docs/latex/wx/tlog.tex

   1 \section{Log classes overview}\label{wxlogoverview}
   2
   3 % Vadim: let's not have references to documentation that doesn't exist yet.
   4 % The docs are allowed to be incomplete, but not containing bad links!
   5 Classes: \helpref{wxLog}{wxlog}, wxLogStderr,
   6 wxLogOstream, wxLogTextCtrl, wxLogWindow, wxLogGui, 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
  20 \begin{itemize}\itemsep=0pt
  21 \item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
  22 terminates the program with the exit code 3 (using {\it abort()} standard
  23 function also terminates the program with this exit code).
  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 \item{\bf wxLogWarning} for warnings - they are also normally shown to the
  28 user, but don't interrupt the program work.
  29 \item{\bf wxLogMessage} is for all normal, informational messages. They also
  30 appear in a message box by default (but it can be changed, see below). Notice
  31 that the standard behaviour is to not show informational messages if there are
  32 any errors later - the logic being that the later error messages make the
  33 informational messages preceding them meaningless.
  34 \item{\bf wxLogVerbose} is for verbose output. Normally, it's suppressed, but
  35 might be activated if the user wishes to know more details about the program
  36 progress (another, but possibly confusing name for the same function is {\bf
  37 wxLogInfo}).
  38 \item{\bf wxLogStatus} is for status messages - they will go into the status
  39 bar of the active or specified (as the first argument) \helpref{wxFrame}{wxframe} if it has one.
  40 \item{\bf wxLogSysError} is mostly used by wxWindows itself, but might be
  41 handy for logging errors after system call (API function) failure. It logs the
  42 specified message text as well as the last system error
  43 code ({\it errno} or {\it ::GetLastError()} depending on the platform) and the corresponding error
  44 message. The second form of this function takes the error code explitly as the
  45 first argument.
  46 \item{\bf wxLogDebug} is {\bf the} right function for debug output. It only
  47 does anything at all in the debug mode (when the preprocessor symbol
  48 \_\_WXDEBUG\_\_ is defined) and expands to nothing in release mode (otherwise).
  49 \item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
  50 build. The reason for making it a separate function from it is that usually
  51 there are a lot of trace messages, so it might make sense to separate them
  52 from other debug messages which would be flooded in them. Moreover, the second
  53 version of this function takes a trace mask as the first argument which allows
  54 to further restrict the amount of messages generated.
  55 \end{itemize}
  56
  57 The usage of these functions should be fairly straightforward, however it may
  58 be asked why not use the other logging facilities, such as C standard stdio
  59 functions or C++ streams. The short answer is that they're all very good
  60 generic mechanisms, but are not really adapted for wxWindows, while the log
  61 classes are. Some of advantages in using wxWindows log functions are:
  62
  63 \begin{itemize}\itemsep=0pt
  64 \item{\bf Portability} It's a common practice to use {\it printf()} statements or
  65 cout/cerr C++ streams for writing out some (debug or otherwise) information.
  66 Although it works just fine under Unix, these messages go strictly nowhere
  67 under Windows where the stdout of GUI programs is not assigned to anything.
  68 Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
  69 printf()}.
  70 \item{\bf Flexibility} The output of wxLog functions can be redirected or
  71 suppressed entirely based on their importance, which is either impossible or
  72 difficult to do with traditional methods. For example, only error messages, or
  73 only error messages and warnings might be logged, filtering out all
  74 informational messages.
  75 \item{\bf Completeness} Usually, an error message should be presented to the user
  76 when some operation fails. Let's take a quite simple but common case of a file
  77 error: suppose that you're writing your data file on disk and there is not
  78 enough space. The actual error might have been detected inside wxWindows code
  79 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
  80 exact reason of the failure, it only knows that the data file couldn't be
  81 written to the disk. However, as wxWindows uses {\it wxLogError()} in this
  82 situation, the exact error code (and the corresponding error message) will be
  83 given to the user together with "high level" message about data file writing
  84 error.
  85 \end{itemize}
  86
  87 After having enumerated all the functions which are normally used to log the
  88 messages, and why would you want to use them we now describe how all this
  89 works.
  90
  91 wxWindows has the notion of a {\it log target}: it's just a class deriving
  92 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
  93 the base class which are called when a message is logged. Only one log target
  94 is {\it active} at any moment, this is the one used by \it{wxLogXXX()}
  95 functions. The normal usage of a log object (i.e. object of a class derived
  96 from wxLog) is to install it as the active target with a call to {\it
  97 SetActiveTarget()} and it will be used automatically by all subsequent calls
  98 to {\it wxLogXXX()} functions.
  99
 100 To create a new log target class you only need to derive it from wxLog and
 101 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 102 second one is enough if you're happy with the standard wxLog message
 103 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 104 to send the messages somewhere else. The first one may be overridden to do
 105 whatever you want but you have to distinguish between the different message
 106 types yourself.
 107
 108 There are some predefined classes deriving from wxLog and which might be
 109 helpful to see how you can create a new log target class and, of course, may
 110 also be used without any change. There are:
 111
 112 \begin{itemize}\itemsep=0pt
 113 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 114 stderr by default as its name suggests.
 115 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 116 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 117 \item{\bf wxLogGui} This is the standard log target for wxWindows
 118 applications (it's used by default if you don't do anything) and provides the
 119 most reasonable handling of all types of messages for given platform.
 120 \item{\bf wxLogWindow} This log target provides a "log console" which
 121 collects all messages generated by the application and also passes them to the
 122 previous active log target. The log window frame has a menu allowing user to
 123 clear the log, close it completely or save all messages to file.
 124 \item{\bf wxLogNull} The last log class is quite particular: it doesn't do
 125 anything. The objects of this class may be instantiated to (temporarily)
 126 suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
 127 non-existing file will usually provoke an error message, but if for some
 128 reasons it's unwanted, just use this construction:
 129
 130 {\small
 131 \begin{verbatim}
 132   wxFile file;
 133
 134   // wxFile.Open() normally complains if file can't be opened, we don't want it
 135   {
 136     wxLogNull logNo;
 137     if ( !file.Open("bar") )
 138       ... process error ourselves ...
 139   } // ~wxLogNull called, old log sink restored
 140
 141   wxLogMessage("..."); // ok
 142 \end{verbatim}
 143 }
 144 \end{itemize}
 145