I have a NSOutlineView bound to a NSTreeController. My data items are a custom class , let's call them "Row", and suppose a Row contains a "name" and a numeric field called "number" . All these "Rows" are found in let's say a "RowContainer" which has a "rows" mutable array holding the parent (level 0) rows. Each row also has a "children" NSMutableArray member which holds it's children. I have this working, and I want to display under the outlineview a textfield with the sum of all the "number" values of the rows. I bound this textfield to a "total" property of the "RowContainer". Now the problem is how or from where to trigger the recalculation of the "total" property, since this involves a recursive walk on the tree of rows, and I always get a "Collection was mutated while being enumerated" error. I've tried making a method "recalculateTotal", and calling it from the "setNumber" method of the "Row" class , but same error occurs. If I put the recalculation logic in the "total" getter, I can't trigger it to do the math. I'm sure the solution is simple but I can't see it