layer tree model can emit messages which will be shown in the message bar

python/core/auto_generated/layertree/qgslayertreemodel.sip.in

@@ -321,6 +321,9 @@ displays.
 .. versionadded:: 3.6
 %End
 
+  signals:
+    void messageEmitted( const QString &message );
+
   protected slots:
     void nodeWillAddChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );
     void nodeAddedChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );

python/gui/auto_generated/layertree/qgslayertreeview.sip.in

@@ -9,6 +9,7 @@
 
 
 
+
 class QgsLayerTreeView : QTreeView
 {
 %Docstring
@@ -211,6 +212,13 @@ Set width of contextual menu mark, at right of layer node items.
 .. seealso:: :py:func:`layerMarkWidth`
 
 .. versionadded:: 3.8
+%End
+
+    void setMessageBar( QgsMessageBar *messageBar );
+%Docstring
+Set the message bar to display messages from the layer tree
+
+.. versionadded:: 3.14
 %End
 
   signals:

src/app/qgisapp.cpp

@@ -1681,12 +1681,13 @@ QgisApp::QgisApp()
   mMapCanvas = new QgsMapCanvas();
   connect( mMapCanvas, &QgsMapCanvas::messageEmitted, this, &QgisApp::displayMessage );
   QgsCanvasRefreshBlocker refreshBlocker;
-  mLayerTreeView = new QgsLayerTreeView( this );
+  QgsLayerTreeView( this );
   QgsLayerTreeModel *model = new QgsLayerTreeModel( QgsProject::instance()->layerTreeRoot(), this );
   mLayerTreeView->setModel( model );
   mUndoWidget = new QgsUndoWidget( nullptr, mMapCanvas );
   mUserInputDockWidget = new QgsUserInputWidget( this );
   mInfoBar = new QgsMessageBar( centralWidget() );
+  mLayerTreeView->setMessageBar( mInfoBar );
   mAdvancedDigitizingDockWidget = new QgsAdvancedDigitizingDockWidget( mMapCanvas, this );
   mPanelMenu = new QMenu( this );
   mProgressBar = new QProgressBar( this );
@@ -4590,6 +4591,8 @@ void QgisApp::initLayerTreeView()
   model->setAutoCollapseLegendNodes( 10 );
 
   mLayerTreeView->setModel( model );
+  mLayerTreeView->setMessageBar( mInfoBar );
+
   mLayerTreeView->setMenuProvider( new QgsAppLayerTreeViewMenuProvider( mLayerTreeView, mMapCanvas ) );
   new QgsLayerTreeViewFilterIndicatorProvider( mLayerTreeView );  // gets parented to the layer view
   new QgsLayerTreeViewEmbeddedIndicatorProvider( mLayerTreeView );  // gets parented to the layer view

src/core/layertree/qgslayertreemodel.cpp

@@ -1109,6 +1109,7 @@ bool QgsLayerTreeModel::dropMimeData( const QMimeData *data, Qt::DropAction acti
     QgsReadWriteContext context;
     QString errorMessage;
     QgsLayerDefinition::loadLayerDefinition( layerDefinitionDoc, QgsProject::instance(), QgsLayerTree::toGroup( nodeParent ), errorMessage, context );
+    emit messageEmitted( tr( "New layers added from another QGIS instance" ) );
   }
   else
   {

src/core/layertree/qgslayertreemodel.h

@@ -286,6 +286,9 @@ class CORE_EXPORT QgsLayerTreeModel : public QAbstractItemModel
      */
     static int scaleIconSize( int standardSize );
 
+  signals:
+    void messageEmitted( const QString &message );
+
   protected slots:
     void nodeWillAddChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );
     void nodeAddedChildren( QgsLayerTreeNode *node, int indexFrom, int indexTo );

src/gui/layertree/qgslayertreeview.cpp

@@ -22,6 +22,8 @@
 #include "qgslayertreeutils.h"
 #include "qgslayertreeviewdefaultactions.h"
 #include "qgsmaplayer.h"
+#include "qgsmessagebar.h"
+
 #include "qgsgui.h"
 
 #include <QMenu>
@@ -77,6 +79,9 @@ void QgsLayerTreeView::setModel( QAbstractItemModel *model )
   connect( model, &QAbstractItemModel::rowsInserted, this, &QgsLayerTreeView::modelRowsInserted );
   connect( model, &QAbstractItemModel::rowsRemoved, this, &QgsLayerTreeView::modelRowsRemoved );
 
+  if ( mMessageBar )
+    connect( layerTreeModel(), &QgsLayerTreeModel::messageEmitted, [ = ]( const QString & message ) {mMessageBar->pushMessage( message );} );
+
   QTreeView::setModel( model );
 
   connect( layerTreeModel()->rootGroup(), &QgsLayerTreeNode::expandedChanged, this, &QgsLayerTreeView::onExpandedChanged );
@@ -490,6 +495,17 @@ void QgsLayerTreeView::collapseAllNodes()
   collapseAll();
 }
 
+void QgsLayerTreeView::setMessageBar( QgsMessageBar *messageBar )
+{
+  if ( mMessageBar == messageBar )
+    return;
+
+  mMessageBar = messageBar;
+
+  if ( mMessageBar )
+    connect( layerTreeModel(), &QgsLayerTreeModel::messageEmitted, [ = ]( const QString & message ) {mMessageBar->pushMessage( message );} );
+}
+
 void QgsLayerTreeView::mouseReleaseEvent( QMouseEvent *event )
 {
   // we need to keep last mouse position in order to know whether to emit an indicator's clicked() signal

src/gui/layertree/qgslayertreeview.h

@@ -29,6 +29,8 @@ class QgsLayerTreeViewDefaultActions;
 class QgsLayerTreeViewIndicator;
 class QgsLayerTreeViewMenuProvider;
 class QgsMapLayer;
+class QgsMessageBar;
+
 
 /**
  * \ingroup gui
@@ -206,6 +208,12 @@ class GUI_EXPORT QgsLayerTreeView : public QTreeView
      */
     void setLayerMarkWidth( int width ) { mLayerMarkWidth = width; }
 
+    /**
+     * Set the message bar to display messages from the layer tree
+     * \since QGIS 3.14
+     */
+    void setMessageBar( QgsMessageBar *messageBar );
+
   signals:
     //! Emitted when a current layer is changed
     void currentLayerChanged( QgsMapLayer *layer );
@@ -253,6 +261,9 @@ class GUI_EXPORT QgsLayerTreeView : public QTreeView
     //! Width of contextual menu mark for layer nodes
     int mLayerMarkWidth;
 
+  private:
+    QgsMessageBar *mMessageBar = nullptr;
+
     // friend so it can access viewOptions() method and mLastReleaseMousePos without making them public
     friend class QgsLayerTreeViewItemDelegate;
 };