docs/latex/wx/tlog.tex

   1 \section{wxLog classes overview}\label{wxlogoverview}
   2
   3 Classes: \helpref{wxLog}{wxlog},\\
   4 \helpref{wxLogStderr}{wxlogstderr},\\
   5 \helpref{wxLogStream}{wxlogstream},\\
   6 \helpref{wxLogTextCtrl}{wxlogtextctrl},\\
   7 \helpref{wxLogWindow}{wxlogwindow},\\
   8 \helpref{wxLogGui}{wxloggui},\\
   9 \helpref{wxLogNull}{wxlognull},\\
  10 \helpref{wxLogBuffer}{wxlogbuffer},\\
  11 \helpref{wxLogChain}{wxlogchain},\\
  12 \helpref{wxLogInterposer}{wxloginterposer},\\
  13 \helpref{wxLogInterposerTemp}{wxloginterposertemp},\\
  14 \helpref{wxStreamToTextRedirector}{wxstreamtotextredirector}
  15
  16 This is a general overview of logging classes provided by wxWidgets. The word
  17 logging here has a broad sense, including all of the program output, not only
  18 non-interactive messages. The logging facilities included in wxWidgets provide
  19 the base {\it wxLog} class which defines the standard interface for a {\it log
  20 target} as well as several standard implementations of it and a family of
  21 functions to use with them.
  22
  23 First of all, no knowledge of {\it wxLog} classes is needed to use them. For
  24 this, you should only know about {\it wxLogXXX()} functions. All of them have
  25 the same syntax as {\it printf()} or {\it vprintf()} , i.e. they take the
  26 format string as the first argument and respectively a variable number of
  27 arguments or a variable argument list pointer. Here are all of them:
  28
  29 \begin{itemize}\itemsep=0pt
  30 \item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
  31 terminates the program with the exit code $3$ (using {\it abort()} standard
  32 function). Unlike for all the other logging functions, this function can't be
  33 overridden by a log target.
  34 \item{\bf wxLogError} is the function to use for error messages, i.e. the
  35 messages that must be shown to the user. The default processing is to pop up a
  36 message box to inform the user about it.
  37 \item{\bf wxLogWarning} for warnings - they are also normally shown to the
  38 user, but don't interrupt the program work.
  39 \item{\bf wxLogMessage} is for all normal, informational messages. They also
  40 appear in a message box by default (but it can be changed, see below).
  41 \item{\bf wxLogVerbose} is for verbose output. Normally, it is suppressed, but
  42 might be activated if the user wishes to know more details about the program
  43 progress (another, but possibly confusing name for the same function is {\bf
  44 wxLogInfo}).
  45 \item{\bf wxLogStatus} is for status messages - they will go into the status
  46 bar of the active or specified (as the first argument) \helpref{wxFrame}{wxframe} if it has one.
  47 \item{\bf wxLogSysError} is mostly used by wxWidgets itself, but might be
  48 handy for logging errors after system call (API function) failure. It logs the
  49 specified message text as well as the last system error
  50 code ({\it errno} or {\it ::GetLastError()} depending on the platform) and
  51 the corresponding error message. The second form of this function takes the
  52 error code explicitly as the first argument.
  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 {\bf Tip:} under Windows, you must either run the program under debugger or
  57 use a 3rd party program such as \urlref{DbgView}{http://www.sysinternals.com}
  58 to actually see the debug output.
  59 \item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
  60 build. The reason for making it a separate function from it is that usually
  61 there are a lot of trace messages, so it might make sense to separate them
  62 from other debug messages which would be flooded in them. Moreover, the second
  63 version of this function takes a trace mask as the first argument which allows
  64 to further restrict the amount of messages generated.
  65 \end{itemize}
  66
  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 wxWidgets, while the log
  71 classes are. Some of advantages in using wxWidgets log functions are:
  72
  73 \begin{itemize}\itemsep=0pt
  74 \item{\bf Portability} It is a common practice to use {\it printf()}
  75 statements or cout/cerr C++ streams for writing out some (debug or otherwise)
  76 information.
  77 Although it works just fine under Unix, these messages go strictly nowhere
  78 under Windows where the stdout of GUI programs is not assigned to anything.
  79 Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
  80 printf()}.
