& element ) /Factory/;
+    //! Read layer node from XML. Returns new instance.
+    //! Also resolves textual references to layers from the project (calls resolveReferences() internally).
+    //! @note added in 3.0
+    static QgsLayerTreeLayer* readXml( QDomElement& element, const QgsProject* project ) /Factory/;
+
     virtual void writeXml( QDomElement& parentElement );
 
     virtual QString dump() const;
 
     virtual QgsLayerTreeLayer* clone() const /Factory/;
 
-  protected slots:
-    void registryLayersAdded( const QList<QgsMapLayer*>& layers );
-    void registryLayersWillBeRemoved( const QStringList& layerIds );
-    //! Emits a nameChanged() signal if layer's name has changed
+    //! Resolves reference to layer from stored layer ID (if it has not been resolved already)
     //! @note added in 3.0
-    void layerNameChanged();
+    virtual void resolveReferences( const QgsProject* project );
 
   signals:
     //! emitted when a previously unavailable layer got loaded

python/core/layertree/qgslayertreenode.sip

@@ -83,8 +83,14 @@ class QgsLayerTreeNode : QObject
     //! @note added in 3.0
     virtual void setName( const QString& name ) = 0;
 
-    //! Read layer tree from XML. Returns new instance
-    static QgsLayerTreeNode* readXml( QDomElement& element ) /Factory/;
+    //! Read layer tree from XML. Returns new instance.
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
+    static QgsLayerTreeNode *readXml( QDomElement &element ) /Factory/;
+    //! Read layer tree from XML. Returns new instance.
+    //! Also resolves textual references to layers from the project (calls resolveReferences() internally).
+    //! @note added in 3.0
+    static QgsLayerTreeNode* readXml( QDomElement& element, const QgsProject* project ) /Factory/;
+
     //! Write layer tree to XML
     virtual void writeXml( QDomElement &parentElement ) = 0;
 
@@ -94,6 +100,11 @@ class QgsLayerTreeNode : QObject
     //! Create a copy of the node. Returns new instance
     virtual QgsLayerTreeNode *clone() const = 0 /Factory/;
 
+    //! Turn textual references to layers into map layer object from project.
+    //! This method should be called after readXml()
+    //! @note added in 3.0
+    virtual void resolveReferences( const QgsProject* project ) = 0;
+
     //! Returns whether a node is really visible (ie checked and all its ancestors checked as well)
     //! @note added in 3.0
     bool isVisible() const;

src/app/qgisapp.cpp

@@ -11573,7 +11573,7 @@ void QgisApp::writeProject( QDomDocument &doc )
 
   QgsLayerTreeNode* clonedRoot = QgsProject::instance()->layerTreeRoot()->clone();
   QgsLayerTreeUtils::replaceChildrenOfEmbeddedGroups( QgsLayerTree::toGroup( clonedRoot ) );
-  QgsLayerTreeUtils::updateEmbeddedGroupsProjectPath( QgsLayerTree::toGroup( clonedRoot ) ); // convert absolute paths to relative paths if required
+  QgsLayerTreeUtils::updateEmbeddedGroupsProjectPath( QgsLayerTree::toGroup( clonedRoot ), QgsProject::instance() ); // convert absolute paths to relative paths if required
   QDomElement oldLegendElem = QgsLayerTreeUtils::writeOldLegend( doc, QgsLayerTree::toGroup( clonedRoot ),
                               mLayerTreeCanvasBridge->hasCustomLayerOrder(), mLayerTreeCanvasBridge->customLayerOrder() );
   delete clonedRoot;

src/core/composer/qgscomposerlegend.cpp

@@ -537,7 +537,7 @@ bool QgsComposerLegend::readXml( const QDomElement& itemElem, const QDomDocument
   {
     // QGIS >= 2.6
     QDomElement layerTreeElem = itemElem.firstChildElement( QStringLiteral( "layer-tree-group" ) );
-    setCustomLayerTree( QgsLayerTreeGroup::readXml( layerTreeElem ) );
+    setCustomLayerTree( QgsLayerTreeGroup::readXml( layerTreeElem, mComposition->project() ) );
   }
 
