// Purpose: topic overview
// Author: wxWidgets team
// RCS-ID: $Id$
-// Licence: wxWindows license
+// Licence: wxWindows licence
/////////////////////////////////////////////////////////////////////////////
/**
@page overview_log wxLog Classes Overview
-Classes:
-@li wxLog
-@li wxLogStderr
-@li wxLogStream
-@li wxLogTextCtrl
-@li wxLogWindow
-@li wxLogGui
-@li wxLogNull
-@li wxLogBuffer
-@li wxLogChain
-@li wxLogInterposer
-@li wxLogInterposerTemp
-@li wxStreamToTextRedirector
+Classes: wxLog, wxLogStderr, wxLogStream, wxLogTextCtrl, wxLogWindow, wxLogGui, wxLogNull, wxLogBuffer,
+ wxLogChain, wxLogInterposer, wxLogInterposerTemp, wxStreamToTextRedirector, wxLogFormatter
+
+Table of contents:
+@li @ref overview_log_introduction
+@li @ref overview_log_enable
+@li @ref overview_log_targets
+@li @ref overview_log_mt
+@li @ref overview_log_customize
+@li @ref overview_log_tracemasks
+<hr>
+
+
+@section overview_log_introduction Introduction
This is a general overview of logging classes provided by wxWidgets. The word
logging here has a broad sense, including all of the program output, not only
use with them.
First of all, no knowledge of wxLog classes is needed to use them. For this,
-you should only know about @e wxLogXXX() functions. All of them have the same
-syntax as @e printf() or @e vprintf() , i.e. they take the format string as the
-first argument and respectively a variable number of arguments or a variable
-argument list pointer. Here are all of them:
+you should only know about @ref group_funcmacro_log "wxLogXXX() functions".
+All of them have the same syntax as @e printf() or @e vprintf() , i.e. they
+take the format string as the first argument and respectively a variable number
+of arguments or a variable argument list pointer. Here are all of them:
-@li wxLogFatalError which is like wxLogError, but also terminates the program
+@li wxLogFatalError() which is like wxLogError(), but also terminates the program
with the exit code 3 (using @e abort() standard function). Unlike for all
the other logging functions, this function can't be overridden by a log
target.
-@li wxLogError is the function to use for error messages, i.e. the messages
+@li wxLogError() is the function to use for error messages, i.e. the messages
that must be shown to the user. The default processing is to pop up a
message box to inform the user about it.
-@li wxLogWarning for warnings. They are also normally shown to the user, but
+@li wxLogWarning() for warnings. They are also normally shown to the user, but
don't interrupt the program work.
-@li wxLogMessage is for all normal, informational messages. They also appear in
+@li wxLogMessage() is for all normal, informational messages. They also appear in
a message box by default (but it can be changed, see below).
-@li wxLogVerbose is for verbose output. Normally, it is suppressed, but might
+@li wxLogVerbose() is for verbose output. Normally, it is suppressed, but might
be activated if the user wishes to know more details about the program
progress (another, but possibly confusing name for the same function is
wxLogInfo).
-@li wxLogStatus is for status messages. They will go into the status bar of the
+@li wxLogStatus() is for status messages. They will go into the status bar of the
active or specified (as the first argument) wxFrame if it has one.
-@li wxLogSysError is mostly used by wxWidgets itself, but might be handy for
+@li wxLogSysError() is mostly used by wxWidgets itself, but might be handy for
logging errors after system call (API function) failure. It logs the
specified message text as well as the last system error code (@e errno or
- ::GetLastError() depending on the platform) and the corresponding error
+ Windows' @e GetLastError() depending on the platform) and the corresponding error
message. The second form of this function takes the error code explicitly
as the first argument.
-@li wxLogDebug is @b the right function for debug output. It only does anything
- at all in the debug mode (when the preprocessor symbol __WXDEBUG__ is
- defined) and expands to nothing in release mode (otherwise). @b Tip: under
- Windows, you must either run the program under debugger or use a 3rd party
- program such as DebugView to actually see the debug output.
- - DebugView: http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx
-@li wxLogTrace as wxLogDebug only does something in debug build. The reason for
+@li wxLogDebug() is @b the right function for debug output. It only does anything
+ at all in the debug mode (when the preprocessor symbol @c __WXDEBUG__ is
+ defined) and expands to nothing in release mode (otherwise).
+ Note that under Windows, you must either run the program under debugger or
+ use a 3rd party program such as DebugView
+ (http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx)
+ to actually see the debug output.
