Add method to transfer all layers from one map store to another

With a note and assert that both stores must have the same
thread affinity

python/core/qgsmaplayerstore.sip

@@ -208,6 +208,13 @@ class QgsMapLayerStore : QObject
 .. seealso:: removeMapLayers()
 %End
 
+    void transferLayersFromStore( QgsMapLayerStore *other );
+%Docstring
+ Transfers all the map layers contained within another map layer store and adds
+ them to this store.
+ Note that ``other`` and this store must have the same thread affinity.
+%End
+
   signals:
 
     void layersWillBeRemoved( const QStringList &layerIds );

src/core/qgsmaplayerstore.cpp

@@ -180,6 +180,23 @@ void QgsMapLayerStore::removeAllMapLayers()
   mMapLayers.clear();
 }
 
+void QgsMapLayerStore::transferLayersFromStore( QgsMapLayerStore *other )
+{
+  if ( !other || other == this )
+    return;
+
+  Q_ASSERT_X( other->thread() == thread(), "QgsMapLayerStore::transferLayersFromStore", "Cannot transfer layers from store with different thread affinity" );
+
+  QMap<QString, QgsMapLayer *> otherLayers = other->mapLayers();
+  QMap<QString, QgsMapLayer *>::const_iterator it = otherLayers.constBegin();
+  for ( ; it != otherLayers.constEnd(); ++it )
+  {
+    QgsMapLayer *layer = other->takeMapLayer( it.value() );
+    if ( layer )
+      addMapLayer( layer );
+  }
+}
+
 void QgsMapLayerStore::onMapLayerDeleted( QObject *obj )
 {
   QString id = mMapLayers.key( static_cast<QgsMapLayer *>( obj ) );

src/core/qgsmaplayerstore.h

@@ -238,6 +238,13 @@ class CORE_EXPORT QgsMapLayerStore : public QObject
      */
     void removeAllMapLayers();
 
+    /**
+     * Transfers all the map layers contained within another map layer store and adds
+     * them to this store.
+     * Note that \a other and this store must have the same thread affinity.
+     */
+    void transferLayersFromStore( QgsMapLayerStore *other );
+
   signals:
 
     /**

tests/src/python/test_qgsmaplayerstore.py

@@ -495,6 +495,36 @@ class TestQgsMapLayerStore(unittest.TestCase):
         store = None
         self.assertTrue(l1.isValid())
 
+    def testTransferLayers(self):
+        # test transferring all layers from another store
+        store1 = QgsMapLayerStore()
+        store2 = QgsMapLayerStore()
+
+        # empty stores
+        store1.transferLayersFromStore(store2)
+
+        # silly behavior checks
+        store1.transferLayersFromStore(None)
+        store1.transferLayersFromStore(store1)
+
+        l1 = createLayer('l1')
+        l2 = createLayer('l2')
+        store1.addMapLayer(l1)
+        store1.addMapLayer(l2)
+
+        l3 = createLayer('l3')
+        store2.addMapLayer(l3)
+
+        store2.transferLayersFromStore(store1)
+        self.assertFalse(store1.mapLayers()) # no layers left
+        self.assertEqual(len(store2.mapLayers()), 3)
+        self.assertEqual(store2.mapLayers(), {l1.id(): l1, l2.id(): l2, l3.id(): l3})
+
+        store1.transferLayersFromStore(store2)
+        self.assertFalse(store2.mapLayers()) # no layers left
+        self.assertEqual(len(store1.mapLayers()), 3)
+        self.assertEqual(store1.mapLayers(), {l1.id(): l1, l2.id(): l2, l3.id(): l3})
+
 
 if __name__ == '__main__':
     unittest.main()