Variables and Properties
Variables and properties are introduced in previous chapters in this document. You use them in script
objects to store and manipulate values.
The following sections cover common issues in working with variables and properties, including how to declare them and how AppleScript interprets their scope in a script:
Defining Properties
Property labels follow the rules described in Identifiers.
Property definitions use the following syntax:
property
propertyLabel :
expression
- propertyLabel
An identifier.
- expression
An AppleScript expression that sets the initial value for the property. Property definitions are evaluated before variable assignments, so property definitions cannot contain variables.
The following are examples of valid property definitions:
property windowCount : 0 |
property defaultName : "Barry" |
property strangeValue : (pi * 7)^2 |
After you define a property, you can change its value with the copy
or set
command.
The value set by a property definition is not reset each time the script is run; instead, it persists until the script is recompiled.
You cannot declare a property in a handler but a handler can access a property defined in its containing script
object.
Declaring Variables
Variable names follow the rules described in Identifiers.
To create a variable in AppleScript, you assign it a value using the copy
or set
command. For example, the following statements create and initialize two variables, one named circumference
and one named savedResult
:
set circumference to pi * 3.5 --result: 10.995574287564 |
copy circumference to savedResult --result: 10.995574287564 (copy of 1st variable) |
As shown in this example, a variable assignment can make use of a previously defined variable. It can also make use of properties declared in the same script
object.
There are some obvious, and some more subtle, differences in using copy
and set
to create a variable—see Using the copy and set Commands for more information.
If you assign a new value to a variable that is already in use, it replaces the old value. You can assign a simple value, an expression, or an object specifier—expressions are evaluated and object specifiers are resolved to obtain the value to assign. To create a variable whose value is an object specifier itself, rather than the value of the object specified, use the a reference to
operator.
The next two sections describe how you can explicitly define a local
or a global
variable. These variable types differ primarily in their scope. Scope, which refers to where a variable is accessible within a script, is described in detail in Scope of Variables and Properties.
Local Variables
You can declare explicit local
variables using the following syntax:
local
variableName [,
variableName ]…
- variableName
An identifier.
The following are examples of valid local
variable declarations:
local windowCount -- defines one variable |
local agentName, agentNumber, agentHireDate -- defines three variables |
You cannot assign an initial value to a local
variable in its declaration, nor can you declare a class for the variable. Instead, you use the copy
or set
command to initialize a variable and set its class. For example:
set windowCount to 0 -- initialize to zero; an integer |
set agentName to "James Smith" -- assign agent name; a text string |
set agentNumber to getAgentNumber(agentName) -- call handler; an integer |
copy current date to agentHireDate -- call current date command; a date |
Global Variables
The syntax for global
variables is nearly identical to that for local
variables:
global
variableName [,
variableName ]…
- variableName
An identifier.
The following are examples of valid global
variable declarations:
global gAgentCount |
global gStatementDate, gNextAgentNumber |
As with local
variables, you use the copy
or set
command to initialize global
variables and set their class types. For example:
set gAgentCount to getCurrentAgentCount() -- call handler to get count |
set gStatementDate to current date -- get date from current date command |
set gNextAgentNumber to getNextAvailNumber() -- call handler to get number |
Using the copy and set Commands
As its name implies, when you use the copy
command to create a variable, it always creates a separate copy (though note that a copy of an object specifier still specifies the same object). However, when you use the set
command to create a variable, the new variable always refers to the original object or value. You have essentially created another name for the same object.
When more than one variable refers to a changeable (or mutable) object, a change to the object is observable through any of the variables. The types of AppleScript objects that are mutable are date
, list
, record
, and script
objects.
For objects that cannot be modified (immutable objects), variables created with the set
command may seem like copies—there’s no way to change the object the variables point to, so they seem independent. This is demonstrated in the example in the next section that creates the variables myName
and yourName
.
Declaring Variables with the set Command
You can use the set
command to set a variable to any type of object. If the variable doesn’t exist, it is created; if it does exist, its current value is replaced:
set numClowns to 5 --result: 5 |
set myList to { 1, 2, "four" } --result: {1, 2, "four"} |
tell application "TextEdit" |
set word1 to word 1 of front document --result: some word |
end tell |
The following example uses a mutable object. It creates two variables that refer to the same list, then modifies the list through one of the variables:
set myList to { 1, 2, 3 } |
set yourList to myList |
set item 1 of myList to 4 |
After executing these statements, the statements item 1 of myList
and item 1 of yourList
both yield 4
, because both variables refer to the same list.
