Behaviors · Lessons · OXT-Beyond

One script, many objects

A game has twenty enemies; a form has a dozen fields that check what is typed; a toolbar has eight buttons that light up under the pointer. Copying the same script into each works until you find a bug in it, and must fix it twenty times.

A behavior is the answer. It is an object (usually a button, or a script-only stack) whose script works for every object whose behavior property is set to it. Change the behavior's script, and every object that uses it changes at once.

A first behavior

In a stack you would type the behavior's script into a button with the Script Editor and hide the button. So that the example runs by itself, it sets the script from text, a line at a time:

The stage, with three buttons in a row: Tally 1 says 2 clicks, Tally 2 says 0 clicks, and Tally 3 says 1 click.

Three things to notice:

Inside a behavior, me is the object using it, not the behavior's button. The one script sets each button's own label.
Each object has its own script locals. sClicks counts each button's clicks apart: Tally 1's two clicks did not touch Tally 3's count. (sClicks + 0 shows a count still empty as 0.)
The buttons' own scripts are empty. A message that finds no handler in an object goes on to its behavior, before anything further along the message path.

The object's own handler, then the behavior's

An object can still have a script of its own. Its handlers run first, and a handler that ends with pass hands the message on to the behavior, so an object can add to what all of them do:

ExampleRun it in the playground

create invisible button "Shared"
set the script of it to "on mouseUp" & return & "put the short name of me && " & quote & "does what all buttons do" & quote & return & "end mouseUp"
put it into tBehavior

create button "Special"
set the behavior of it to tBehavior
set the script of it to "on mouseUp" & return & "put " & quote & "Special does its own thing first" & quote & return & "pass mouseUp" & return & "end mouseUp"

send "mouseUp" to button "Special"

Output

Special does its own thing first
Special does what all buttons do

Without the pass, the object's handler would replace the behavior's for that object: a simple way to make one exception.

Before and after

A behavior can also have before and after handlers, which run around the object's own handler, whether or not it passes the message. They suit work that must always happen, such as logging, or a sound on every click:

ExampleRun it in the playground

put "before mouseUp" & return into tScript
put "   put " & quote & "before: the behavior" & quote & return after tScript
put "end mouseUp" & return after tScript
put "after mouseUp" & return after tScript
put "   put " & quote & "after: the behavior" & quote & return after tScript
put "end mouseUp" after tScript
create invisible button "Logger"
set the script of it to tScript
put it into tBehavior

create button "Save"
set the behavior of it to tBehavior
set the script of it to "on mouseUp" & return & "put " & quote & "the button saves" & quote & return & "end mouseUp"
send "mouseUp" to button "Save"

Output

before: the behavior
the button saves
after: the behavior

Behaviors of behaviors

A behavior can have a behavior of its own, which makes a chain: from the most particular to the most general. A message goes along the chain until a handler takes it. Here the general behavior knows how to shout a description, and the particular one says what the description is. A call from the general script still finds the particular handler first, because the search starts again at the object:

ExampleRun it in the playground

put "function describe" & return & "return " & quote & "a thing" & quote & return & "end describe" & return into tScript
put "function shout" & return & "return toUpper(describe()) & " & quote & "!" & quote & return & "end shout" after tScript
create invisible button "Thing"
set the script of it to tScript
put it into tThing

create invisible button "Animal"
set the behavior of it to tThing
set the script of it to "function describe" & return & "return " & quote & "an animal called " & quote & " & the short name of me" & return & "end describe"
put it into tAnimal

create button "Rex"
set the behavior of it to tAnimal
create button "Box"
set the behavior of it to tThing

put value("shout()", button "Rex")
put value("shout()", button "Box")

Output

AN ANIMAL CALLED REX!
A THING!

value with an object works out an expression as if in that object's script, which is one way to call an object's function from outside it. dispatch function is the other (Messages and events).

Behaviors in script-only stacks

In a real project, behaviors usually live in script-only stacks: stacks with nothing but a script, saved as plain text files (.livecodescript) that version control can compare line by line (Shipping your program shows how a program finds them). A behavior's long id can be a stack's as well as a button's:

ExampleRun it in the playground

create script only stack "Greeter"
set the script of stack "Greeter" to "on mouseUp" & return & "put " & quote & "Hello from " & quote & " & the short name of me" & return & "end mouseUp"

create button "Ada"
set the behavior of it to the long id of stack "Greeter"
send "mouseUp" to button "Ada"
put the scriptOnly of stack "Greeter"
delete stack "Greeter"

Output

Hello from Ada
true

A program opens its behavior stacks before the objects that use them: list them in the stack's stackFiles, or start using them as libraries, which the next lesson explains.

Try it yourself

Write one behavior for toggle buttons: a click flips the button's hilite and sets its label to its name followed by on or off. Give it to three buttons, Sound, Music and Hints, click Sound and Hints, and put the three labels.

Show a solution

The stage, with three buttons down the left side, labelled Sound on, Music off and Hints on; Sound and Hints are highlighted.

The solution keeps each button's state in a script local, sOn, rather than reading its hilite back: a real click lights a button up while it is pressed and turns it off again as the mouse comes up, so the hilite says nothing about the toggle by the time mouseUp arrives.

Words in this lesson

behaviormepassbeforeafterscript only stackthe long iddispatch functionvalue

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