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 <hr>
  32
  33
  34 @section overview_log_introduction Introduction
  35
  36 This is a general overview of logging classes provided by wxWidgets. The word
  37 logging here has a broad sense, including all of the program output, not only
  38 non-interactive messages. The logging facilities included in wxWidgets provide
  39 the base wxLog class which defines the standard interface for a @e log target
  40 as well as several standard implementations of it and a family of functions to
  41 use with them.
  42
  43 First of all, no knowledge of wxLog classes is needed to use them. For this,
  44 you should only know about @e wxLogXXX() functions. All of them have the same
  45 syntax as @e printf() or @e vprintf() , i.e. they take the format string as the
  46 first argument and respectively a variable number of arguments or a variable
  47 argument list pointer. Here are all of them:
  48
  49 @li wxLogFatalError which is like wxLogError, but also terminates the program
  50     with the exit code 3 (using @e abort() standard function). Unlike for all
  51     the other logging functions, this function can't be overridden by a log
  52     target.
  53 @li wxLogError is the function to use for error messages, i.e. the messages
  54     that must be shown to the user. The default processing is to pop up a
  55     message box to inform the user about it.
  56 @li wxLogWarning for warnings. They are also normally shown to the user, but
  57     don't interrupt the program work.
  58 @li wxLogMessage is for all normal, informational messages. They also appear in
  59     a message box by default (but it can be changed, see below).
  60 @li wxLogVerbose is for verbose output. Normally, it is suppressed, but might
  61     be activated if the user wishes to know more details about the program
  62     progress (another, but possibly confusing name for the same function is
  63     wxLogInfo).
  64 @li wxLogStatus is for status messages. They will go into the status bar of the
  65     active or specified (as the first argument) wxFrame if it has one.
  66 @li wxLogSysError is mostly used by wxWidgets itself, but might be handy for
  67     logging errors after system call (API function) failure. It logs the
  68     specified message text as well as the last system error code (@e errno or
  69     ::GetLastError() depending on the platform) and the corresponding error
  70     message. The second form of this function takes the error code explicitly
  71     as the first argument.
  72 @li wxLogDebug is @b the right function for debug output. It only does anything
  73     at all in the debug mode (when the preprocessor symbol __WXDEBUG__ is
  74     defined) and expands to nothing in release mode (otherwise). @b Tip: under
  75     Windows, you must either run the program under debugger or use a 3rd party
  76     program such as DebugView to actually see the debug output.
  77     - DebugView: http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx
  78 @li wxLogTrace as wxLogDebug only does something in debug build. The reason for
  79     making it a separate function from it is that usually there are a lot of
  80     trace messages, so it might make sense to separate them from other debug
  81     messages which would be flooded in them. Moreover, the second version of
  82     this function takes a trace mask as the first argument which allows to
  83     further restrict the amount of messages generated.
  84
  85 The usage of these functions should be fairly straightforward, however it may
  86 be asked why not use the other logging facilities, such as C standard stdio
  87 functions or C++ streams. The short answer is that they're all very good
  88 generic mechanisms, but are not really adapted for wxWidgets, while the log
  89 classes are. Some of advantages in using wxWidgets log functions are:
  90
  91 @li @b Portability: It is a common practice to use @e printf() statements or
  92     cout/cerr C++ streams for writing out some (debug or otherwise)
  93     information. Although it works just fine under Unix, these messages go
  94     strictly nowhere under Windows where the stdout of GUI programs is not
  95     assigned to anything. Thus, you might view wxLogMessage() as a simple
  96     substitute for @e printf().
  97     You can also redirect the @e wxLogXXX calls to @e cout by just writing:
  98     @code
  99     wxLog* logger = new wxLogStream(&cout);
 100     wxLog::SetActiveTarget(logger);
 101     @endcode
 102     Finally, there is also a possibility to redirect the output sent to @e cout
 103     to a wxTextCtrl by using the wxStreamToTextRedirector class.
 104 @li @b Flexibility: The output of wxLog functions can be redirected or
 105     suppressed entirely based on their importance, which is either impossible
 106     or difficult to do with traditional methods. For example, only error
 107     messages, or only error messages and warnings might be logged, filtering
 108     out all informational messages.