Now suppose you’re working with an immutable object, such as a text
object:
set myName to "Sheila" |
set yourName to myName |
Both variables refer to the same text
object, but text
objects are not mutable, so there is no way to change the the value myName
such that it affects the value of yourName
. (If you assign new text to one of the variables, you are just creating a new, separate text
object.)
The set
command can assign several variables at once using a pattern, which may be a list or record: a list or record of variables on one side, and a list or record of values on the other. Values are matched to variables based on their position for a list, or based on their keys for a record. Not having enough values is an error; if there are too many values, the extra ones are ignored. The order in which the values are evaluated and the variables are assigned is unspecified, but all values are evaluated before any assignments are made.
The Examples section of the set
command shows some simple pattern assignments. Here is an example with more complex patterns:
set x to {8, 94133, {firstName:"John", lastName:"Chapman"}} |
set {p, q, r} to x |
(* now p, q, and r have these values: |
p = 8 |
q = 94133 |
r = {firstName:"John", lastName:"Chapman"} *) |
set {p, q, {lastName:r}} to x |
(* now p, q, and r have these values: p = 8 |
q = 94133 |
r = "Chapman" *) |
In the final assignment statement above, {lastName:r}
is a record that hasn’t been used before in the script, and contains an item with label lastName
and value r
(a previously defined variable). The variable x
has previously been set to have a record that has an item with label lastName
and value "Chapman"
. During the assignment, the value of the item labeled lastName
in the new record is set to the value of the item labeled lastName
in x
—hence it now has the value "Chapman"
.
As this example demonstrates, the properties of a record need not be given in the same order and need not all be used when you set a pattern to a pattern, as long as the patterns match. For details, see the set
command.
Declaring Variables with the copy Command
You can use the copy
command to set a variable to any type of object. If the variable doesn’t exist, it is created; if it does exist, its current value is replaced. The copy
command creates a new copy that is independent of the original—a subsequent change does not change the original value (though note that a copy of an object specifier still specifies the same object).
To copy within an application, you should use the application’s duplicate
command, if it has one. To copy between applications, you can use the get
command to obtain information from one application and the set
command to set it in another.
The copy
command creates a deep copy—that is, if you copy a nested data structure, such as a list that contains another list, the entire structure is copied, as shown in the following example. This example creates a record (alpha
), then a list (beta
), then a list that contains the first record and list (gamma
), then finally a copy of gamma
(delta
). It then changes a property in the original record, alpha
. The result shows that the property is changed wherever alpha
appears, except in the copy, delta
:
set alpha to {property1:10, property2:20} |
set beta to {1, 2, "Hello"} |
set gamma to {alpha, beta, "Goodbye"} |
copy gamma to delta |
set property1 of alpha to 42 |
{alpha, beta, gamma, delta} -- List variables to show contents |
(*result: {{property1:42, property2:20}, {1, 2, "Hello"}, {{property1:42, property2:20}, {1, 2, "Hello"}, "Goodbye"}, {{property1:10, property2:20}, {1, 2, "Hello"}, "Goodbye"}} *) |
If you make a copy of a reference
object, it refers to the same object as the original (because both contain the same object specifier):
set windowRef to a reference to window 1 of application "Finder" |
name of windowRef --result: "Script testing folder" |
copy windowRef to currentWindowRef --result: a new object specifier |
name of currentWindowRef --result: "Script testing folder" |
Scope of Variables and Properties
The declaration of a variable or property identifier is the first valid occurrence of the identifier in a script
object. The form and location of the declaration determine how AppleScript treats the identifier in that script
object.
The scope is the range over which AppleScript recognizes a declared identifier within a script
object. The scope of a variable depends on where you declare it and whether you declare it as global
or local
. The scope of a property extends to the entire script
object in which it is declared. After declaring a property, you can reuse the same identifier as a separate variable only if you first declare it as a local
variable.
Lifetime refers to the period of time over which a variable or property is in existence. Only the values of properties and global
variables can persist after a script is run.
In the discussions that follow, declarations and statements in a script
object that occur outside of any handlers or nested script
objects are identified as outside.
