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From: <po...@us...> - 2003-03-22 15:58:53
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wxPython.txt
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New wxPython documentation.
--- NEW FILE: wxPython.txt ---
========================
The wxPython Reference
========================
--------------------------------------------------------
A guide to the wxPython toolkit for Python programmers
--------------------------------------------------------
:Author: Patrick K. O'Brien
:Contact: po...@or...
:Date: $Date: 2003/03/22 15:58:49 $
:Revision: $Revision: 1.1 $
:License: wxWindows Free Documentation Licence, Version 3
.. contents::
Introduction
============
This is a guide to the wxPython GUI toolkit, written **by** a Python
programmer **for** his fellow Python programmers. It began as a
simple translation of the wxWindows documentation (which is written
for C++ programmers), and evolved from there. And while there's
nothing wrong with C++...
Okay, you got me there. I hate C++. That's why I use Python. If you
like C++, go read the wxWindows documentation. If you'd rather read a
guide that's written with Python programmers in mind, keep reading
this one. If you like it, feel free to send me freshly roasted coffee
beans, dark chocolate, and large denomination currency. Better yet,
buy huge quantities of my wxPython book (written with Robin Dunn) and
send one to each of your friends, relatives, and coworkers.
Source Document
===============
The source document is named wxPython.txt and is located in the
PyCrust/wxd directory. It is written using a fantastic formatting
convention called reStructuredText. The wxPython.html file is created
using the Docutils utilities, which can turn reStructuredText
documents into html, xml, pdf, and even OpenOffice files.
Some items in the source text file look like this::
.. This is text from the wxWindows documentation that needs to be
translated into something appropriate for the wxPython version.
The two dots followed by uniformly indented text turns this
paragraph into a reStructuredText comment, so it doesn't appear
in any output file, such as the html file.
They have been commented out and are awaiting editorial review and a
rewrite so that they make sense in the context of wxPython. Feel free
to send me suggestions for rewording these, or any other parts of this
document that you think need improving. I will be eternally grateful
to you and will show my gratitude by adding your name to the list of
contributors. (Contributors who also send me gifts of coffee,
chocolate, or currency will have their names listed in bold.)
What is wxWindows?
==================
wxWindows is a C++ framework providing GUI (Graphical User Interface)
and other facilities on more than one platform. Version 2 currently
supports all desktop versions of MS Windows, Unix with GTK+, Unix with
Motif, and MacOS. An OS/2 port is in progress.
wxWindows was originally developed at the Artificial Intelligence
Applications Institute, University of Edinburgh, for internal use, and
was first made publicly available in 1992. Version 2 is a vastly
improved version written and maintained by Julian Smart, Robert
Roebling, Vadim Zeitlin, Vaclav Slavik and many others.
Please note that in the following, "MS Windows" often refers to all
platforms related to Microsoft Windows, including 16-bit and 32-bit
variants, unless otherwise stated. All trademarks are acknowledged.
Why another cross-platform development tool?
============================================
wxWindows was developed to provide a cheap and flexible way to
maximize investment in GUI application development. While a number of
commercial class libraries already existed for cross-platform
development, none met all of the following criteria:
* low price
* source availability
* simplicity of programming
* support for a wide range of compilers
Since wxWindows was started, several other free or almost-free GUI
frameworks have emerged. However, none has the range of features,
flexibility, documentation and the well-established development team
that wxWindows has.
As open source software, wxWindows has benefited from comments, ideas,
bug fixes, enhancements and the sheer enthusiasm of users. This gives
wxWindows a certain advantage over its commercial competitors (and
over free libraries without an independent development team), plus a
robustness against the transience of one individual or company. This
openness and availability of source code is especially important when
the future of thousands of lines of application code may depend upon
the longevity of the underlying class library.
Version 2 goes much further than previous versions in terms of
generality and features, allowing applications to be produced that are
often indistinguishable from those produced using single-platform
toolkits such as Motif, GTK+ and MFC.
The importance of using a platform-independent class library cannot be
overstated, since GUI application development is very time-consuming,
and sustained popularity of particular GUIs cannot be guaranteed.
Code can very quickly become obsolete if it addresses the wrong
platform or audience. wxWindows helps to insulate the programmer from
these winds of change. Although wxWindows may not be suitable for
every application (such as an OLE-intensive program), it provides
access to most of the functionality a GUI program normally requires,
plus many extras such as network programming, PostScript output, and
HTML rendering; and it can of course be extended as needs dictate. As
a bonus, it provides a far cleaner and easier programming interface
than the native APIs. Programmers may find it worthwhile to use
wxWindows even if they are developing on only one platform.
