Transcript Chapter 7 - Emunix Emich

1
Chapter 7
VBA Modules,
Functions, Variables,
and Constants
7
Chapter Objectives
• Design VBA code that is organized into
standard modules, independent class
modules, and form and report class modules
• Determine when each type of module is
appropriate
• Develop simple sub procedures, functions,
and property procedures
7
Chapter Objectives
• Determine when a sub procedure or a
function is most appropriate
• Declare arguments, variables, and constants
• Invoke procedures and pass arguments
between procedures
• Cancel and manage events through VBA
7
Modules
• Procedure
 Collection of VBA statements combined in a
manner to accomplish a processing task
• Module
 Group of procedures related to a specific
process
 Makes maintaining the application easier in the
long run
7
Modules
• Form and report class modules
 Include code for all event procedures triggered by
events occurring on a specific form or report, or the
controls on that form or report
Figure 7-1
Procedures
within
modules
7
Modules
• Standard module
 Module you can place sub and function procedures and
declarations in to make available to other procedures
throughout your application
 Procedures can be executed whenever the application is
running
• Independent class modules
 Not connected to a form or report
 Procedures inside an independent class module can be
executed as long as the application is open
7
Selecting Module Types
• Selection of the best module type for a
procedure depends on:
 Runtime efficiency of the application
 Amount of primary memory expected on the
computer executing the application
 Number of forms and reports that use a
procedure
7
Selecting Module Types
• Standard modules and independent class modules
are loaded into primary memory when an
application opens
 Use primary memory even when they are not needed,
yet the time needed to load the module occurs once
• Form and class modules are loaded into primary
memory when the form or report opens
 Do not use memory until form or report is opened, but
the form or report takes longer to open because module
must be loaded
7
Sub Procedures
• Sub procedures
 Containers for programming code
 Invoked at any time by writing a statement
within a different procedure to invoke the
necessary sub procedure
 Place its name, and if needed, the values for its
arguments on a line within the invoking procedure
7
Functions
• Functions
 Return values
 Invoked by placing it in an expression or using
it as a value to display in a control
 Application can use the value returned by a function
in the expression
7
Property Procedures
• Property procedure
 Defines a property that is attached to an object
through dot notation
 Activated by assigning or using the value of a
property
 Used if the procedure will correspond to
something that could be a property
7
Creating and Using Sub
Procedures
Table 7-1
Data types in
VBA and their
corresponding
types in
Access tables
7
Creating and Using Sub
Procedures
Table 7-1
Data types in
VBA and their
corresponding
types in
Access tables
(continued)
7
Passing Arguments by
Reference and by Value
• When arguments are processed by reference:
 The memory address containing the value to be
used is “referenced” by the argument
• When arguments are processed by value:
 A copy of the value, instead of a reference to
the memory location containing the value, is
supplied to the procedure
7
Optional Keyword
• Optional keyword
 Can be used with arguments
 Specifies that the invoking procedure does not
need to provide a value for the argument
• Named arguments approach
 Arguments can be placed in any order and
commas do not have to be used to represent
blank arguments
7
Nuances of Argument Use
• A procedure must be invoked using the
same number of actual arguments as formal
arguments in the sub statement, unless
optional arguments are used
 The actual arguments will supply values for the
formal arguments in the order in which they are
listed, so the data types must be compatible
7
Event Procedure Arguments
• Event procedures
 Special types of sub procedures located in form and
report class modules
 Most do not take arguments, but some have one or
more predefined arguments
• Flags
 Arguments that contain values that Access evaluates
during and after the procedure executes to determine
how to continue the application’s processing
7
Event Procedure Arguments
Table 7-2 Event procedures with single arguments
7
Event Procedure Arguments
• Intrinsic constants
 Represent the legal values for arguments that
specify actions other than canceling events
 True
 Intrinsic constant that represents value –1
7
Event Procedure Arguments
Table 7-3 Event
procedure with
multiple or
special
arguments
7
Event Procedure Arguments
Table 7-3 Event
procedure with
multiple or
special
arguments
(continued)
7
Creating and Using Functions
• Function Procedures
 Similar to sub procedures
 Use formal arguments, can be declared as Public or
Private
 Can contain variable and constant declaration
statements
• In a function procedure, the function is assigned a
value
 Assignment is fundamental difference between sub
procedures and functions
7
Property Procedures –
Let, Set, and Get
• Property Let and Set procedures
 Accept information from an invoking
procedure, but do not return a value
• Property Get procedures
 Returns a value to the calling procedure
• Property Let and Get usually appear in pairs
7
Property Procedures – Let,
Set, and Get
