docs/latex/wx/tlog.tex

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