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<HTML><LINK HREF="style.css" REL="STYLESHEET" TYPE="text/css"><HEAD><TITLE>Tutorial: Understanding the Message Loop</TITLE></HEAD><BODY><FONT SIZE="-1">[ <A HREF="./index.html">contents</A>| <A HREF="http://www.winprog.org/">#winprog</A>]</FONT><HR><H1>Understanding the Message Loop</H1><P>Understanding the message loop and entire message sending structure of windowsprograms is essential in order to write anything but the most trivial programs.Now that we've tried out message handling a little, we should look a little deeperinto the whole process, as things can get very confusing later on if you don't understand why things happen the way they do.<H3>What is a Message?</H3>A message is an integer value. If you look up in your header files (which isgood and common practice when investigating the workings of API's) you can findthings like:<PRE CLASS="SNIP">#define WM_INITDIALOG 0x0110#define WM_COMMAND 0x0111#define WM_LBUTTONDOWN 0x0201</PRE>...and so on. Messages are used to communicate pretty much everything inwindows at least on basic levels. If you want a window or control (which isjust a specialized window) to do something you send it a message. If anotherwindow wants you to do something it sends you a message. If an event happenssuch as the user typing on the keyboard, moving the mouse, clicking a button,then messages are sent by the system to the windows affected. If you areone of those windows, you handle the message and act accordingly.<P>Each windows message may have up to two parameters, <CODE>wParam</CODE> and <CODE>lParam</CODE>.Originally <CODE>wParam</CODE> was 16 bit and <CODE>lParam</CODE> was 32 bit, but in Win32 they areboth 32 bit. Not every message uses these parameters, and each message usesthem differently. For example the <CODE>WM_CLOSE</CODE> message doesn't use either, andyou should ignore them both. The <CODE>WM_COMMAND</CODE> message uses both, <CODE>wParam</CODE> contains<I>two</I> values, <CODE>HIWORD(wParam)</CODE> is the notification message (if applicable)and <CODE>LOWORD(wParam)</CODE> is the control or menu id that sent the message. <CODE>lParam</CODE>is the <CODE>HWND</CODE> (window handle) to the control which sent the message or <CODE>NULL</CODE> ifthe messages isn't from a control.<P><CODE>HIWORD()</CODE> and <CODE>LOWORD()</CODE> are macros defined by windows that singleout the two high bytes (High Word) of a 32 bit value (<CODE>0x<B>FFFF</B>0000</CODE>) and the low word(<CODE>0x0000<B>FFFF</B></CODE>) respectively. In Win32 a <CODE>WORD</CODE> is a 16bit value, making<CODE>DWORD</CODE> (or Double Word) a 32bit value. <P>To send a message you can use <CODE>PostMessage()</CODE> or <CODE>SendMessage()</CODE>. <CODE>PostMessage()</CODE> puts the message into the <I>Message Queue</I> and returns immediatly. That means once the call to <CODE>PostMessage()</CODE> is done the message may or may not have beenprocessed yet. <CODE>SendMessage()</CODE> sends the message directly to the window anddoes not return untill the window has finished processing it. If we wantedto close a window we could send it a <CODE>WM_CLOSE</CODE> message like this<CODE>PostMessage(hwnd, WM_CLOSE, 0, 0);</CODE> which wouldhave the same effect as clicking on the <IMG SRC="images/button_close.gif" ALT="[x]"> button on the top of the window.Notice that <CODE>wParam</CODE> and <CODE>lParam</CODE> are both <CODE>0</CODE>. This is because, as mentioned,they aren't used for <CODE>WM_CLOSE</CODE>.<H3>Dialogs</H3><P>Once you begin to use dialog boxes, you will need to send messages to the controls in orderto communicate with them. You can do this either by using <CODE>GetDlgItem()</CODE> firstto get the handle to the control using the ID and then use <CODE>SendMessage()</CODE>, ORyou can use <CODE>SendDlgItemMessage()</CODE> which combines the steps. You give it a windowhandle and a child ID and it will get the child handle, and then send it the message. <CODE>SendDlgItemMessage()</CODE> and similar APIs like <CODE>GetDlgItemText()</CODE> willwork on all windows, not just dialog boxes.<H3>What is the Message Queue</H3><P>Lets say you were busy handling the <CODE>WM_PAINT</CODE> message and suddenly the user typesa bunch of stuff on the keyboard. What should happen? Should you be interruptedin your drawing to handle the keys or should the keys just be discarded?Wrong! Obviously neither of these options is reasonable, so we have the messagequeue, when messages are posted they are added to the message queue and whenyou handle them they are removed. This ensure that you aren't going to missmessages, if you are handling one, the others will be queued up untill you get tothem.<H3>What is a Message Loop</H3><PRE CLASS="SNIP">while(GetMessage(&Msg, NULL, 0, 0) > 0){ TranslateMessage(&Msg); DispatchMessage(&Msg);}</PRE><OL><LI>The message loop calls <CODE>GetMessage()</CODE>, which looks in your message queue. If themessage queue is empty your program basically stops and waits for one (it <I>Blocks</I>).<LI>When an event occures causing a message to be added to the queue (for example the system registers a mouse click) <CODE>GetMessages()</CODE> returns <B>a positive value</B> indicating there is a messageto be processed, and that it has filled in the members of the <CODE>MSG</CODE> structure we passed it. It returns <CODE>0</CODE> if it hits <CODE>WM_QUIT</CODE>, and <B>a negative value</B> if an error occured.