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{wxLogChain}{wxlogchain},\\
  11 \helpref{wxLogPassThrough}{wxlogpassthrough},\\
  12 \helpref{wxStreamToTextRedirector}{wxstreamtotextredirector}
  13
  14 This is a general overview of logging classes provided by wxWindows. The word
  15 logging here has a broad sense, including all of the program output, not only
  16 non interactive messages. The logging facilities included in wxWindows provide
  17 the base {\it wxLog} class which defines the standard interface for a {\it log
  18 target} as well as several standard implementations of it and a family of
  19 functions to use with them.
  20
  21 First of all, no knowledge of {\it wxLog} classes is needed to use them. For
  22 this, you should only know about {\it wxLogXXX()} functions. All of them have
  23 the same syntax as {\it printf()} or {\it vprintf()} , i.e. they take the
  24 format string as the first argument and respectively a variable number of
  25 arguments or a variable argument list pointer. Here are all of them:
  26
  27 \begin{itemize}\itemsep=0pt
  28 \item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
  29 terminates the program with the exit code $3$ (using {\it abort()} standard
  30 function). Unlike for all the other logging functions, this function can't be
  31 overridden by a log target.
  32 \item{\bf wxLogError} is the function to use for error messages, i.e. the
  33 messages that must be shown to the user. The default processing is to pop up a
  34 message box to inform the user about it.
  35 \item{\bf wxLogWarning} for warnings - they are also normally shown to the
  36 user, but don't interrupt the program work.
  37 \item{\bf wxLogMessage} is for all normal, informational messages. They also
  38 appear in a message box by default (but it can be changed, see below). Notice
  39 that the standard behaviour is to not show informational messages if there are
  40 any errors later - the logic being that the later error messages make the
  41 informational messages preceding them meaningless.
  42 \item{\bf wxLogVerbose} is for verbose output. Normally, it is suppressed, but
  43 might be activated if the user wishes to know more details about the program
  44 progress (another, but possibly confusing name for the same function is {\bf
  45 wxLogInfo}).
  46 \item{\bf wxLogStatus} is for status messages - they will go into the status
  47 bar of the active or specified (as the first argument) \helpref{wxFrame}{wxframe} if it has one.
  48 \item{\bf wxLogSysError} is mostly used by wxWindows itself, but might be
  49 handy for logging errors after system call (API function) failure. It logs the
  50 specified message text as well as the last system error
  51 code ({\it errno} or {\it ::GetLastError()} depending on the platform) and
  52 the corresponding error message. The second form of this function takes the
  53 error code explicitly as the first argument.
  54 \item{\bf wxLogDebug} is {\bf the} right function for debug output. It only
  55 does anything at all in the debug mode (when the preprocessor symbol
  56 \_\_WXDEBUG\_\_ is defined) and expands to nothing in release mode (otherwise).
  57 {\bf Tip:} under Windows, you must either run the program under debugger or
  58 use a 3rd party program such as \urlref{DbgView}{http://www.sysinternals.com}
  59 to actually see the debug output.
  60 \item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
  61 build. The reason for making it a separate function from it is that usually
  62 there are a lot of trace messages, so it might make sense to separate them
  63 from other debug messages which would be flooded in them. Moreover, the second
  64 version of this function takes a trace mask as the first argument which allows
  65 to further restrict the amount of messages generated.
  66 \end{itemize}
  67
  68 The usage of these functions should be fairly straightforward, however it may
  69 be asked why not use the other logging facilities, such as C standard stdio
  70 functions or C++ streams. The short answer is that they're all very good
  71 generic mechanisms, but are not really adapted for wxWindows, while the log
  72 classes are. Some of advantages in using wxWindows log functions are:
  73
  74 \begin{itemize}\itemsep=0pt
  75 \item{\bf Portability} It is a common practice to use {\it printf()}
  76 statements or cout/cerr C++ streams for writing out some (debug or otherwise)
  77 information.
  78 Although it works just fine under Unix, these messages go strictly nowhere
  79 under Windows where the stdout of GUI programs is not assigned to anything.
  80 Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
  81 printf()}.
  82
  83 You can also redirect the {\it wxLogXXX} calls to {\it cout} by just writing:
  84 {\small
  85 \begin{verbatim}
  86   wxLog *logger=new wxLogStream(&cout);
  87   wxLog::SetActiveTarget(logger);
  88 \end{verbatim}
  89 }
  90
  91 Finally, there is also a possibility to redirect the output sent to {\it cout}
  92 to a \helpref{wxTextCtrl}{wxtextctrl} by using the
  93 \helpref{wxStreamToTextRedirector}{wxstreamtotextredirector} class.
  94
  95 \item{\bf Flexibility} The output of wxLog functions can be redirected or
  96 suppressed entirely based on their importance, which is either impossible or
  97 difficult to do with traditional methods. For example, only error messages, or
  98 only error messages and warnings might be logged, filtering out all
  99 informational messages.
 100 \item{\bf Completeness} Usually, an error message should be presented to the user
 101 when some operation fails. Let's take a quite simple but common case of a file
 102 error: suppose that you're writing your data file on disk and there is not
 103 enough space. The actual error might have been detected inside wxWindows code
 104 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
 105 exact reason of the failure, it only knows that the data file couldn't be
 106 written to the disk. However, as wxWindows uses {\it wxLogError()} in this
 107 situation, the exact error code (and the corresponding error message) will be
 108 given to the user together with "high level" message about data file writing
 109 error.
 110 \end{itemize}
 111
 112 After having enumerated all the functions which are normally used to log the
 113 messages, and why would you want to use them we now describe how all this
 114 works.
 115
 116 wxWindows has the notion of a {\it log target}: it is just a class deriving
 117 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
 118 the base class which are called when a message is logged. Only one log target
 119 is {\it active} at any moment, this is the one used by {\it wxLogXXX()}
 120 functions. The normal usage of a log object (i.e. object of a class derived
 121 from wxLog) is to install it as the active target with a call to {\it
 122 SetActiveTarget()} and it will be used automatically by all subsequent calls
 123 to {\it wxLogXXX()} functions.
 124
 125 To create a new log target class you only need to derive it from wxLog and
 126 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 127 second one is enough if you're happy with the standard wxLog message
 128 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 129 to send the messages somewhere else. The first one may be overridden to do
 130 whatever you want but you have to distinguish between the different message
 131 types yourself.
 132
 133 There are some predefined classes deriving from wxLog and which might be
 134 helpful to see how you can create a new log target class and, of course, may
 135 also be used without any change. There are:
 136
 137 \begin{itemize}\itemsep=0pt
 138 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 139 stderr by default as its name suggests.
 140 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 141 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 142 \item{\bf wxLogGui} This is the standard log target for wxWindows
 143 applications (it is used by default if you don't do anything) and provides the
 144 most reasonable handling of all types of messages for given platform.
 145 \item{\bf wxLogWindow} This log target provides a "log console" which
 146 collects all messages generated by the application and also passes them to the
 147 previous active log target. The log window frame has a menu allowing user to
 148 clear the log, close it completely or save all messages to file.
 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} and
 174 \helpref{wxLogPassThrough}{wxlogpassthrough} can be used.
 175