docs/doxygen/overviews/log.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        log.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /**
  10
  11 @page overview_log wxLog Classes Overview
  12
  13 Classes:
  14 @li wxLog
  15 @li wxLogStderr
  16 @li wxLogStream
  17 @li wxLogTextCtrl
  18 @li wxLogWindow
  19 @li wxLogGui
  20 @li wxLogNull
  21 @li wxLogBuffer
  22 @li wxLogChain
  23 @li wxLogInterposer
  24 @li wxLogInterposerTemp
  25 @li wxStreamToTextRedirector
  26
  27 @li @ref overview_log_introduction
  28 @li @ref overview_log_targets
  29 @li @ref overview_log_customize
  30
  31
  32 @section overview_log_introduction Introduction
  33
  34 This is a general overview of logging classes provided by wxWidgets. The word
  35 logging here has a broad sense, including all of the program output, not only
  36 non-interactive messages. The logging facilities included in wxWidgets provide
  37 the base wxLog class which defines the standard interface for a @e log target
  38 as well as several standard implementations of it and a family of functions to
  39 use with them.
  40
  41 First of all, no knowledge of wxLog classes is needed to use them. For this,
  42 you should only know about @e wxLogXXX() functions. All of them have the same
  43 syntax as @e printf() or @e vprintf() , i.e. they take the format string as the
  44 first argument and respectively a variable number of arguments or a variable
  45 argument list pointer. Here are all of them:
  46
  47 @li wxLogFatalError which is like wxLogError, but also terminates the program
  48     with the exit code 3 (using @e abort() standard function). Unlike for all
  49     the other logging functions, this function can't be overridden by a log
  50     target.
  51 @li wxLogError is the function to use for error messages, i.e. the messages
  52     that must be shown to the user. The default processing is to pop up a
  53     message box to inform the user about it.
  54 @li wxLogWarning for warnings. They are also normally shown to the user, but
  55     don't interrupt the program work.
  56 @li wxLogMessage is for all normal, informational messages. They also appear in
  57     a message box by default (but it can be changed, see below).
  58 @li wxLogVerbose is for verbose output. Normally, it is suppressed, but might
  59     be activated if the user wishes to know more details about the program
  60     progress (another, but possibly confusing name for the same function is
  61     wxLogInfo).
  62 @li wxLogStatus is for status messages. They will go into the status bar of the
  63     active or specified (as the first argument) wxFrame if it has one.
  64 @li wxLogSysError is mostly used by wxWidgets itself, but might be handy for
  65     logging errors after system call (API function) failure. It logs the
  66     specified message text as well as the last system error code (@e errno or
  67     ::GetLastError() depending on the platform) and the corresponding error
  68     message. The second form of this function takes the error code explicitly
  69     as the first argument.
  70 @li wxLogDebug is @b the right function for debug output. It only does anything
  71     at all in the debug mode (when the preprocessor symbol __WXDEBUG__ is
  72     defined) and expands to nothing in release mode (otherwise). @b Tip: under
  73     Windows, you must either run the program under debugger or use a 3rd party
  74     program such as DebugView to actually see the debug output.
  75     - DebugView: http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx
  76 @li wxLogTrace as wxLogDebug only does something in debug build. The reason for
  77     making it a separate function from it is that usually there are a lot of
  78     trace messages, so it might make sense to separate them from other debug
  79     messages which would be flooded in them. Moreover, the second version of
  80     this function takes a trace mask as the first argument which allows to
  81     further restrict the amount of messages generated.
  82
  83 The usage of these functions should be fairly straightforward, however it may
  84 be asked why not use the other logging facilities, such as C standard stdio
  85 functions or C++ streams. The short answer is that they're all very good
  86 generic mechanisms, but are not really adapted for wxWidgets, while the log
  87 classes are. Some of advantages in using wxWidgets log functions are:
  88
  89 @li @b Portability: It is a common practice to use @e printf() statements or
  90     cout/cerr C++ streams for writing out some (debug or otherwise)
  91     information. Although it works just fine under Unix, these messages go
  92     strictly nowhere under Windows where the stdout of GUI programs is not
  93     assigned to anything. Thus, you might view wxLogMessage() as a simple
  94     substitute for @e printf().
  95     You can also redirect the @e wxLogXXX calls to @e cout by just writing:
  96     @code
  97     wxLog* logger = new wxLogStream(&cout);
  98     wxLog::SetActiveTarget(logger);
  99     @endcode
 100     Finally, there is also a possibility to redirect the output sent to @e cout
 101     to a wxTextCtrl by using the wxStreamToTextRedirector class.
 102 @li @b Flexibility: The output of wxLog functions can be redirected or
 103     suppressed entirely based on their importance, which is either impossible
 104     or difficult to do with traditional methods. For example, only error
 105     messages, or only error messages and warnings might be logged, filtering
 106     out all informational messages.