+@li wxLogTrace() as wxLogDebug() only does something in debug build. The reason for
making it a separate function from it is that usually there are a lot of
trace messages, so it might make sense to separate them from other debug
messages which would be flooded in them. Moreover, the second version of
error message) will be given to the user together with "high level" message
about data file writing error.
+
+@section overview_log_enable Log Messages Selection
+
+By default, most log messages are enabled. In particular, this means that
+errors logged by wxWidgets code itself (e.g. when it fails to perform some
+operation, for instance wxFile::Open() logs an error when it fails to open a
+file) will be processed and shown to the user. To disable the logging entirely
+you can use wxLog::EnableLogging() method or, more usually, wxLogNull class
+which temporarily disables logging and restores it back to the original setting
+when it is destroyed.
+
+To limit logging to important messages only, you may use wxLog::SetLogLevel()
+with e.g. wxLOG_Warning value -- this will completely disable all logging
+messages with the severity less than warnings, so wxLogMessage() output won't
+be shown to the user any more.
+
+Moreover, the log level can be set separately for different log components.
+Before showing how this can be useful, let us explain what log components are:
+they are simply arbitrary strings identifying the component, or module, which
+generated the message. They are hierarchical in the sense that "foo/bar/baz"
+component is supposed to be a child of "foo". And all components are children
+of the unnamed root component.
+
+By default, all messages logged by wxWidgets originate from "wx" component or
+one of its subcomponents such as "wx/net/ftp", while the messages logged by
+your own code are assigned empty log component. To change this, you need to
+define @c wxLOG_COMPONENT to a string uniquely identifying each component, e.g.
+you could give it the value "MyProgram" by default and re-define it as
+"MyProgram/DB" in the module working with the database and "MyProgram/DB/Trans"
+in its part managing the transactions. Then you could use
+wxLog::SetComponentLevel() in the following ways:
+ @code
+ // disable all database error messages, everybody knows databases never
+ // fail anyhow
+ wxLog::SetComponentLevel("MyProgram/DB", wxLOG_FatalError);
+
+ // but enable tracing for the transactions as somehow our changes don't
+ // get committed sometimes
+ wxLog::SetComponentLevel("MyProgram/DB/Trans", wxLOG_Trace);
+
+ // also enable tracing messages from wxWidgets dynamic module loading
+ // mechanism
+ wxLog::SetComponentLevel("wx/base/module", wxLOG_Trace);
+ @endcode
+Notice that the log level set explicitly for the transactions code overrides
+the log level of the parent component but that all other database code
+subcomponents inherit its setting by default and so won't generate any log
+messages at all.
+
+@section overview_log_targets Log Targets
+
After having enumerated all the functions which are normally used to log the
-messages, and why would you want to use them we now describe how all this
+messages, and why would you want to use them, we now describe how all this
works.
wxWidgets has the notion of a <em>log target</em>: it is just a class deriving
from wxLog. As such, it implements the virtual functions of the base class
which are called when a message is logged. Only one log target is @e active at
-any moment, this is the one used by @e wxLogXXX() functions. The normal usage
-of a log object (i.e. object of a class derived from wxLog) is to install it as
-the active target with a call to @e SetActiveTarget() and it will be used
-automatically by all subsequent calls to @e wxLogXXX() functions.
+any moment, this is the one used by @ref group_funcmacro_log "wxLogXXX() functions".
+The normal usage of a log object (i.e. object of a class derived from wxLog) is
+to install it as the active target with a call to @e SetActiveTarget() and it will be used
+automatically by all subsequent calls to @ref group_funcmacro_log "wxLogXXX() functions".
To create a new log target class you only need to derive it from wxLog and
-implement one (or both) of @e DoLog() and @e DoLogString() in it. The second
-one is enough if you're happy with the standard wxLog message formatting
-(prepending "Error:" or "Warning:", timestamping @&c) but just want to send
-the messages somewhere else. The first one may be overridden to do whatever
-you want but you have to distinguish between the different message types
-yourself.
+override one or several of wxLog::DoLogRecord(), wxLog::DoLogTextAtLevel() and
+wxLog::DoLogText() in it. The first one is the most flexible and allows you to
+change the formatting of the messages, dynamically filter and redirect them and
+so on -- all log messages, except for those generated by wxLogFatalError(),
+pass by this function. wxLog::DoLogTextAtLevel() should be overridden if you
+simply want to redirect the log messages somewhere else, without changing their
+formatting. Finally, it is enough to override wxLog::DoLogText() if you only
+want to redirect the log messages and the destination doesn't depend on the
+message log level.
