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Introduction to Messages as to vc++

王朝vc·作者佚名  2006-01-10
窄屏简体版  字體: |||超大  

The computer is a machine that only follows

instructions. It almost doesn't know anything. Because of this, the

computer cannot predict what a user wants to do with the computer. In

fact, a great deal of the responsibility is left to the programmer who

must decide what can and cannot or should not be done on an application. To help

the users with computer interaction, the operating system provides a

series of objects called Windows controls. The programmer decides what

objects are necessary for a given application.

Each computer application is equipped with Windows

controls that allow the user to interact with the computer. Because the

computer cannot and would not predict what the user wants to do when using

the computer, the operating system lets each object tell it when it needs

something from Windows. To do this, a control sends a message to

the operating system every time something is new. Because there can be so many

messages a control can send and because many controls can send various

messages, there is a formula each message or almost every one of them must

follow, just like there are rules the post office wants you to follow in

order to send a letter.

A message to Windows must provide four pieces of

information:

WHO sent the message? Every object you will need in your program,

just like everything in the computer, must have a name. The operating

system needs this name to identify every object, for any reason. An

object in Microsoft Windows is identified as a Handle. For Windows

controls, the handle is called HWND WHAT message? The object that sends a message must let the operating

system know what message it is sending. As we will learn, there are

various types of messages for different circumstances. Nevertheless,

to make matters a little easier, each message is a constant positive

natural number (unsigned int) identified with a particular name.

Therefore, the message identifier is passed as UINT Accompanying items: Because there are so many types of messages, you

must provide two additional pieces of information to help process the

message. These two items depend on the type of message and could be

anything. The first accompanying item is a 32-bit type (unsigned int)

called WPARAM (stands for WORD Parameter; in other

words, it is a WORD (unsigned int) argument). The second accompanying item is a 32-bit type

of value (long) calle LPARAM (stands for LONG Parameter; in

other words, it is a LONG (long in C/C++) argument). Remember that these two can be different things

for different messages.

To manage the messages sent to Windows, they are

communicated through a function pointer called a Windows procedure. The

name of the function is not important but it must return a 32-bit integer,

in fact a C/C++ long or Win32 LONG.

Therefore, it is declared as LRESULT (LONG Result). Because this is a function pointer,

it must be declared and defined as CALLBACK . The messages can

be carried in a function defined as follows:

LRESULT CALLBACK MessageProcedure (HWND hWnd , UINT uMsg , WPARAM wParam , LPARAM lParam );

To process the messages, and because there can be so

many of them, this function typically uses a switch control to list

all necessary messages and process each one in turn. After processing a

message, its case must return a value indicating that the message was

successfully processed or not.

No matter how many messages you processed, there will

still be messages that you did not deal with. It could be because they

were not sent even though they are part of the Windows controls used on an

application. If you didn't process some messages, you should/must let the

operating system know so it can take over. What happens is that the

operating system is aware of all messages and it has a default behavior or

processing for each one of them. Therefore, you should/must return a value

for this to happen. The value returned can be placed in the default

section of the switch condition and must simply be a DefWindowProc()

function. Its syntax is:

LRESULT DefWindowProc(HWND hWnd , UINT uMsg , WPARAM wParam , LPARAM lParam );

This function is returned to Windows, saying

"There are messages I couldn't process. Do what you want with

them". The operating system would simply apply a default processing

to them. The values returned by the DefWindowProc() function should be the

same passed to the procedure.

The most basic message you can process is to make sure

a user can close a window after using it. This can be done with a function

called PostQuitMessage() . Its syntax is:

VOID PostQuitMessage(int nExitCode )

This function takes one argument which is the value of

the LPARAM argument. To close a window, you can pass the argument as

WM_QUIT .

Based on this, a simple Windows procedure can be

defined as follows:

LRESULT CALLBACK WndProcedure(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam)

{

switch(Msg)

{

case WM_DESTROY:

PostQuitMessage(WM_QUIT);

break;

default:

return DefWindowProc(hWnd, Msg, wParam, lParam);

}

return 0;

}

<img>

A basic program with one message can be written as

follows:

//---------------------------------------------------------------------------

#include <windows.h>

//---------------------------------------------------------------------------

HWND hWnd;

const char ClsName[] = "WndMsg";

const char WindowCaption[] = "Windows and Controls Messages";

LRESULT CALLBACK WndProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam);

//---------------------------------------------------------------------------

INT WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,

LPSTR lpCmdLine, int nCmdShow)

{

MSG Msg;

WNDCLASSEX WndClsEx;

WndClsEx.cbSize = sizeof(WNDCLASSEX);

WndClsEx.style = CS_HREDRAW | CS_VREDRAW;

WndClsEx.lpfnWndProc = WndProc;

WndClsEx.cbClsExtra = NULL;

WndClsEx.cbWndExtra = NULL;

WndClsEx.hInstance = hInstance;

WndClsEx.hIcon = LoadIcon(NULL, IDI_APPLICATION);

WndClsEx.hCursor = LoadCursor(NULL, IDC_ARROW);

WndClsEx.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);

WndClsEx.lpszMenuName = NULL;

WndClsEx.lpszClassName = ClsName;

WndClsEx.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

RegisterClassEx(&WndClsEx);

hWnd = CreateWindowEx(WS_EX_OVERLAPPEDWINDOW,

ClsName,

WindowCaption,

WS_OVERLAPPEDWINDOW,

100,

120,

640,

480,

NULL,

NULL,

hInstance,

NULL);

ShowWindow(hWnd, nCmdShow);

UpdateWindow(hWnd);

while( GetMessage(&Msg, NULL, 0, 0) )

{

TranslateMessage(&Msg);

DispatchMessage(&Msg);

}

return Msg.wParam;

}

//---------------------------------------------------------------------------

LRESULT CALLBACK WndProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam)

{

switch(Msg)

{

case WM_DESTROY:

PostQuitMessage(WM_QUIT);

break;

default:

return DefWindowProc(hWnd, Msg, wParam, lParam);

}

return 0;

}

 
 
 
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