 107 @li @b Completeness: Usually, an error message should be presented to the user
 108     when some operation fails. Let's take a quite simple but common case of a
 109     file error: suppose that you're writing your data file on disk and there is
 110     not enough space. The actual error might have been detected inside
 111     wxWidgets code (say, in wxFile::Write), so the calling function doesn't
 112     really know the exact reason of the failure, it only knows that the data
 113     file couldn't be written to the disk. However, as wxWidgets uses
 114     wxLogError() in this situation, the exact error code (and the corresponding
 115     error message) will be given to the user together with "high level" message
 116     about data file writing error.
 117
 118
 119 @section overview_log_targets Log Targets
 120
 121 After having enumerated all the functions which are normally used to log the
 122 messages, and why would you want to use them we now describe how all this
 123 works.
 124
 125 wxWidgets has the notion of a <em>log target</em>: it is just a class deriving
 126 from wxLog. As such, it implements the virtual functions of the base class
 127 which are called when a message is logged. Only one log target is @e active at
 128 any moment, this is the one used by @e wxLogXXX() functions. The normal usage
 129 of a log object (i.e. object of a class derived from wxLog) is to install it as
 130 the active target with a call to @e SetActiveTarget() and it will be used
 131 automatically by all subsequent calls to @e wxLogXXX() functions.
 132
 133 To create a new log target class you only need to derive it from wxLog and
 134 implement one (or both) of @e DoLog() and @e DoLogString() in it. The second
 135 one is enough if you're happy with the standard wxLog message formatting
 136 (prepending "Error:" or "Warning:", timestamping @&c) but just want to send
 137 the messages somewhere else. The first one may be overridden to do whatever
 138 you want but you have to distinguish between the different message types
 139 yourself.
 140
 141 There are some predefined classes deriving from wxLog and which might be
 142 helpful to see how you can create a new log target class and, of course, may
 143 also be used without any change. There are:
 144
 145 @li wxLogStderr: This class logs messages to a <tt>FILE *</tt>, using stderr by
 146     default as its name suggests.
 147 @li wxLogStream: This class has the same functionality as wxLogStderr, but uses
 148     @e ostream and cerr instead of <tt>FILE *</tt> and stderr.
 149 @li wxLogGui: This is the standard log target for wxWidgets applications (it is
 150     used by default if you don't do anything) and provides the most reasonable
 151     handling of all types of messages for given platform.
 152 @li wxLogWindow: This log target provides a "log console" which collects all
 153     messages generated by the application and also passes them to the previous
 154     active log target. The log window frame has a menu allowing user to clear
 155     the log, close it completely or save all messages to file.
 156 @li wxLogBuffer: This target collects all the logged messages in an internal
 157     buffer allowing to show them later to the user all at once.
 158 @li wxLogNull: The last log class is quite particular: it doesn't do anything.
 159     The objects of this class may be instantiated to (temporarily) suppress
 160     output of @e wxLogXXX() functions. As an example, trying to open a
 161     non-existing file will usually provoke an error message, but if for some
 162     reasons it is unwanted, just use this construction:
 163     @code
 164     wxFile file;
 165
 166     // wxFile.Open() normally complains if file can't be opened, we don't want it
 167     {
 168         wxLogNull logNo;
 169         if ( !file.Open("bar") )
 170         {
 171             // ... process error ourselves ...
 172         }
 173     } // ~wxLogNull called, old log sink restored
 174
 175     wxLogMessage("..."); // ok
 176     @endcode
 177
 178 The log targets can also be combined: for example you may wish to redirect the
 179 messages somewhere else (for example, to a log file) but also process them as
 180 normally. For this the wxLogChain, wxLogInterposer, and wxLogInterposerTemp can
 181 be used.
 182
 183
 184 @section overview_log_customize Logging Customization
 185
 186 To completely change the logging behaviour you may define a custom log target.
 187 For example, you could define a class inheriting from wxLog which shows all the
 188 log messages in some part of your main application window reserved for the
 189 message output without interrupting the user work flow with modal message
 190 boxes.
 191
 192 To use your custom log target you may either call wxLog::SetActiveTarget() with
 193 your custom log object or create a wxAppTraits-derived class and override
 194 CreateLogTarget() virtual method in it and also override wxApp::CreateTraits()
 195 to return an instance of your custom traits object. Notice that in the latter
 196 case you should be prepared for logging messages early during the program
 197 startup and also during program shutdown so you shouldn't rely on existence of
 198 the main application window, for example. You can however safely assume that
 199 GUI is (already/still) available when your log target as used as wxWidgets
 200 automatically switches to using wxLogStderr if it isn't.
 201
 202 The dialog sample illustrates this approach by defining a custom log target
 203 customizing the dialog used by wxLogGui for the single messages.
 204
 205 */
 206