1. In a computer system which implements a view model including a plurality of coordinate systems, a computer-implemented method for updating an item within a first coordinate system of said plurality of coordinate systems responsive to a change in another coordinate system of said plurality of coordinate systems, comprising the following steps: a. storing in said computer system an acyclic graph, said acyclic graph representing dependencies between various items in each of said plurality of coordinate systems; b. upon creation of said item in said first coordinate system of said plurality of coordinate systems, setting a flag corresponding to said item to indicate whether every item of a plurality of items in others of said plurality of coordinate systems is valid in its respective coordinate system, wherein said plurality of items are items upon which said item depends; c. setting said flag corresponding to said item to indicate that at least one item of said plurality of items is not valid in its respective coordinate system if said change causes any item of said plurality of items to become invalid in its respective coordinate system; and d. when an application program references said item, d1. determining whether said flag corresponding to said item indicates that at least one item of said plurality of items is not valid in its respective coordinate system; d2. if said flag corresponding to said item indicates that at least one item of said plurality of items is not valid in its respective coordinate system, then i. descending said acyclic graph to determine a valid item of said plurality of items, ii. when said valid item is determined, ascending said acyclic graph to determine each item of said plurality of items which depends on said valid item, iii. calculating each item of said plurality of items which depends on said valid item, iv. calculating said item, v. setting said flag corresponding to said item to indicate that each item of said plurality of items is valid in its respective coordinate system, and vi. presenting said one item to said application program.