The following examples show the four basic forms for declaring variables and properties in AppleScript:
property x: 3
The scope of a property definition is the
script
object in which it is declared, including any handlers or nestedscript
objects. A property definition specifies an initial value. You cannot declare a property in a handler.The value set by a property definition is not reset each time the script is run; instead, it persists until the script is recompiled.
global x
The scope of a
global
variable can be limited to specific handlers or containedscript
objects or it can extend throughout a top-levelscript
object. Aglobal
declaration doesn’t set an initial value—it must be initialized by acopy
orset
command before a script can access its value.The value of a
global
variable is not reset each time a script is run, unless its initialization statement is executed.local x
The scope of a
local
variable can be limited to specific handlers or containedscript
objects or it can extend throughout a top-levelscript
object. Alocal
declaration doesn’t set an initial value—it must be initialized by acopy
orset
command before a script can access its value.The value of a
local
variable is reset each time the handler is run (either therun
handler for the script, or the specific handler in which the variable is declared).set x to 3
In the absence of a
global
variable declaration, the scope of a variable declared with thecopy
orset
command is normally restricted to therun
handler for the script, making it implicitly local to that run handler. However, a handler or nested script object can declare the same variable with aglobal
declaration to gain access to it.The value of a variable declared with the
copy
orset
command is reset each time a script is run.
If you want to use the same identifier in several different places in a script, you should either declare it as a property or as a global
variable.
It is often convenient to limit the scope of a particular identifier to a single handler or nested script
object, which you can do by defining it as a local
variable in the handler or script
object. Outside, the identifier has no value associated with it and can be reused elsewhere in the script. When used this way, a local
variable is said to shadow (or block access to) a global
variable or property with the same name, making the global version inaccessible in the scope of the handler or script
object where the local
variable is declared.
The following sections provide additional information about scope:
Scope of Properties and Variables Declared in a Script Object
Table 3-1 shows the scope and lifetime for variables and properties that are declared at the top level in a script
object (outside any handlers or nested script
objects).
Declaration type | Scope (visibility) | Lifetime |
---|---|---|
| Everywhere in script | Reset when script is recompiled |
| Everywhere in script | Reset when reinitialized in script or when script is recompiled |
| Within | Reset when script is run |
| Within | Reset when script is run |
The scope of a property in a script
object extends to any subsequent statements anywhere in the script. Consider the following example:
property currentCount : 0 |
increment() |
on increment() |
set currentCount to currentCount + 1 |
display dialog "Count is now " & currentCount & "." |
end increment |
When it encounters the identifier currentCount
anywhere in this script, AppleScript associates it with the currentCount
property.
The value of a property persists after the script in which the property is defined has been run. Thus, the value of currentCount
is 0 the first time this script is run, 1 the next time it is run, and so on. The property’s current value is saved with the script
object and is not reset to 0 until the script is recompiled—that is, modified and then run again, saved, or checked for syntax.
The value of a global
variable also persists after the script in which it is defined has been run. However, depending on how it is initialized, a global
variable may be reset each time the script is run again. The next example shows how to initialize a global
variable so that it is initialized only the first time a script is run, and thus produces the same result as using a property in the previous example:
global currentCount |
increment() |
on increment() |
try |
set currentCount to currentCount + 1 |
on error |
set currentCount to 1 |
end try |
display dialog "Count is now " & currentCount & "." |
end increment |
The first time the script is run, the statement set currentCount to currentCount + 1
generates an error because the global
variable currentCount
has not been initialized. When the error occurs, the on error
block initializes currentCount
. When the script is run again, the variable has already been initialized, so the error branch is not executed, and the variable keeps its previous value. Persistence is accomplished, but not as simply as in the previous example.
If you don’t want the value associated with an identifier to persist after a script is run but you want to use the same identifier throughout a script, declare a global
variable and use the set
command to set its value each time the script is run:
global currentCount |
set currentCount to 0 |
on increment() |
set currentCount to currentCount + 1 |
end increment |
increment() --result: 1 |
increment() --result: 2 |
Each time the on increment
handler is called within the script, the global
variable currentCount
increases by 1. However, when you run the entire script again, currentCount
is reset to 0.
In the absence of a global
variable declaration, the scope of a variable declaration using the set
command is normally restricted to the run
handler for the script. For example, this script declares two separate currentCount
variables:
set currentCount to 10 |
on increment() |
set currentCount to 5 |
end increment |
increment() --result: 5 |
currentCount --result: 10 |
The scope of the first currentCount
variable’s declaration is limited to the run
handler for the script. Because this script has no explicit run
handler, outside statements are part of its implicit run
handler, as described in run Handlers. The scope of the second currentCount
declaration, within the on increment
handler, is limited to that handler. AppleScript keeps track of each variable independently.
