FLASH: making the call action behave more like a function call

[Damian Morton]

If you begin to use the call action a lot, things can get kind of hairy as

you try to manage passing paramaters to and from called frames. Often, you

end up clobbering variables that you wanted to maintain.


Heres my stab at how to make the call action behave more like a regular

function call.


This set of actionscripts sort of simulates a stack based function call.

There are two clip instances that hold variables at any point in a call

action. The two instances are /var and /call. /var holds local variables.

/call holds the variables you want to pass to the function you are about to

call. The first thing the called function does is Call "/stack:save", which

saves away /var, renames /call to /var, and creates a new, empty, /call

instance. The last thing a function does is to call "/stack:restore" which

basically does the reverse of the "/stack:save" operation.


The upshot of all this juggling is that each "function" now has temporary

local variables in "/var", and a set of variables to communicate to and from

called functions in "/call". If a function calls itself, or calls a function

that calls the caller, each function call has its own set of variables, and

there are no collisions.


Even if you arent using functions calling themselves (recursion), it can be

very usefull to differentiate between temporary local variables, parameters

passed to functions, and variables belonging to clip instances, especially

if there is a chain of functions calling each other.


For the object oriented types out there, clip instances correspond to object

instances, labeled actions correspond to methods, "/var:x" corresponds to a

local variable of a method, "/call:y" corresponds to an in/out parameter,

and "z" corresponds to an object property.


Heres an example of a recursive fibbonacci function implemented in

actionscript.



Library

	Clip: "Stack Manager"


		Label: "init"

			Set Variable: "top" = 1

			Duplicate Movie Clip ("/_frame_", "call", 1)

			Duplicate Movie Clip ("/_frame_", "var", 0)

			Set Property ("/call", Visibility) = 0

			Set Property ("/var", Visibility) = 0

			Stop


		Label: "save"

			Set Variable: "top" = top + 1

			Set Property ("/var", Name) = "_stack_"&top

			Set Property ("/call", Name) = "var"

			Duplicate Movie Clip ("/_frame_", "call", top)

			Set Property ("/call", Visibility) = 0


		Label: "restore"

			Remove Movie Clip ("/call")

			Set Property ("/var", Name) = "call"

			Set Property ("/_stack_"&top, Name) = "var"

			Set Variable: "top" = top - 1



	Clip: "Fibbonacci"


		Label: "fib"

			Comment: fib(n)

			Call ("/stack:save")

			If (/var:n > 1)

				  Set Variable: "/call:n" = /var:n - 1

				  Call ("fib")

				  Set Variable: "/var:fib" = /call:fib * /var:n

			Else

				  Set Variable: "/var:fib" = 1

			End If

			Call ("/stack:restore")



	Clip: "Stack Frame"

		empty


you will need to lay out your movie something like this:


Main Movie

  Movie Clip: Target="/stack" ID="Stack Manager"

  Movie Clip: Target="/fib" ID="Fibbonacci"

  Movie Clip: Target="/_frame_" ID="Stack Frame"



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