   //restore general composer item properties

src/core/layertree/qgslayertreegroup.cpp

@@ -18,7 +18,6 @@
 #include "qgslayertree.h"
 #include "qgslayertreeutils.h"
 #include "qgsmaplayer.h"
-#include "qgsproject.h"
 
 #include <QDomElement>
 #include <QStringList>
@@ -73,9 +72,9 @@ QgsLayerTreeGroup* QgsLayerTreeGroup::addGroup( const QString &name )
   return grp;
 }
 
-QgsLayerTreeLayer*QgsLayerTreeGroup::insertLayer( int index, QgsMapLayer* layer )
+QgsLayerTreeLayer* QgsLayerTreeGroup::insertLayer( int index, QgsMapLayer* layer )
 {
-  if ( !layer || QgsProject::instance()->mapLayer( layer->id() ) != layer )
+  if ( !layer )
     return nullptr;
 
   QgsLayerTreeLayer* ll = new QgsLayerTreeLayer( layer );
@@ -85,7 +84,7 @@ QgsLayerTreeLayer*QgsLayerTreeGroup::insertLayer( int index, QgsMapLayer* layer
 
 QgsLayerTreeLayer* QgsLayerTreeGroup::addLayer( QgsMapLayer* layer )
 {
-  if ( !layer || QgsProject::instance()->mapLayer( layer->id() ) != layer )
+  if ( !layer )
     return nullptr;
 
   QgsLayerTreeLayer* ll = new QgsLayerTreeLayer( layer );
@@ -276,6 +275,14 @@ QgsLayerTreeGroup* QgsLayerTreeGroup::readXml( QDomElement& element )
   return groupNode;
 }
 
+QgsLayerTreeGroup* QgsLayerTreeGroup::readXml( QDomElement& element, const QgsProject* project )
+{
+  QgsLayerTreeGroup* node = readXml( element );
+  if ( node )
+    node->resolveReferences( project );
+  return node;
+}
+
 void QgsLayerTreeGroup::writeXml( QDomElement& parentElement )
 {
   QDomDocument doc = parentElement.ownerDocument();
@@ -329,6 +336,12 @@ QgsLayerTreeGroup* QgsLayerTreeGroup::clone() const
   return new QgsLayerTreeGroup( *this );
 }
 
+void QgsLayerTreeGroup::resolveReferences( const QgsProject* project )
+{
+  Q_FOREACH ( QgsLayerTreeNode* node, mChildren )
+    node->resolveReferences( project );
+}
+
 static bool _nodeIsChecked( QgsLayerTreeNode* node )
 {
   return node->itemVisibilityChecked();

src/core/layertree/qgslayertreegroup.h

@@ -81,11 +81,18 @@ class CORE_EXPORT QgsLayerTreeGroup : public QgsLayerTreeNode
     //! Find group node with specified name. Searches recursively the whole sub-tree.
     QgsLayerTreeGroup* findGroup( const QString& name );
 
-    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error)
+    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error).
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
     static QgsLayerTreeGroup* readXml( QDomElement& element );
+    //! Read group (tree) from XML element <layer-tree-group> and return the newly created group (or null on error).
+    //! Also resolves textual references to layers from the project (calls resolveReferences() internally).
+    //! @note added in 3.0
+    static QgsLayerTreeGroup* readXml( QDomElement& element, const QgsProject* project );
+
     //! Write group (tree) as XML element <layer-tree-group> and add it to the given parent element
     virtual void writeXml( QDomElement& parentElement ) override;
     //! Read children from XML and append them to the group.
+    //! Does not resolve textual references to layers. Call resolveReferences() afterwards to do it.
     void readChildrenFromXml( QDomElement& element );
 
     //! Return text representation of the tree. For debugging purposes only.
@@ -94,6 +101,10 @@ class CORE_EXPORT QgsLayerTreeGroup : public QgsLayerTreeNode
     //! Return a clone of the group. The children are cloned too.
     virtual QgsLayerTreeGroup* clone() const override;
 
+    //! Calls resolveReferences() on child tree nodes
+    //! @note added in 3.0
+    virtual void resolveReferences( const QgsProject* project ) override;
+
     //! Check or uncheck a node and all its children (taking into account exclusion rules)
     virtual void setItemVisibilityCheckedRecursive( bool checked ) override;