To associate a variable in a handler with the same variable declared with the set
command outside the handler, you can use a global
declaration in the handler, as shown in the next example. (This approach also works to associate a variable in a nested script
object.)
set currentCount to 0 |
on increment() |
global currentCount |
set currentCount to currentCount + 1 |
end increment |
increment() --result: 1 |
currentCount --result: 1 |
To restrict the context of a variable to a script’s run
handler regardless of subsequent global
declarations, you must declare it explicitly as a local
variable, as shown in this example:
local currentCount |
set currentCount to 10 |
on increment() |
global currentCount |
set currentCount to currentCount + 2 |
end increment |
increment() --error: "The variable currentCount is not defined" |
Because the currentCount
variable in this example is declared as local to the script, and hence to its implicit run
handler, any subsequent attempt to declare the same variable as global
results in an error.
If you declare an outside variable with the set
command and then declare the same identifier as a property, the declaration with the set
command overrides the property definition. For example, the following script returns 10, not 5. This occurs because AppleScript evaluates property definitions before it evaluates set
command declarations:
set numClowns to 10 -- evaluated after property definition |
property numClowns: 5 -- evaluated first |
numClowns --result: 10 |
The next example, shows how to use a global
variable declaration in a script
object to associate a global
variable with an outside property:
property currentCount : 0 |
script Paula |
property currentCount : 20 |
script Joe |
global currentCount |
on increment() |
set currentCount to currentCount + 1 |
return currentCount |
end increment |
end script |
tell Joe to increment() |
end script |
run Paula --result: 1 |
run Paula --result: 2 |
currentCount --result: 2 |
currentCount of Paula --result: 20 |
This script declares two separate currentCount
properties: one outside any handlers (and script
objects) in the main script and one in the script
object Paula
but outside of any handlers or script
objects within Paula
. Because the script Joe
declares the global
variable currentCount
, AppleScript looks for currentCount
at the top level of the script, thus treating Joe’s currentCount
and currentCount
at the top level of the script as the same variable.
Scope of Variables Declared in a Handler
A handler can’t declare a property, although it can refer to a property that is declared outside any handler in the script
object. (A handler can contain script objects, but it can’t contain another handler, except in a contained script object.)
Table 3-2 summarizes the scope of variables declared in a handler. Examples of each form of declaration follow.
Declaration type | Scope (visibility) | Lifetime |
---|---|---|
| Within handler only | Reset when script is recompiled; if initialized in handler, then reset when handler is run |
| Within handler only | Reset when handler is run |
| Within handler only | Reset when handler is run |
The scope of a global
variable declared in a handler is limited to that handler, although AppleScript looks beyond the handler when it tries to locate an earlier occurrence of the same variable. Here’s an example:
set currentCount to 10 |
on increment() |
global currentCount |
set currentCount to currentCount + 2 |
end increment |
increment() --result: 12 |
currentCount --result: 12 |
When AppleScript encounters the currentCount
variable within the on increment
handler, it doesn’t restrict its search for a previous occurrence to that handler but keeps looking until it finds the declaration outside any handler. However, the use of currentCount
in any subsequent handler in the script is local to that handler unless the handler also explicitly declares currentCount
as a global
variable.
The scope of a local
variable declaration in a handler is limited to that handler, even if the same identifier has been declared as a property outside the handler:
property currentCount : 10 |
on increment() |
local currentCount |
set currentCount to 5 |
end increment |
increment() --result: 5 |
currentCount --result: 10 |
The scope of a variable declaration using the set
command in a handler is limited to that handler:
script Henry |
set currentCount to 10 -- implicit local variable in script object |
on increment() |
set currentCount to 5-- implicit local variable in handler |
end increment |
return currentCount |
end script |
tell Henry to increment() --result: 5 |
run Henry --result: 10 |
The scope of the first declaration of the first currentCount
variable in the script
object Henry
is limited to the run
handler for the script
object (in this case, an implicit run
handler, consisting of the last two statements in the script). The scope of the second currentCount
declaration, within the on increment
handler, is limited to that handler. The two instances of currentCount
are independent variables.
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