+
There are some predefined classes deriving from wxLog and which might be
helpful to see how you can create a new log target class and, of course, may
normally. For this the wxLogChain, wxLogInterposer, and wxLogInterposerTemp can
be used.
+
+@section overview_log_mt Logging in Multi-Threaded Applications
+
+Starting with wxWidgets 2.9.1, logging functions can be safely called from any
+thread. Messages logged from threads other than the main one will be buffered
+until wxLog::Flush() is called in the main thread (which usually happens during
+idle time, i.e. after processing all pending events) and will be really output
+only then. Notice that the default GUI logger already only output the messages
+when it is flushed, so by default messages from the other threads will be shown
+more or less at the same moment as usual. However if you define a custom log
+target, messages may be logged out of order, e.g. messages from the main thread
+with later timestamp may appear before messages with earlier timestamp logged
+from other threads. wxLog does however guarantee that messages logged by each
+thread will appear in order in which they were logged.
+
+Also notice that wxLog::EnableLogging() and wxLogNull class which uses it only
+affect the current thread, i.e. logging messages may still be generated by the
+other threads after a call to @c EnableLogging(false).
+
+
+@section overview_log_customize Logging Customization
+
+To completely change the logging behaviour you may define a custom log target.
+For example, you could define a class inheriting from wxLog which shows all the
+log messages in some part of your main application window reserved for the
+message output without interrupting the user work flow with modal message
+boxes.
+
+To use your custom log target you may either call wxLog::SetActiveTarget() with
+your custom log object or create a wxAppTraits-derived class and override
+wxAppTraits::CreateLogTarget() virtual method in it and also override wxApp::CreateTraits()
+to return an instance of your custom traits object. Notice that in the latter
+case you should be prepared for logging messages early during the program
+startup and also during program shutdown so you shouldn't rely on existence of
+the main application window, for example. You can however safely assume that
+GUI is (already/still) available when your log target as used as wxWidgets
+automatically switches to using wxLogStderr if it isn't.
+
+There are several methods which may be overridden in the derived class to
+customize log messages handling: wxLog::DoLogRecord(), wxLog::DoLogTextAtLevel()
+and wxLog::DoLogText().
+
+The last method is the simplest one: you should override it if you simply
+want to redirect the log output elsewhere, without taking into account the
+level of the message. If you do want to handle messages of different levels
+differently, then you should override wxLog::DoLogTextAtLevel().
+
+Additionally, you can customize the way full log messages are constructed from
+the components (such as time stamp, source file information, logging thread ID
+and so on). This task is performed by wxLogFormatter class so you need to
+derive a custom class from it and override its Format() method to build the log
+messages in desired way. Notice that if you just need to modify (or suppress)
+the time stamp display, overriding FormatTime() is enough.
+
+Finally, if even more control over the output format is needed, then
+DoLogRecord() can be overridden as it allows to construct custom messages
+depending on the log level or even do completely different things depending
+on the message severity (for example, throw away all messages except
+warnings and errors, show warnings on the screen and forward the error
+messages to the user's (or programmer's) cell phone -- maybe depending on
+whether the timestamp tells us if it is day or night in the current time
+zone).
+
+The @e dialog sample illustrates this approach by defining a custom log target
+customizing the dialog used by wxLogGui for the single messages.
+
+
+@section overview_log_tracemasks Using trace masks
+
+Notice that the use of log trace masks is hardly necessary any longer in
+current wxWidgets version as the same effect can be achieved by using
+different log components for different log statements of any level. Please
+see @ref overview_log_enable for more information about the log components.
+
+The functions below allow some limited customization of wxLog behaviour
+without writing a new log target class (which, aside from being a matter of
+several minutes, allows you to do anything you want).
+The verbose messages are the trace messages which are not disabled in the
+release mode and are generated by wxLogVerbose().
+They are not normally shown to the user because they present little interest,
+but may be activated, for example, in order to help the user find some program
+problem.
+
+As for the (real) trace messages, their handling depends on the currently
+enabled trace masks: if wxLog::AddTraceMask() was called for the mask of the given
+message, it will be logged, otherwise nothing happens.
+
+For example,
+@code
+wxLogTrace( wxTRACE_OleCalls, "IFoo::Bar() called" );
+@endcode
+
+will log the message if it was preceded by:
+
+@code
+wxLog::AddTraceMask( wxTRACE_OleCalls );
+@endcode
+
+The standard trace masks are given in wxLogTrace() documentation.
+
*/
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