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()}, i.e. they take the format string as the
  24 first argument and a variable number of arguments. Here are all of them:
  25
  26 \begin{itemize}\itemsep=0pt
  27 \item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
  28 terminates the program with the exit code 3 (using {\it abort()} standard
  29 function also terminates the program with this exit code).
  30 \item{\bf wxLogError} is the function to use for error messages, i.e. the
  31 messages that must be shown to the user. The default processing is to pop up a
  32 message box to inform the user about it.
  33 \item{\bf wxLogWarning} for warnings - they are also normally shown to the
  34 user, but don't interrupt the program work.
  35 \item{\bf wxLogMessage} is for all normal, informational messages. They also
  36 appear in a message box by default (but it can be changed, see below). Notice
  37 that the standard behaviour is to not show informational messages if there are
  38 any errors later - the logic being that the later error messages make the
  39 informational messages preceding them meaningless.
  40 \item{\bf wxLogVerbose} is for verbose output. Normally, it is suppressed, but
  41 might be activated if the user wishes to know more details about the program
  42 progress (another, but possibly confusing name for the same function is {\bf
  43 wxLogInfo}).
  44 \item{\bf wxLogStatus} is for status messages - they will go into the status
  45 bar of the active or specified (as the first argument) \helpref{wxFrame}{wxframe} if it has one.
  46 \item{\bf wxLogSysError} is mostly used by wxWindows itself, but might be
  47 handy for logging errors after system call (API function) failure. It logs the
  48 specified message text as well as the last system error
  49 code ({\it errno} or {\it ::GetLastError()} depending on the platform) and the corresponding error
  50 message. The second form of this function takes the error code explicitly as the
  51 first argument.
  52 \item{\bf wxLogDebug} is {\bf the} right function for debug output. It only
  53 does anything at all in the debug mode (when the preprocessor symbol
  54 \_\_WXDEBUG\_\_ is defined) and expands to nothing in release mode (otherwise).
  55 {\bf Tip:} under Windows, you must either run the program under debugger or
  56 use a 3rd party program such as \urlref{DbgView}{http://www.sysinternals.com}
  57 to actually see the debug output.
  58 \item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
  59 build. The reason for making it a separate function from it is that usually
  60 there are a lot of trace messages, so it might make sense to separate them
  61 from other debug messages which would be flooded in them. Moreover, the second
  62 version of this function takes a trace mask as the first argument which allows
  63 to further restrict the amount of messages generated.
  64 \end{itemize}
  65
  66 The usage of these functions should be fairly straightforward, however it may
  67 be asked why not use the other logging facilities, such as C standard stdio
  68 functions or C++ streams. The short answer is that they're all very good
  69 generic mechanisms, but are not really adapted for wxWindows, while the log
  70 classes are. Some of advantages in using wxWindows log functions are:
  71
  72 \begin{itemize}\itemsep=0pt
  73 \item{\bf Portability} It is a common practice to use {\it printf()} statements or
  74 cout/cerr C++ streams for writing out some (debug or otherwise) information.
  75 Although it works just fine under Unix, these messages go strictly nowhere
  76 under Windows where the stdout of GUI programs is not assigned to anything.
  77 Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
  78 printf()}.
  79
  80 You can also redirect the {\it wxLogXXX} calls to {\it cout} by just writing:
  81 {\small
  82 \begin{verbatim}
  83   wxLog *logger=new wxLogStream(&cout);
  84   wxLog::SetActiveTarget(logger);
  85 \end{verbatim}
  86 }
  87
  88 Finally, there is also a possibility to redirect the output sent to {\it cout}
  89 to a \helpref{wxTextCtrl}{wxtextctrl} by using the
  90 \helpref{wxStreamToTextRedirector}{wxstreamtotextredirector} class.
  91
  92 \item{\bf Flexibility} The output of wxLog functions can be redirected or
  93 suppressed entirely based on their importance, which is either impossible or
  94 difficult to do with traditional methods. For example, only error messages, or
  95 only error messages and warnings might be logged, filtering out all
  96 informational messages.
  97 \item{\bf Completeness} Usually, an error message should be presented to the user
  98 when some operation fails. Let's take a quite simple but common case of a file
  99 error: suppose that you're writing your data file on disk and there is not
 100 enough space. The actual error might have been detected inside wxWindows code
 101 (say, in {\it wxFile::Write}), so the calling function doesn't really know the
 102 exact reason of the failure, it only knows that the data file couldn't be
 103 written to the disk. However, as wxWindows uses {\it wxLogError()} in this
 104 situation, the exact error code (and the corresponding error message) will be
 105 given to the user together with "high level" message about data file writing
 106 error.
 107 \end{itemize}
 108
 109 After having enumerated all the functions which are normally used to log the
 110 messages, and why would you want to use them we now describe how all this
 111 works.
 112
 113 wxWindows has the notion of a {\it log target}: it is just a class deriving
 114 from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
 115 the base class which are called when a message is logged. Only one log target
 116 is {\it active} at any moment, this is the one used by {\it wxLogXXX()}
 117 functions. The normal usage of a log object (i.e. object of a class derived
 118 from wxLog) is to install it as the active target with a call to {\it
 119 SetActiveTarget()} and it will be used automatically by all subsequent calls
 120 to {\it wxLogXXX()} functions.
 121
 122 To create a new log target class you only need to derive it from wxLog and
 123 implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
 124 second one is enough if you're happy with the standard wxLog message
 125 formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
 126 to send the messages somewhere else. The first one may be overridden to do
 127 whatever you want but you have to distinguish between the different message
 128 types yourself.
 129
 130 There are some predefined classes deriving from wxLog and which might be
 131 helpful to see how you can create a new log target class and, of course, may
 132 also be used without any change. There are:
 133
 134 \begin{itemize}\itemsep=0pt
 135 \item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
 136 stderr by default as its name suggests.
 137 \item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
 138 but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
 139 \item{\bf wxLogGui} This is the standard log target for wxWindows
 140 applications (it is used by default if you don't do anything) and provides the
 141 most reasonable handling of all types of messages for given platform.
 142 \item{\bf wxLogWindow} This log target provides a "log console" which
 143 collects all messages generated by the application and also passes them to the
 144 previous active log target. The log window frame has a menu allowing user to
 145 clear the log, close it completely or save all messages to file.
 146 \item{\bf wxLogNull} The last log class is quite particular: it doesn't do
 147 anything. The objects of this class may be instantiated to (temporarily)
 148 suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
 149 non-existing file will usually provoke an error message, but if for some
 150 reasons it is unwanted, just use this construction:
 151
 152 {\small
 153 \begin{verbatim}
 154   wxFile file;
 155
 156   // wxFile.Open() normally complains if file can't be opened, we don't want it
 157   {
 158     wxLogNull logNo;
 159     if ( !file.Open("bar") )
 160       ... process error ourselves ...
 161   } // ~wxLogNull called, old log sink restored
 162
 163   wxLogMessage("..."); // ok
 164 \end{verbatim}
 165 }
 166 \end{itemize}
 167
 168 The log targets can also be combined: for example you may wish to redirect the
 169 messages somewhere else (for example, to a log file) but also process them as
 170 normally. For this the \helpref{wxLogChain}{wxlogchain} and
 171 \helpref{wxLogPassThrough}{wxlogpassthrough} can be used.
 172