 109 @li @b Completeness: Usually, an error message should be presented to the user
 110     when some operation fails. Let's take a quite simple but common case of a
 111     file error: suppose that you're writing your data file on disk and there is
 112     not enough space. The actual error might have been detected inside
 113     wxWidgets code (say, in wxFile::Write), so the calling function doesn't
 114     really know the exact reason of the failure, it only knows that the data
 115     file couldn't be written to the disk. However, as wxWidgets uses
 116     wxLogError() in this situation, the exact error code (and the corresponding
 117     error message) will be given to the user together with "high level" message
 118     about data file writing error.
 119
 120
 121 @section overview_log_targets Log Targets
 122
 123 After having enumerated all the functions which are normally used to log the
 124 messages, and why would you want to use them we now describe how all this
 125 works.
 126
 127 wxWidgets has the notion of a <em>log target</em>: it is just a class deriving
 128 from wxLog. As such, it implements the virtual functions of the base class
 129 which are called when a message is logged. Only one log target is @e active at
 130 any moment, this is the one used by @e wxLogXXX() functions. The normal usage
 131 of a log object (i.e. object of a class derived from wxLog) is to install it as
 132 the active target with a call to @e SetActiveTarget() and it will be used
 133 automatically by all subsequent calls to @e wxLogXXX() functions.
 134
 135 To create a new log target class you only need to derive it from wxLog and
 136 implement one (or both) of @e DoLog() and @e DoLogString() in it. The second
 137 one is enough if you're happy with the standard wxLog message formatting
 138 (prepending "Error:" or "Warning:", timestamping @&c) but just want to send
 139 the messages somewhere else. The first one may be overridden to do whatever
 140 you want but you have to distinguish between the different message types
 141 yourself.
 142
 143 There are some predefined classes deriving from wxLog and which might be
 144 helpful to see how you can create a new log target class and, of course, may
 145 also be used without any change. There are:
 146
 147 @li wxLogStderr: This class logs messages to a <tt>FILE *</tt>, using stderr by
 148     default as its name suggests.
 149 @li wxLogStream: This class has the same functionality as wxLogStderr, but uses
 150     @e ostream and cerr instead of <tt>FILE *</tt> and stderr.
 151 @li wxLogGui: This is the standard log target for wxWidgets applications (it is
 152     used by default if you don't do anything) and provides the most reasonable
 153     handling of all types of messages for given platform.
 154 @li wxLogWindow: This log target provides a "log console" which collects all
 155     messages generated by the application and also passes them to the previous
 156     active log target. The log window frame has a menu allowing user to clear
 157     the log, close it completely or save all messages to file.
 158 @li wxLogBuffer: This target collects all the logged messages in an internal
 159     buffer allowing to show them later to the user all at once.
 160 @li wxLogNull: The last log class is quite particular: it doesn't do anything.
 161     The objects of this class may be instantiated to (temporarily) suppress
 162     output of @e wxLogXXX() functions. As an example, trying to open a
 163     non-existing file will usually provoke an error message, but if for some
 164     reasons it is unwanted, just use this construction:
 165     @code
 166     wxFile file;
 167
 168     // wxFile.Open() normally complains if file can't be opened, we don't want it
 169     {
 170         wxLogNull logNo;
 171         if ( !file.Open("bar") )
 172         {
 173             // ... process error ourselves ...
 174         }
 175     } // ~wxLogNull called, old log sink restored
 176
 177     wxLogMessage("..."); // ok
 178     @endcode
 179
 180 The log targets can also be combined: for example you may wish to redirect the
 181 messages somewhere else (for example, to a log file) but also process them as
 182 normally. For this the wxLogChain, wxLogInterposer, and wxLogInterposerTemp can
 183 be used.
 184
 185
 186 @section overview_log_customize Logging Customization
 187
 188 To completely change the logging behaviour you may define a custom log target.
 189 For example, you could define a class inheriting from wxLog which shows all the
 190 log messages in some part of your main application window reserved for the
 191 message output without interrupting the user work flow with modal message
 192 boxes.
 193
 194 To use your custom log target you may either call wxLog::SetActiveTarget() with
 195 your custom log object or create a wxAppTraits-derived class and override
 196 CreateLogTarget() virtual method in it and also override wxApp::CreateTraits()
 197 to return an instance of your custom traits object. Notice that in the latter
 198 case you should be prepared for logging messages early during the program
 199 startup and also during program shutdown so you shouldn't rely on existence of
 200 the main application window, for example. You can however safely assume that
 201 GUI is (already/still) available when your log target as used as wxWidgets
 202 automatically switches to using wxLogStderr if it isn't.
 203
 204 The dialog sample illustrates this approach by defining a custom log target
 205 customizing the dialog used by wxLogGui for the single messages.
 206
 207 */
 208