It is impossible to sum up the functionality of wxWindows in a few
paragraphs, but here are some of the benefits:
* Low cost (free, in fact!)
* You get the source.
* Available on a variety of popular platforms.
* Works with almost all popular C++ compilers and Python.
* Over 50 example programs.
* Over 1000 pages of printable and on-line documentation.
* Includes Tex2RTF, to allow you to produce your own documentation in
Windows Help, HTML and Word RTF formats.
* Simple-to-use, object-oriented API.
* Flexible event system.
* Graphics calls include lines, rounded rectangles, splines,
polylines, etc.
* Constraint-based and sizer-based layouts.
* Print/preview and document/view architectures.
* Toolbar, notebook, tree control, advanced list control classes.
* PostScript generation under Unix, normal MS Windows printing on the
PC.
* MDI (Multiple Document Interface) support.
* Can be used to create DLLs under Windows, dynamic libraries on Unix.
* Common dialogs for file browsing, printing, colour selection, etc.
* Under MS Windows, support for creating metafiles and copying them to
the clipboard.
* An API for invoking help from applications.
* Ready-to-use HTML window (supporting a subset of HTML).
* Dialog Editor for building dialogs.
* Network support via a family of socket and protocol classes.
* Support for platform independent image processing.
* Built-in support for many file formats (BMP, PNG, JPEG, GIF, XPM,
PNM, PCX).
What is wxPython?
=================
wxPython is a GUI toolkit for the Python programming language. It
allows Python programmers to create programs with a robust, highly
functional graphical user interface, simply and easily. It is
implemented as a Python extension module (native code) that wraps the
popular wxWindows cross platform GUI library, which is written in C++.
Like Python and wxWindows, wxPython is Open Source, which means that
it is free for anyone to use and the source code is available for
anyone to look at and modify. And anyone can contribute fixes or
enhnacments to the project.
wxPython is a cross-platform toolkit. This means that the same
program will run on multiple platforms without modification.
Currently supported platforms are 32-bit Microsoft Windows, most Unix
or unix-like systems, and Macintosh OS X.
Since the language is Python, wxPython programs are simple, easy to
write and easy to understand.
wxPython requirements
=====================
To make use of wxPython, you currently need one of the following
setups.
MS-Windows
----------
* A 486 or higher PC running MS Windows.
* At least ?? MB of disk space.
Linux or Unix
-------------
* Almost any C++ compiler, including GNU C++ (EGCS 1.1.1 or above).
* Almost any Unix workstation, and one of: GTK+ 1.2, GTK+ 2.0, Motif
1.2 or higher, Lesstif.
* At least ?? MB of disk space.
Mac OS X
--------
* A PowerPC Mac running Mac OS X 10.x.
* At least ?? MB of disk space.
wxPython Overview
=================
To set a wxPython application going, you will need to derive an App
class and override App.OnInit.
An application must have a top-level Frame or Dialog window. Each
frame may contain one or more instances of classes such as Panel,
SplitterWindow or other windows and controls.
A frame can have a MenuBar, a ToolBar, a status line, and an Icon for
when the frame is iconized.
A Panel is used to place controls (classes derived from Control) which
are used for user interaction. Examples of controls are Button,
CheckBox, Choice, ListBox, RadioBox, Slider.
Instances of Dialog can also be used for controls, and they have the
advantage of not requiring a separate frame.
Instead of creating a dialog box and populating it with items, it is
possible to choose one of the convenient common dialog classes, such
as MessageDialog and FileDialog.
You never draw directly onto a window. Instead, you use a device
context (DC). DC is the base for ClientDC, PaintDC, MemoryDC,
PostScriptDC, MemoryDC, MetafileDC and PrinterDC. If your drawing
functions have DC as a parameter, you can pass any of these DCs to the
function, and thus use the same code to draw to several different
devices. You can draw using the member functions of DC, such as
DC.DrawLine and DC.DrawText. Control colour on a window (Colour) with
brushes (Brush) and pens (Pen).
.. To intercept events, you add a DECLARE_EVENT_TABLE macro to the
window class declaration, and put a BEGIN_EVENT_TABLE
... END_EVENT_TABLE block in the implementation file. Between these
macros, you add event macros which map the event (such as a mouse
click) to a member function. These might override predefined event
handlers such as for KeyEvent and MouseEvent.
