Introduction to the Windows API API - Application Programming Interface an API is the software interface for

things such as the OS an API is the fundamental level of

higher-level programming in high-level programming, a program

has an intermediate to execute tasks Microsoft Word does not directly control

the printing it puts a print task on the OS’s print queue (an API call)

Benefits of API Programming

Using API calls in Visual Basic can cut down on the number of dependency files and thus the size of the application package which you need to deploy to users.

Sometimes now and especially in previous versions of VB API calls were the only way to do certain things like starting and running executable files to do this now use the FSO File System Objects in the Scripting Runtime Library

Benefits of API Programming Designers of software do not have to

worry about the basic chores involved in every program (such as disk access, memory allocation, displaying graphics

Every time there is an improvement in the way hardware is accessed each program would have to be rewritten to take advantage of these changes.

API calls are more powerful than VB methods Powerful meaning can do more / has more features

API Drawbacks

API functions are significantly more error-prone

API functions are prone to fail spectacularly - shut down with a GPF

Visual Basic comes with almost no API documentation

syntactically challenging for non C++ programmers

Where is the Windows API? C:\Windows\System directory

Win32 API another term for these DLLs user32.dll (user interface functions)

most functions we use are in here kernel32.dll (o.s. kernel functions) gdi32.dll (graphics device interface

functions) shell32.dll (Windows shell functions).

Accessing the Microsoft Windows API

The Windows API contains thousands of functions, subs, types, and constants that you can declare and use in your projects

These procedures are written in the C language so they must be declared before you can use them with VB

the easiest way to access the Windows API is by using the predefined declares included with Visual Basic

API Viewer Application

API Viewer is a VB Add-In. It uses Win32api.txt, located in the \Winapi subdirectory of the main VB directory. You need to load this file!

The viewer allows you to search through this text file to put together a series of dependant function calls.

Sometimes you need to use functions that get values for the function you’re really interested in using

Components of the Win API Functions - provide API functionality Structures multiple individual variables

passed between functions Named Constants - numeric codes for

information Callback Functions defined completely

in your program a way to process each item found belonging to the group

Messages are sent to objects to tell them to do something

Declaring the Function

Before an API function can be using in Visual Basic, it must first be declared

The Declare statement can only appear in the (declarations) section of a form or module

If it appears in a form, the declaration must be Private

In a module, the declaration can be either Public or Private

[{Public | Private}] Declare Function function_name Lib "DLL_filename" [Alias "function_alias"] (argument_list) As data_type

function_name - safest to make this the same as the"official" name

DLL_filename - name of the DLL file which stores the function. This does not include the path

function_alias almost every function which has a string as a parameter has two versions ANSI or Unicode for

English speakers use ANSI ANSI version ends with the letter A Unicode version ends with the letter W

argument_list same as VB data_type the return type

almost always a long

[{Public | Private}] Declare Function function_name Lib "DLL_filename" [Alias "function_alias"] (argument_list) As data_type

[{ByVal | ByRef}] argument_name As data_type, ...

argument_name - gives clue as to what the argument represents can use any name but best to use “official” name

data type specifies the size and format Allowed Data types

Byte An 8-bit integer. Integer A 16-bit integer. Long A 32-bit integer. String A variable-length string.

ByVal and ByRef

the method used to pass a parameter to the API function

ByVal This method prevents the function from altering the contents

ByRef this method passes a sort of reference to the variable itself

Strings are always passed ByVal structures are always passed ByRef Entire Arrays are always passed ByRef

hDC & hWnd– hWnd Handle A unique 32 bit integer

defined by the operating environment and used by a program to identify and switch to an object, such as a form or control.

– hDC device context similar in appearance to handles can be the intermediary between your program and a physical device also windows themselves are considered to be devices need hDC to draw (use graphical methods) on an object

Pointers and Flags

pointer a 32-bit integer variable which holds a memory address usually the location of some other object

VB has little support for pointers 99.5% of the time Visual Basic handles pointer for you

A flag is simply a type of named constant. The special thing about flags is that they can be combined with other related flags like (shift control alt) mask

Using API Structures

Structures allow a function to receive or return a large amount of information without cluttering the argument list

Structures almost always group related information

To define a structure in Visual Basic, the Type block is used

[(Public | Private}] Type type_name member1 As data_type1 member2 As data_type2 ...End Type

type_name The name of the structure member1, member2, ...

– The name of an individual member of the structure

data_type1, data_type2, ... – The data type of a particular item in the structure

Type EXAMPLESTRUCT longvar As Long another As Long astruct As RECTEnd Type

To access a data member of the structure use the . (period) operator between the variable name and the member name

notice one of the members is another structure the RECT angle structure

defined a variable to use the structure Dim ex As EXAMPLESTRUCT

ex.longvar = 54 ‘ store 54 here

Rect is a predefined structure

If you want to use it you must define it like this Type Rect

left As Long top As Long

right As Long bottom As Long End Type

Rect a convenient way to keep the necessary coordinates of a rectangle grouped together

Using API Callback Functions

a powerful tool, giving great flexibility to some API functions

allows your program to build its own routines to handle events generated by the API functions themselves

Windows does not define any "default" callback functions

The most common examples of callback functions occur in enumeration

Enumeration

During an enumeration, the invoked API function locates all objects which fit the desired category

However, the API function does not know what to do with all the handles it finds

Callback functions typically process some data with these handles during the middle of a API function call

The AddressOf Operator

The only specific pointer in VB It is a pointer to an address of a

function defined by your program This function must be Public and be

defined in a module (not a form). can only be used inside of the

argument list of a call to a function; it cannot be used any other time