docs/latex/wx/tlog.tex

   1 \section{Log classes overview}\label{wxlogoverview}
   2
   3 Classes: \helpref{wxLog}wxlog, \helpref{wxLogStderr}wxlogstderr,
   4 \helpref{wxLogOstream}wxlogostream, \helpref{wxLogTextCtrl}wxlogtextctrl,
   5 \helpref{wxLogWindow}{wxlogwindow}, \helpref{wxLogGui}{wxloggui},
   6 \helpref{wxLogNull}{wxlognull}
   7
   8 This is a general overview of logging classes provided by wxWindows. The word
   9 logging here has a broad sense, including all of the program output, not only
  10 non interactive messages. The logging facilities included in wxWindows provide
  11 the base {\it wxLog} class which defines the standard interface for a {\it log
  12 target} as well as several standard implementations of it and a family of
  13 functions to use with them.
  14
  15 First of all, no knowledge of {\it wxLog} classes is needed to use them. For
  16 this, you should only know about {\it wxLogXXX()} functions. All of them have
  17 the same syntax as {\it printf()}, i.e. they take the format string as the
  18 first argument and a variable number of arguments. Here are all of them:
  19 \begin{itemize}\itemsep=0pt
  20 \item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
  21 terminates the program with the exit code 3 (using {\it abort()} standard
  22 function also terminates the program with this exit code).
  23
  24 \item{\bf wxLogError} is the function to use for error messages, i.e. the
  25 messages that must be shown to the user. The default processing is to pop up a
  26 message box to inform the user about it.
  27
  28 \item{\bf wxLogWarning} for warnings - they are also normally shown to the
  29 user, but don't interrupt the program work.
  30
  31 \item{\bf wxLogMessage} is for all normal, informational messages. They also
  32 appear in a message box by default (but it can be changed, see below). Notice
  33 that the standard behaviour is to not show informational messages if there are
  34 any errors later - the logic being that the later error messages make the
  35 informational messages preceding them meaningless.
  36
  37 \item{\bf wxLogVerbose} is for verbose output. Normally, it's suppressed, but
  38 might be activated if the user wishes to know more details about the program
  39 progress (another, but possibly confusing name for the same function is {\bf
  40 wxLogInfo}
  41
  42 \item{\bf wxLogStatus} is for status messages - they will go into the status
  43 bar of the active or specified (as the first argument)
  44 \helpref{wxFrame}{wxframe} if it has one.
  45
  46 \item{\bf wxLogSysError} is mostly used by wxWindows itself, but might be
  47 handy for logging errors after system call (API function) failure. It logs the
  48 specified message text as well as the last system error code ({\it errno} or
  49 {\it ::GetLastError()} depending on the platform) and the corresponding error
  50 message. The second form of this function takes the error code explitly as the
  51 first argument.
  52
  53 \item{\bf wxLogDebug} is {\bf the} right function for debug output. It only
  54 does anything at all in the debug mode (when the preprocessor symbol
  55 \_\_WXDEBUG\_\_ is defined) and expands to nothing in release mode (otherwise).
  56
  57 \item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
  58 build. The reason for making it a separate function from it is that usually
  59 there are a lot of trace messages, so it might make sense to separate them
  60 from other debug messages which would be flooded in them. Moreover, the second
  61 version of this function takes a trace mask as the first argument which allows
  62 to further restrict the amount of messages generated.
  63
  64 \end{itemize}
  65
  66 % VZ: Julian, am I pushing too much here?
  67 The usage of these functions should be fairly straightforward, however it may
  68 be asked why not use the other logging facilities, such as C standard stdio
  69 functions or C++ streams. The short answer is that they're all very good
  70 generic mechanisms, but are not really adapted for wxWindows, while the log
  71 classes are. Some of advantages in using wxWindows log functions are:
  72 \begin{itemize}\itemsep=0pt
  73
  74 \item{Portability} It's a common practice to use {\it printf()} statements or
  75 cout/cerr C++ streams for writing out some (debug or otherwise) information.
  76 Although it works just fine under Unix, these messages go strictly nowever
  77 under Windows where the stdout of GUI programs is not assigned to anything.
  78 Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
  79 printf()}.
  80
  81 \item{Flexibility} The output of wxLog functions can be redirected or
  82 suppressed entirely based on their importance, which is either impossible or
  83 difficult to do with traditional methods. For example, only error messages, or
  84 only error messages and warnings might be logged, filtering out all
  85 informational messages.
  86
  87 \item{Comlpeteness} Usually, an error message should be presented to the user
  88 when some operation fails. Let's take a quite simple but common case of a file
  89 error: suppose that you're writing your data file on disk and there is not
  90 enough space. The actual error might have been detected inside wxWindows code
  91 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
  92 exact reason of the failure, it only knows that the data file couldn't be
  93 written to the disk. However, as wxWindows uses {\it wxLogError()} in this
  94 situation, the exact error code (and the corresponding error message) will be
  95 given to the user together with "high level" message about data file writing
  96 error.
  97
  98 \end{itemize}
  99
 100 After having enumerated all the functions which are normally used to log the
 101 messages, and why would you want to use them we now describe how all this
 102 works.
 103
 104 wxWindows has the notion of a {\it log target}: it's just a class deriving
 105 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
 106 the base class which are called when a message is logged. Only one log target
 107 is {\it active} at any moment, this is the one used by \it{wxLogXXX()}
 108 functions. The normal usage of a log object (i.e. object of a class derived
 109 from wxLog) is to install it as the active target with a call to {\it
 110 SetActiveTarget()} and it will be used automatically by all subsequent calls
 111 to {\it wxLogXXX()} functions.
 112
 113 To create a new log target class you only need to derive it from wxLog and
 114 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 115 second one is enough if you're happy with the standard wxLog message
 116 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 117 to send the messages somewhere else. The first one may be overridden to do
 118 whatever you want but you have to distinguish between the different message
 119 types yourself.
 120
 121 There are some predefined classes deriving from wxLog and which might be
 122 helpful to see how you can create a new log target class and, of course, may
 123 also be used without any change. There are:
 124 \begin{itemize}\itemsep=0pt
 125 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 126 stderr by default as its name suggests.
 127
 128 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 129 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 130
 131 \item{\bf wxLogGui} This is the standard log target for wxWindows
 132 applications (it's used by default if you don't do anything) and provides the
 133 most reasonable handling of all types of messages for given platform.
 134
 135 \item{\bf wxLogWindow} This log target provides a "log console" which
 136 collects all messages generated by the application and also passes them to the
 137 previous active log target. The log window frame has a menu allowing user to
 138 clear the log, close it completely or save all messages to file.
 139
 140 \item{\bf wxLogNull} The last log class is quite particular: it doesn't do
 141 anything. The objects of this class may be instantiated to (temporarily)
 142 suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
 143 non-existing file will usually provoke an error message, but if you for some
 144 reason it's unwanted, just use this construction:
 145 \begin{verbatim}
 146   wxFile file;
 147
 148   // wxFile.Open() normally complains if file can't be opened, we don't want it
 149   {
 150     wxLogNull logNo;
 151     if ( !file.Open("bar") )
 152       ... process error ourselves ...
 153   } // ~wxLogNull called, old log sink restored
 154
 155   wxLogMessage("..."); // ok
 156 \end{verbatim}
 157
 158 \end{itemize}