Excel Commands, Functions, and States
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Microsoft Office Excel recognizes two very different types of added functionality: commands and functions.
In Excel, commands have the following characteristics:
They perform actions in the same way that users do.
They can do anything a user can do (subject to the limits of the interface used), such as altering Excel settings, opening, closing, and editing documents, initiating recalculations, and so on.
They can be set up to be called when certain trapped events occur.
They can display dialog boxes and interact with the user.
They can be linked to control objects so that they are called when some action is taken on that object, such as left-clicking.
They are never called by Excel during a recalculation.
They cannot be called by functions during a recalculation.
Functions in Excel do the following:
They usually take arguments and always return a result.
They can be entered into one or more cells as part of an Excel formula.
They can be used in defined name definitions.
They can be used in conditional formatting limit and threshold expressions.
They can be called by commands.
They cannot call commands.
Excel makes a further distinction between user-defined worksheet functions and user-defined functions that are designed to work on macro sheets. Excel does not limit user-defined macro sheet functions only to being used on macro sheets: these functions can be used anywhere a normal worksheet function can be used.
The following is true of Excel worksheet functions:
They cannot access macro sheet information functions.
They cannot obtain the values of uncalculated cells.
They can be written and registered as thread-safe in Excel 2007.
The following is true of Excel macro-sheet functions:
They can access macro sheet information functions.
They can obtain the values of uncalculated cells including the values of the calling cells.
They are not considered thread safe in Excel 2007.
How Excel treats a user-defined function (UDF), what it permits the function to do, and how it recalculates the function are all determined when you register the function. If a function is registered as a worksheet function but tries to do something that only a macro-sheet function can do, the operation fails. In Office Excel 2007, if a worksheet function registered as thread safe tries to call a macro sheet function, again, the operation fails.
Excel treats Microsoft Visual Basic for Applications (VBA) UDFs as macro sheet-equivalent functions, in that they can access workspace information and the value of uncalculated cells, and they are not considered as thread safe in Excel 2007.
Excel can be in one of a number of states at any given time depending on the actions of the user, an external process, a trapped event running a macro, or a timed Excel housekeeping event such as Autosave.
The states that the user experiences are as follows:
Ready state: No commands or macros are being run. No dialog boxes are being displayed. No cells are being edited and the user is not in the middle of a cut/copy and paste operation. No embedded object has focus.
Edit mode: The user has started to type valid input characters into an unlocked or unprotected cell, or has pressed F2 on one or more unlocked or unprotected cells.
Cut/copy and paste mode: The user has cut or copied a cell or range of cells and has not yet pasted them, or has pasted them using the paste-special dialog box, which enables multiple paste operations.
Point mode: The user is editing a formula and is selecting cells whose addresses are added to the formula being edited.
The user can clear the edit, point, and cut/copy modes by pressing the ESC key, which returns Excel to its ready state. Other events can clear these states, such as the following:
The user opens a built-in dialog box.
The user initiates a recalculation.
The user runs a command.
Excel performs an Autosave operation.
A timer event is trapped.
The last example is of importance to add-in developers. You should consider the impact of the normal usability of Excel where frequent timer event traps are being set and executed. When this is an important part of your add-in’s functionality, you should provide users with an easily accessible way of suspending it, so that they can cut/copy and paste normally when they need to.