Add method to take layer and ownership from a project

Author: nyalldawson

python/core/qgsproject.sip

@@ -480,6 +480,8 @@ class QgsProject : QObject, QgsExpressionContextGenerator
      */
     void removeMapLayer( QgsMapLayer* layer );
 
+    QgsMapLayer *takeMapLayer( QgsMapLayer *layer ) /TransferBack/;
+
     /**
      * Removes all registered layers. If the registry has ownership
      * of any layers these layers will also be deleted.

src/core/qgsproject.cpp

@@ -2201,6 +2201,27 @@ void QgsProject::removeMapLayer( QgsMapLayer *layer )
     removeMapLayers( QList<QgsMapLayer *>() << layer );
 }
 
+QgsMapLayer *QgsProject::takeMapLayer( QgsMapLayer *layer )
+{
+  if ( !layer )
+    return nullptr;
+
+  if ( mMapLayers.contains( layer->id() ) )
+  {
+    emit layersWillBeRemoved( QStringList() << layer->id() );
+    emit layersWillBeRemoved( QList<QgsMapLayer *>() << layer );
+    emit layerWillBeRemoved( layer->id() );
+    emit layerWillBeRemoved( layer );
+
+    mMapLayers.remove( layer->id() );
+    layer->setParent( nullptr );
+    emit layerRemoved( layer->id() );
+    emit layersRemoved( QStringList() << layer->id() );
+    return layer;
+  }
+  return nullptr; //don't return layer - it wasn't owned and accordingly we aren't transferring ownership
+}
+
 void QgsProject::removeAllMapLayers()
 {
   emit removeAll();

src/core/qgsproject.h

@@ -699,6 +699,15 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
      */
     void removeMapLayer( QgsMapLayer *layer );
 
+    /**
+     * Takes a layer from the registry. If the layer was owned by the project, the
+     * layer will be returned without deleting it. The caller takes ownership of
+     * the layer and is responsible for deleting it.
+     * \see removeMapLayer()
+     * \since QGIS 3.0
+     */
+    QgsMapLayer *takeMapLayer( QgsMapLayer *layer );
+
     /**
      * Removes all registered layers. If the registry has ownership
      * of any layers these layers will also be deleted.

tests/src/python/test_qgsmaplayerregistry.py

@@ -502,6 +502,35 @@ def test_RemoveLayerShouldNotSegFault(self):
         for k, layer in list(reg.mapLayers().items()):
             assert(layer is not None)
 
+    def testTakeLayer(self):
+        # test taking ownership of a layer from the project
+        l1 = createLayer('l1')
+        l2 = createLayer('l2')
+        p = QgsProject()
+
+        # add one layer to project
+        p.addMapLayer(l1)
+        self.assertEqual(p.mapLayers(), {l1.id(): l1})
+        self.assertEqual(l1.parent(), p)
+
+        # try taking some layers which don't exist in project
+        self.assertFalse(p.takeMapLayer(None))
+        self.assertFalse(p.takeMapLayer(l2))
+        # but l2 should still exist..
+        self.assertTrue(l2.isValid())
+
+        # take layer from project
+        self.assertEqual(p.takeMapLayer(l1), l1)
+        self.assertFalse(p.mapLayers()) # no layers left
+        # but l1 should still exist
+        self.assertTrue(l1.isValid())
+        # layer should have no parent now
+        self.assertFalse(l1.parent())
+
+        # destroy project
+        p = None
+        self.assertTrue(l1.isValid())
+
 
 if __name__ == '__main__':
     unittest.main()