  81
  82 You can also redirect the {\it wxLogXXX} calls to {\it cout} by just writing:
  83 {\small
  84 \begin{verbatim}
  85   wxLog *logger=new wxLogStream(&cout);
  86   wxLog::SetActiveTarget(logger);
  87 \end{verbatim}
  88 }
  89
  90 Finally, there is also a possibility to redirect the output sent to {\it cout}
  91 to a \helpref{wxTextCtrl}{wxtextctrl} by using the
  92 \helpref{wxStreamToTextRedirector}{wxstreamtotextredirector} class.
  93 \item{\bf Flexibility} The output of wxLog functions can be redirected or
  94 suppressed entirely based on their importance, which is either impossible or
  95 difficult to do with traditional methods. For example, only error messages, or
  96 only error messages and warnings might be logged, filtering out all
  97 informational messages.
  98 \item{\bf Completeness} Usually, an error message should be presented to the user
  99 when some operation fails. Let's take a quite simple but common case of a file
 100 error: suppose that you're writing your data file on disk and there is not
 101 enough space. The actual error might have been detected inside wxWidgets code
 102 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
 103 exact reason of the failure, it only knows that the data file couldn't be
 104 written to the disk. However, as wxWidgets uses {\it wxLogError()} in this
 105 situation, the exact error code (and the corresponding error message) will be
 106 given to the user together with "high level" message about data file writing
 107 error.
 108 \end{itemize}
 109
 110 After having enumerated all the functions which are normally used to log the
 111 messages, and why would you want to use them we now describe how all this
 112 works.
 113
 114 wxWidgets has the notion of a {\it log target}: it is just a class deriving
 115 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
 116 the base class which are called when a message is logged. Only one log target
 117 is {\it active} at any moment, this is the one used by {\it wxLogXXX()}
 118 functions. The normal usage of a log object (i.e. object of a class derived
 119 from wxLog) is to install it as the active target with a call to {\it
 120 SetActiveTarget()} and it will be used automatically by all subsequent calls
 121 to {\it wxLogXXX()} functions.
 122
 123 To create a new log target class you only need to derive it from wxLog and
 124 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 125 second one is enough if you're happy with the standard wxLog message
 126 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 127 to send the messages somewhere else. The first one may be overridden to do
 128 whatever you want but you have to distinguish between the different message
 129 types yourself.
 130
 131 There are some predefined classes deriving from wxLog and which might be
 132 helpful to see how you can create a new log target class and, of course, may
 133 also be used without any change. There are:
 134
 135 \begin{itemize}\itemsep=0pt
 136 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 137 stderr by default as its name suggests.
 138 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 139 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 140 \item{\bf wxLogGui} This is the standard log target for wxWidgets
 141 applications (it is used by default if you don't do anything) and provides the
 142 most reasonable handling of all types of messages for given platform.
 143 \item{\bf wxLogWindow} This log target provides a "log console" which
 144 collects all messages generated by the application and also passes them to the
 145 previous active log target. The log window frame has a menu allowing user to
 146 clear the log, close it completely or save all messages to file.
 147 \item{\bf wxLogBuffer} This target collects all the logged messages in an
 148 internal buffer allowing to show them later to the user all at once.
 149 \item{\bf wxLogNull} The last log class is quite particular: it doesn't do
 150 anything. The objects of this class may be instantiated to (temporarily)
 151 suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
 152 non-existing file will usually provoke an error message, but if for some
 153 reasons it is unwanted, just use this construction:
 154
 155 {\small%
 156 \begin{verbatim}
 157   wxFile file;
 158
 159   // wxFile.Open() normally complains if file can't be opened, we don't want it
 160   {
 161     wxLogNull logNo;
 162     if ( !file.Open("bar") )
 163       ... process error ourselves ...
 164   } // ~wxLogNull called, old log sink restored
 165
 166   wxLogMessage("..."); // ok
 167 \end{verbatim}
 168 }%
 169 \end{itemize}
 170
 171 The log targets can also be combined: for example you may wish to redirect the
 172 messages somewhere else (for example, to a log file) but also process them as
 173 normally. For this the \helpref{wxLogChain}{wxlogchain}, \helpref{wxLogInterposer}{wxloginterposer} and\rtfsp
 174 \helpref{wxLogInterposerTemp}{wxloginterposertemp} can be used.
 175