CollectScenes : Check for cancellation while building RootTree

Changes.md

@@ -12,6 +12,7 @@ Improvements
 - MatchPatternPathFilterWidget : Added the name of the property being filtered to the placeholder text.
 - OpenImageIOReader : The `availableFrames` plug no longer errors for a missing file sequence - instead it outputs an empty list.
 - PathListingWidget : Header data is now computed asynchronously, without locking the UI.
+- CollectScenes : Improved cancellation responsiveness for large lists of `rootNames`.
 
 Fixes
 -----

src/GafferScene/CollectScenes.cpp

@@ -95,12 +95,14 @@ class RootTree : public IECore::Data
 
 		};
 
-		RootTree( const IECore::StringVectorData *roots )
+		RootTree( const IECore::StringVectorData *roots, const IECore::Canceller *canceller )
 			:	m_treeRoot( new Location( 0 ) )
 		{
 			ScenePlug::ScenePath path;
 			for( const auto &root : roots->readable() )
 			{
+				IECore::Canceller::check( canceller );
+
 				ScenePlug::stringToPath( root, path );
 				if( path.empty() )
 				{
@@ -415,7 +417,7 @@ void CollectScenes::compute( Gaffer::ValuePlug *output, const Gaffer::Context *c
 	{
 		ConstStringVectorDataPtr roots = rootNamesPlug()->getValue();
 		static_cast<ObjectPlug *>( output )->setValue(
-			new RootTree( roots.get() )
+			new RootTree( roots.get(), context->canceller() )
 		);
 		return;
 	}