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 wxWidgets. 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 wxWidgets 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 wxWidgets 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 wxWidgets, while the log
  72 classes are. Some of advantages in using wxWidgets 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 \item{\bf Flexibility} The output of wxLog functions can be redirected or
  95 suppressed entirely based on their importance, which is either impossible or
  96 difficult to do with traditional methods. For example, only error messages, or
  97 only error messages and warnings might be logged, filtering out all
  98 informational messages.
  99 \item{\bf Completeness} Usually, an error message should be presented to the user
 100 when some operation fails. Let's take a quite simple but common case of a file
 101 error: suppose that you're writing your data file on disk and there is not
 102 enough space. The actual error might have been detected inside wxWidgets code
 103 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
 104 exact reason of the failure, it only knows that the data file couldn't be
 105 written to the disk. However, as wxWidgets uses {\it wxLogError()} in this
 106 situation, the exact error code (and the corresponding error message) will be
 107 given to the user together with "high level" message about data file writing
 108 error.
 109 \end{itemize}
 110
 111 After having enumerated all the functions which are normally used to log the
 112 messages, and why would you want to use them we now describe how all this
 113 works.
 114
 115 wxWidgets has the notion of a {\it log target}: it is just a class deriving
 116 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
 117 the base class which are called when a message is logged. Only one log target
 118 is {\it active} at any moment, this is the one used by {\it wxLogXXX()}
 119 functions. The normal usage of a log object (i.e. object of a class derived
 120 from wxLog) is to install it as the active target with a call to {\it
 121 SetActiveTarget()} and it will be used automatically by all subsequent calls
 122 to {\it wxLogXXX()} functions.
 123
 124 To create a new log target class you only need to derive it from wxLog and
 125 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 126 second one is enough if you're happy with the standard wxLog message
 127 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 128 to send the messages somewhere else. The first one may be overridden to do
 129 whatever you want but you have to distinguish between the different message
 130 types yourself.
 131
 132 There are some predefined classes deriving from wxLog and which might be
 133 helpful to see how you can create a new log target class and, of course, may
 134 also be used without any change. There are:
 135
 136 \begin{itemize}\itemsep=0pt
 137 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 138 stderr by default as its name suggests.
 139 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 140 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 141 \item{\bf wxLogGui} This is the standard log target for wxWidgets
 142 applications (it is used by default if you don't do anything) and provides the
 143 most reasonable handling of all types of messages for given platform.
 144 \item{\bf wxLogWindow} This log target provides a "log console" which
 145 collects all messages generated by the application and also passes them to the
 146 previous active log target. The log window frame has a menu allowing user to
 147 clear the log, close it completely or save all messages to file.
 148 \item{\bf wxLogNull} The last log class is quite particular: it doesn't do
 149 anything. The objects of this class may be instantiated to (temporarily)
 150 suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
 151 non-existing file will usually provoke an error message, but if for some
 152 reasons it is unwanted, just use this construction:
 153
 154 {\small%
 155 \begin{verbatim}
 156   wxFile file;
 157
 158   // wxFile.Open() normally complains if file can't be opened, we don't want it
 159   {
 160     wxLogNull logNo;
 161     if ( !file.Open("bar") )
 162       ... process error ourselves ...
 163   } // ~wxLogNull called, old log sink restored
 164
 165   wxLogMessage("..."); // ok
 166 \end{verbatim}
 167 }%
 168 \end{itemize}
 169
 170 The log targets can also be combined: for example you may wish to redirect the
 171 messages somewhere else (for example, to a log file) but also process them as
 172 normally. For this the \helpref{wxLogChain}{wxlogchain} and
 173 \helpref{wxLogPassThrough}{wxlogpassthrough} can be used.
 174