Figure 7-2 Property
procedures for
frmIDSearchWindow
7
Declaring Variables
• Dim Statements
 Define variables that will be used within
procedures
• Variables are frequently used to store
immediate results as execution of the
procedure moves from one line to another
7
Declaring Variables
Figure 7-3 Part of
txtPaymentNo_BeforeUpdate
in frmProcessPayment
7
Declaring Variables
• Private
 Used in the Declarations section of a module to
declare variables that are used only within the
procedure contained by the module
• Public
 Used in the Declarations section to declare
variables that can be used by any procedure in
any module
7
Declaring Variables
• Static
 Used in a procedure to declare variables that
retain their value even after the procedure
finishes executing
 May be declared only within procedures
7
The Variant Data Type
• Variant data type
 Can contain almost any type of data, including
numbers, strings, and dates
 Only data type in VBA that can store null and
empty values
• VarType built-in function
 Used to determine the type of data stored in a
Variant
7
The Variant Data Type
Table 7-4
Values
returned by
VarType and
variable
testing
functions
7
The User-Defined Type
• VBA provides a way to create a user-defined
type with a Type statement
 Allows you to define a complex data structure
 Useful when the values of many similar
variables need to be copied to other variables
7
Single- and MultipleDimension Arrays
• Arrays
 Collections of variables given a single name but
indexed by one or more values
• Array indexes begin at zero
• ReDim statement
 Can change the size of the array
7
Scope, Life, and the Dim
Statement
• Where and how variables are declared
determine which other procedures can use
the variable
 Scope of a variable
• How long the variable actually exists
 Life of a variable
7
Scope, Life, and the Dim
Statement
Table 7-5 Variable and constant scope prefixes
7
Scope, Life, and the Dim
Statement
Figure 7-4 Procedure that facilitates the reuse of values
7
Scope, Life, and the Dim
Statement
• Any variable declared in the Declarations section
of a module maintains its stored value as long as:
 The application continues to run or
 The form or report remains open
• Variables declared with the Dim statement in a
procedure are available to that procedure only as
long as the procedure is running
7
Scope, Life, and the Static
Statement
• Variables declared with the Static statement
in a procedure continue to exist after the
procedure finishes
• Programmers frequently use static variables
to determine whether a newly entered value
is the same as a previous value
7
Intrinsic and Symbolic
Constants
• Constant
 Named item that retains the same value
throughout the execution of a program
• Intrinsic constant
 Integer values that have some particular
meaning
 Makes programming easier for the developer
and facilitates program maintenance
7
Intrinsic and Symbolic
Constants
• Symbolic constant
 Constant that is defined by the programmer
 Retains a constant value throughout the
execution of a procedure
 Similar to a variable except that its value is
specified as soon as the constant is declared
7
Intrinsic and Symbolic
Constants
Figure 7-5 Public constants in the Declarations section of the
basUtilities module
7
Controlling Processing in the
Declarations Section
• Option Compare statement
 Automatically included in Declarations section
 Defines the default method to use when
comparing text data
• Three types of methods used when
comparing text data:
 Binary method
 Comparisons will be case-sensitive
7
Controlling Processing in the
Declarations Section
• Three types of methods used when
comparing text data (continued):
 Text method
 Comparisons will not be case sensitive
 Database method
 Comparisons are based on character’s relative
positions according to the sort order specified when
the database was created or compacted
7
Controlling Processing in the
Declarations Section
• If the Declarations section of the module
containing the procedure contains an Option
Explicit statement:
 Variables must be explicitly defined prior to
their use
• If it does not:
 Variables do not need to be declared before they
are used
7
Chapter Summary
• VBA statements are written inside functions
procedures, sub procedures, and property
procedures
• Standard modules
 Contain functions and sub procedures
 Loaded into memory as soon as the application opens
• Form and Report class modules
 Contain all three types of procedures
7
Chapter Summary
• Functions differ from sub procedures in two
ways
 A function returns a value
 A function has an associated data type
• Functions should be written in cases where
a value is being returned
7
Chapter Summary
• All procedures have arguments
• Argument values can be passed
 by reference
 Changes made to the value passed into the
procedure are reflected in the environment where
the procedure was originally invoked
 by value
 Make a copy of the actual argument value available
to the procedure
7
Chapter Summary
• Variables
 Can be local or they can be public
 Usually only available while the code in which
they have been declared is executing
 Deleted from memory unless they have been
declared as static
 May be declared as one of a number of
different data types
7
Chapter Summary
• Arrays
 Collections of indexed variables with a
common name, and user defined types, which
allow programmers to create their own data
structures
• Constants
 Help make the program code more
understandable