Excel Add-in Development in C/C++: Applications in ... - F9

26 Excel Add-in Development in C/C++2.11.2 Triggering functions to be called by Excel – the trigger argumentThere are times when you want things to be calculated in a very specific order, or forsomething to be triggered by the change in value of some cell or other. Of course, Exceldoes this automatically, you might say. True, but the trigger is the change in value ofsome input to the calculation. This is fine as long as you only want that to be the trigger.What if you want something else to be the trigger? What if the function you want totrigger doesn’t need any arguments? For example, what if you want to have a cell thatshows the time that another cell’s value last changed so that an observer can see howfresh the information is?The solution is simple: the trigger argument. This is a dummy argument that is ofabsolutely no use to the function being triggered other than to force Excel to call it.(Section 9.1 Timing function execution in VB and C/C++ on page 285 relies heavily onthis idea.) The VB function NumCalls_1() in the above section uses the argument solelyto trigger Excel to call the code.In the case of wanting to record the time a static numeric cell’s value changes, a simpleVB function like this would have the desired effect:Function Get_Time(trigger As Double) As DoubleGet_Time = NowEnd FunctionThe argument trigger is not used in the calculation which simply returns the currentdate and time as the number of days from 1st January 1900 inclusive by calling VB’sNow function. It just ensures the calculation is done whenever the static trigger changesvalue (or when Excel decides it needs to do a brute-force recalculation of everything onthe sheet). 3The concept of a trigger argument can, of course, usefully be applied to C/C++ add-infunctions too, and is extensively used in later sections of this book.2.11.3 Volatile functionsExcel supports the concept of a volatile function, one whose value cannot be assumed tobe the same from one moment to the next even if none of its arguments (if it takes any) haschanged. Excel re-evaluates cells containing volatile functions, along with all dependents,every time it recalculates, usually any time anything in the workbook changes, or whenthe user presses {F9}.It is easy to create user-defined functions that are optionally volatile (see the VB macroNumCalls_1() in the above section), by using a built-in volatile function as a triggerargument. Additionally, VB and the C API both support ways to tell Excel that an add-infunction should be treated as volatile. With VB, Excel only learns this when it first callsthe function. With the C API a function can be registered as volatile before its first call.Among the standard worksheet functions, there are five volatile functions:• NOW();• TODAY();3 If the trigger were itself the result of a formula, this function might be called even when the value of the triggerhad not changed. See section 2.11.5 User-defined functions (VB Macros) and add-in functions on page 25.