This chapter is from the book
This chapter is from the book
This chapter is from the book
Programming Objects
Although this chapter isn't a programming primer per se, I'd like to take some time now to run through a few quick notes about programming objects. This will serve you well throughout the rest of the chapter as I take you on a tour of the Windows Script Host object model.
The dictionary definition of an object is "anything perceptible by one or more of the senses, especially something that can be seen and felt." In scripting, an object is an application element that exposes an interface to the programmer, who can then perform the programming equivalent of seeing and feeling:
- You can make changes to the object's properties (this is the seeing part).
- You can make the object perform a task by activating a method associated with the object (this is the feeling part).
Working with Object Properties
Every programmable object has a defining set of characteristics. These characteristics are the object's properties, and they control the appearance and position of the object. For example, the WScript object (the top-level Windows Script Host object) has an Interactive property that determines whether the script runs in interactive mode or batch mode.
When you refer to a property, you use the following syntax:
Object.Property
|
Object |
The name of the object |
|
Property |
The name of the property with which you want to work |
For example, the following expression refers to the Interactive property of the WScript object:
WScript.Interactive
Setting the Value of a Property
To set a property to a certain value, you use the following syntax:
Object.Property = value
Here, value is an expression that specifies the value to which you want to set the property. As such, it can be any of the scripting language's recognized data types, which usually include the following:
- A numeric value
- A string value, enclosed in double quotation marks (such as "My Script Application")
- A logical value (in VBScript: True or False; in JavaScript: true or false)
For example, the following VBScript statement tells the Windows Script Host to run the script using interactive mode:
WScript.Interactive = True
Returning the Value of a Property
Sometimes you need to know the current setting of a property before changing the property or performing some other action. You can find out the current value of a property by using the following syntax:
variable = Object.Property
Here, variable is a variable name or another property. For example, the following statement stores the current script mode (batch or interactive) in a variable named currentMode:
currentMode = WScript.Interactive
Working with Object Methods
An object's properties describe what the object is, whereas its methods describe what the object does. For example, the WScript object has a Quit method that enables you to stop the execution of a script.
How you refer to a method depends on whether the method requires any arguments. If it doesn't, the syntax is similar to that of properties:
Object.Method
|
Object |
The name of the object |
|
Method |
The name of the method you want to run |
For example, the following statement shuts down a script:
WScript.Quit
If the method requires arguments, you use the following syntax:
Object.Method(Argument1, Argument2, ...)
For example, the WshShell object has a RegWrite method that you use to write a key or value to the Registry. (I discuss this object and method in detail later in this chapter; see "Working with Registry Entries.") Here's the syntax:
WshShell.RegWrite strName, anyValue[, strType]
|
strName |
The name of the Registry key or value |
|
anyValue |
The value to write, if strName is a Registry value |
|
strType |
The data type of the value |
For example, the following statement creates a new value named Test and sets it equal to Foo:
WshShell.RegWrite "HKCU\Software\Microsoft\Windows Script Host\Test", "Foo", "REG_SZ"
Assigning an Object to a Variable
If you're using JavaScript, you assign an object to a variable using a standard variable assignment:
var variableName = ObjectName
|
variableName |
The name of the variable |
|
ObjectName |
The object you want to assign to the variable |
In VBScript, you assign an object to a variable by using the Set statement. Set has the following syntax:
Set variableName = ObjectName
|
variableName |
The name of the variable |
|
ObjectName |
The object you want to assign to the variable |
You'll see later on that you must often use Automation to access external objects. For example, if you want to work with files and folders in your script, you must access the scripting engine object named FileSystemObject. To get this access, you use the CreateObject method and store the resulting object in a variable, like so:
Set fs = CreateObject("Scripting.FileSystemObject")
Working with Object Collections
A collection is a set of similar objects. For example, WScript.Arguments is the set of all the arguments specified on the script's command line. Collections are objects, too, so they have their own properties and methods, and you can use these properties and methods to manipulate one or more objects in the collection.
The members of a collection are elements. You can refer to individual elements by using an index. For example, the following statement refers to the first command-line argument (collection indexes always begin at 0):
WScript.Arguments(0)
If you don't specify an element, the Windows Script Host assumes that you want to work with the entire collection.
VBScript: Using For Each...Next Loops for Collections
As you might know, VBScript provides the For...Next loop that enables you to cycle through a chunk of code a specified number of times. For example, the following code loops 10 times:
For counter = 1 To 10
Code entered here is repeated 10 times
Next counter
A useful variation on this theme is the For Each...Next loop, which operates on a collection of objects. You don't need a loop counter because VBScript loops through the individual elements in the collection and performs on each element whatever operations are inside the loop. Here's the structure of the basic For Each...Next loop:
For Each element In collection
[statements]
Next
|
element |
A variable used to hold the name of each element in the collection |
|
collection |
The name of the collection |
|
statements |
The statements to execute for each element in the collection |
The following code loops through all the arguments specified on the script's command line and displays each one:
For Each arg In WScript.Arguments
WScript.Echo arg
Next
JavaScript: Using Enumerators and for Loops for Collections
To iterate through a collection in JavaScript, you must do two things: create a new Enumerator object and use a for loop to cycle through the enumerated collection.
To create a new Enumerator object, use the new keyword to set up an object variable (where collection is the name of the collection you want to work with):
var enum = new Enumerator(collection)
Then set up a special for loop:
for (; !enumerator.atEnd(); enumerator.moveNext())
{
[statements];
}
|
enumerator |
The Enumerator object you created |
|
statements |
The statements to execute for each element in the collection |
The Enumerator object's moveNext method runs through the elements in the collection, whereas the atEnd method shuts down the loop after the last item has been processed. The following code loops through all the arguments specified on the script's command line and displays each one:
var args = new Enumerator(WScript.Arguments);
for (; !args.atEnd(); args.moveNext())
{
WScript.Echo(args.item());
}