introduction to the windows api n api - application programming interface n an api is the software...
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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