PathListingWidget : Add column context menu signal

This follows the pattern established by other context menus, in allowing multiple client slots to collaborate on the building of a context menu.

I debated whether or not the signal should pass the `path` that is currently under the mouse, but decided against. That would encourage the creation of menu items that operate only on that path, but with multiple selection I believe the user expectation would be that the entire selection is operated on. By forcing clients to go fetch the selection themselves they are more likely to do the expected thing with it.

Changes.md

@@ -14,6 +14,11 @@ Fixes
 - WidgetAlgo : Fixed issue preventing `grab()` from capturing popup menus on Windows.
 - ShowURL : Fixed opening of "file://" URLs on Windows (#5861).
 
+API
+---
+
+- PathListingWidget : Added `columnContextMenuSignal()`, allowing multiple clients to collaborate on the creation of a column-specific context menu.
+
 Documentation
 -------------
 

python/GafferUI/PathListingWidget.py

@@ -148,6 +148,7 @@ def __init__(
 		self.__displayModeChangedSignal = GafferUI.WidgetSignal()
 		self.__expansionChangedSignal = GafferUI.WidgetSignal()
 		self.__updateFinishedSignal = GafferUI.WidgetSignal()
+		self.__columnContextMenuSignal = Gaffer.Signal3()
 
 		# Connections for implementing selection and drag and drop.
 		self.keyPressSignal().connect( Gaffer.WeakMethod( self.__keyPress ), scoped = False )
@@ -157,6 +158,7 @@ def __init__(
 		self.mouseMoveSignal().connect( Gaffer.WeakMethod( self.__mouseMove ), scoped = False )
 		self.dragBeginSignal().connect( Gaffer.WeakMethod( self.__dragBegin ), scoped = False )
 		self.dragEndSignal().connect( Gaffer.WeakMethod( self.__dragEnd ), scoped = False )
+		self.contextMenuSignal().connect( Gaffer.WeakMethod( self.__contextMenu ), scoped = False )
 		self.__dragPointer = "paths"
 
 		self.__path = None
@@ -423,6 +425,19 @@ def pathSelectedSignal( self ) :
 
 		return self.__pathSelectedSignal
 
+	## Signal emitted to generate a context menu for a column. This allows
+	# multiple clients to collaborate in the construction of a menu, with each
+	# providing different items. It should be preferred to the generic
+	# `Widget.contextMenuSignal()`.
+	#
+	# Slots must have the following signature, with the `menuDefinition` being
+	# edited directly in place :
+	#
+	# `slot( column, pathListingWidget, menuDefinition )`
+	def columnContextMenuSignal( self ) :
+
+		return self.__columnContextMenuSignal
+
 	def setDragPointer( self, dragPointer ) :
 
 		self.__dragPointer = dragPointer
@@ -749,6 +764,47 @@ def __dragEnd( self, widget, event ) :
 
 		GafferUI.Pointer.setCurrent( None )
 
+	def __contextMenu( self, widget ) :
+
+		if not self.columnContextMenuSignal().numSlots() :
+			# Allow legacy clients connected to `Widget.contextMenuSignal()` to
+			# do their own thing instead.
+			return False
+
+		# Select the path under the mouse, if it's not already selected.
+		# The user will expect to be operating on the thing under the mouse.
+
+		mousePosition = GafferUI.Widget.mousePosition( relativeTo = self )
+		column = self.columnAt( mousePosition )
+
+		path = self.pathAt( mousePosition )
+		if path is not None :
+			path = str( path )
+			selection = self.getSelection()
+			if isinstance( selection, IECore.PathMatcher ) :
+				# Row or Rows mode.
+				if not selection.match( path ) & IECore.PathMatcher.Result.ExactMatch :
+					selection = IECore.PathMatcher( [ path ] )
+					self.setSelection( selection )
+			else :
+				# Cell or Cells mode.
+				columnIndex = self.getColumns().index( column )
+				if not selection[columnIndex].match( path ) & IECore.PathMatcher.Result.ExactMatch :
+					for i in range( 0, len( selection ) ) :
+						selection[i] = IECore.PathMatcher() if columnIndex != i else IECore.PathMatcher( [ path ] )
+					self.setSelection( selection )
+
+		# Use signals to build menu and display it.
+
+		menuDefinition = IECore.MenuDefinition()
+		self.columnContextMenuSignal()( column, self, menuDefinition )
+
+		if menuDefinition.size() :
+			self.__columnContextMenu = GafferUI.Menu( menuDefinition )
+			self.__columnContextMenu.popup( self )
+
+		return True
+
 	def __indexAt( self, position ) :
 
 		point = self._qtWidget().viewport().mapFrom(