| THE FLASH 4 BIBLE (IDG BOOKS) |
| FLASH 4 ACTIONSCRIPT EXPRESSIONS |
flash 4 actionscript expressions
Flash uses the term "expression" to refer to two separate kinds of code fragments
in ActionScript. An expression is either, 1) a phrase of code used to compare
values in a Conditional or a Loop (these are known as "conditional expressions")
or 2) a snipit of code that is interpreted at run-time (these are known as "numeric
expressions" and "string expressions").
Numeric and string expressions are essentially just segments of ActionScript
code that are dynamically converted to their calculated values when a movie
runs. For instance, suppose you have a variable, y, set to a value of 3. In
the statement x = y + 1, the "y + 1" on the right side of the equal
sign is an expression. Hence, when the movie runs, the statement x = y
+ 1 actually becomes x = 4, because the value of y
(which is 3) is retrieved (or "interpreted") and the calculation
3 + 1 is performed. Numeric and string expressions are an extremely
potent part of ActionScript because they allow nearly any of the options for
Actions to be set based on mathematical calculations and external variables
rather than requiring fixed information. Consider these two examples: 1) the
"Frame" option of a Go to Action could be set as an expression that returns
a random number in a certain range, sending the movie to a random frame and
2) the URL option in a Get URL Action could be made up of a variable
that indicates a server name and a literal string which is the file path-to
change all the URLs in your movie from a staging server to a live server you'd
just have to change the value of the server variable. Anywhere that you see
the word "expression" in any Action options or you see the little "abc" button,
you can use an interpreted ActionScript expression to specify the value of the
option. Just enter the code, then click the "abc" button and select the expression
setting, the two lines (=) or shift click.
Within the context of ActionScript code itself, that little "abc" button plays
an extremely important role: if you write an expression in a text field but
do not change the button to the expression setting, the code will not be executed.
Instead, the actual string of text that makes up the code segment will be used
as the value of the parameter in question. Even if you just want to assign one
variable value to another variable, you must set the button to expression so
that the second variable is interpreted. Here's an example: suppose you have
two variables: x = 5, and y = 10. If you use Set
Variable to make x = y but forget to set the "abc" button
to expression, you're actually literally setting x equal to the letter "y",
not to the number "10". In your code, you can see the difference: expressions
are not quoted, and strings are quoted (eg. x = "y" sets x to the
string "y", but x = y sets x to the value of the variable
y). Be especially careful to set the "abc" button correctly when
assigning dynamic variable values and when using the Trace Action to return
variable values.
To use a string inside an expression, simply add quotation marks around it.
Anything surrounded by quotation marks will be taken as a literal string. For
example, the conditional: if (status eq ready) will wrongly check
to see if the value of the variable status is the same as the value
of the non-existent variable ready. The correct conditional would
check to see if the value of status is the same as the string "ready" by quoting
it, as in: if (status eq "ready").
about this info
this documentation is a short excerpt from colin moock's section of the 600
page book "the flash 4 bible", (idg books). to order, or for general
information about the book, visit colin's flash 4 bible
page.