<LI>We take the message (in the <CODE>Msg</CODE> variable)and pass it to <CODE>TranslateMessage()</CODE>, this does a bit of additional processing,translating virtual key messages into character messages. This step is actually optional, butcertain things won't work if it's not there.<LI>Once that's done we pass the message to <CODE>DispatchMessage()</CODE>. What <CODE>DispatchMessage()</CODE> does is takethe message, checks which window it is for and then looks up the WindowProcedure for the window. It then calls that procedure, sending as parametersthe handle of the window, the message, and <CODE>wParam</CODE> and <CODE>lParam</CODE>.<LI>In your window procedure you check the message and it's parameters, and do whatever youwant with them! If you aren't handling the specific message, you almost always call <CODE>DefWindowProc()</CODE>which will perform the default actions for you (which often means it does nothing).<LI>Once you have finished processing the message, your windows procedure returns, <CODE>DispatchMessage()</CODE>returns, and we go back to the beginning of the loop.</OL><P>This is a very important concept for windows programs. Your window procedure is not magicallycalled by the system, in effect <I>you call it yourself</I> indirectly by calling <CODE>DispatchMessage()</CODE>.If you wanted, you could use <CODE>GetWindowLong()</CODE> on the window handle that the message is destined forto look up the window's procedure and call it directly!<PRE CLASS="SNIP">while(GetMessage(&Msg, NULL, 0, 0) > 0){ WNDPROC fWndProc = (WNDPROC)GetWindowLong(Msg.hwnd, GWL_WNDPROC); fWndProc(Msg.hwnd, Msg.message, Msg.wParam, Msg.lParam);}</PRE><P>I tried this with the previous example code, and it does work, however there are variousissues such as Unicode/ANSI translation, calling timer callbacks and so forth that thismethod will not account for, and very likely will break all but trivial applications. Sodo it to try it, but don't do it in real code :)<P>Notice that we use <CODE>GetWindowLong()</CODE> to retreive the window procedure associatedwith the window. Why don't we just call our <CODE>WndProc()</CODE> directly? Well our messageloop is responsible for ALL of the windows in our program, this includes things like buttonsand list boxes that have their own window procedures, so we need to make sure that wecall the right procedure for the window. Since more than one window can use the same window procedure, the first parameter (the handle to the window) is used to tell thewindow procedure which window the message is intended for.<P>As you can see, your application spends the majority of it's time spinning round and roundin this message loop, where you joyfully send out messages to the happy windows that will process them. But what do you do when you want your program to exit? Since we're usinga <CODE>while()</CODE> loop, if <CODE>GetMessage()</CODE> were to return <CODE>FALSE</CODE> (aka <CODE>0</CODE>), the loop would end and we would reach the end of our <CODE>WinMain()</CODE> thus exiting the program.This is exactly what <CODE>PostQuitMessage()</CODE> accomplishes. It places a <CODE>WM_QUIT</CODE>message into the queue, and instead of returning a positive value, <CODE>GetMessage()</CODE> fills in the Msg structure and returns <CODE>0</CODE>. At this point, the <CODE>wParam</CODE> member of <CODE>Msg</CODE> contains the value that you passed to <CODE>PostQuitMessage()</CODE>and you can either ignore it, or return it from <CODE>WinMain()</CODE> which will then be usedas the exit code when the process terminates.<P><B>IMPORTANT:</B> <CODE>GetMessage()</CODE> will return <CODE><B>-1</B></CODE> if it encounters an error.Make sure you remember this, or it will catch you out at some point... even though <CODE>GetMessage()</CODE>is defined as returning a <CODE>BOOL</CODE>, it can return values other than <CODE>TRUE</CODE> or <CODE>FALSE</CODE>, since<CODE>BOOL</CODE> is defined as <CODE>UINT</CODE> (<CODE>unsigned int</CODE>). The following are examples ofcode that may <I>seem</I> to work, but will not process certian conditions correctly:<PRE CLASS="SNIP"> while(GetMessage(&Msg, NULL, 0, 0))</PRE><PRE CLASS="SNIP"> while(GetMessage(&Msg, NULL, 0, 0) != 0)</PRE><PRE CLASS="SNIP"> while(GetMessage(&Msg, NULL, 0, 0) == TRUE)</PRE><B>The above are all wrong!</B> It may be of note that I used to use the first of these throughout the tutorial,since as I just mentioned, it works fine as long as <CODE>GetMessage()</CODE> never fails, which when your codeis correct it won't. However I failed to take into consideration that if you're reading this, your code probablywon't be correct a lot of the time, and <CODE>GetMessage()</CODE> will fail at some point :) I've gone through and corrected this, but forgive me if I've missed a few spots.<PRE CLASS="SNIP"> while(GetMessage(&Msg, NULL, 0, 0) > 0)</PRE>This, or code that has the same effect should always be used.<P>I hope you now have a better understanding of the windows message loop, if not, do not fear,things will make more sense once you have been using them for a while.<HR><FONT SIZE="-1">Copyright © 1998-2003, Brook Miles (<A HREF="mailto:forger(nospam)winprog.org">theForger</A>). 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