Most modern applications will have an on-line, hypertext help system;
for this, you need Help and the HelpController class to control
Help.
GUI applications aren't all graphical wizardry. You'll also need
lists and hash tables. But since you're working with Python, you
should use the ones Python provides (list, tuple, dict), rather than
the wxWindows versions. Same goes for the database related classes.
The basic rule of thumb is this: If you can do it directly in Python,
you probably should. If there is a reason not to use a Python data
type, wxPython will provide a wrapper for the wxWindows class.
You will undoubtedly need some platform-independent file functions,
and you may find it handy to maintain and search a list of paths using
PathList. There's a miscellany of operating system and other
functions.
See also Classes by Category for a list of classes.
Utilities and libraries supplied with wxPython
==============================================
In addition to the core wxWindows library, a number of further
libraries and utilities are supplied with each distribution.
[Need to list these.]
Creating and deleting wxPython objects
======================================
[This section needs to be reviewed.]
.. In general, classes derived from wxWindow must dynamically
allocated with new and deleted with delete. If you delete a window,
all of its children and descendants will be automatically deleted,
so you don't need to delete these descendants explicitly.
.. When deleting a frame or dialog, use Destroy rather than delete so
that the wxWindows delayed deletion can take effect. This waits
until idle time (when all messages have been processed) to actually
delete the window, to avoid problems associated with the GUI
sending events to deleted windows.
.. If you decide to allocate a C++ array of objects (such as wxBitmap)
that may be cleaned up by wxWindows, make sure you delete the array
explicitly before wxWindows has a chance to do so on exit, since
calling delete on array members will cause memory problems.
.. wxColour can be created statically: it is not automatically cleaned
up and is unlikely to be shared between other objects; it is
lightweight enough for copies to be made.
.. Beware of deleting objects such as a wxPen or wxBitmap if they are
still in use. Windows is particularly sensitive to this: so make
sure you make calls like wxDC::SetPen(wxNullPen) or
wxDC::SelectObject(wxNullBitmap) before deleting a drawing object
that may be in use. Code that doesn't do this will probably work
fine on some platforms, and then fail under Windows.
App overview
============
Classes: wx.App
Application initialization
--------------------------
The OnInit method defined for a class derived from wx.App will usually
create a top window as a bare minimum.
OnInit must return a boolean value to indicate whether processing
should continue (True) or not (False). You call App.SetTopWindow to
let wxPython know about the top window.
An application closes by destroying all windows. Because all frames
must be destroyed for the application to exit, it is advisable to use
parent frames wherever possible when creating new frames, so that
deleting the top level frame will automatically delete child frames.
The alternative is to explicitly delete child frames in the top-level
frame's CloseEvent handler.
In emergencies the wx.Exit() function can be called to kill the
application, however, normally the application shuts down
automatically, see below.
An example of defining an application follows::
import wx
from frame import Frame
class App(wx.App):
"""Application class."""
def OnInit(self):
self.frame = Frame()
self.frame.Show()
self.SetTopWindow(self.frame)
return True
def main():
app = App()
app.MainLoop()
if __name__ == '__main__':
main()
Application shutdown
--------------------
The application normally shuts down when the last of its top level
windows is closed. This is normally the expected behaviour and means
that it is enough to call Close() in response to the "Exit" menu
command if your program has a single top level window. If this
behaviour is not desirable, App.SetExitOnFrameDelete can be called to
change it. Note that such logic doesn't apply for the windows shown
before the program enters the main loop: in other words, you can
safely show a dialog from App.OnInit and not be afraid that your
application terminates when this dialog -- which is the last top level
window for the moment -- is closed.
Another aspect of the application shutdown is the OnExit which is
called when the application exits but before wxPython cleans up its
internal structures. You should delete all wxPython objects that you
created by the time OnExit finishes.
For example, this code may crash:
[Need examples of objects needing cleanup to keep app from crashing.]
Sizer overview
==============
Classes: wx.Sizer, wx.GridSizer, wx.FlexGridSizer, wx.BoxSizer,
wx.StaticBoxSizer, wx.NotebookSizer, wx.CreateButtonSizer
Sizers, as represented by the wx.Sizer class and its descendants in
the wxPython class hierarchy, have become the method of choice to
define the layout of controls in dialogs in wxPython because of their
ability to create visually appealing dialogs independent of the
platform, taking into account the differences in size and style of the
individual controls. Editors such as wxDesigner, wxrcedit, XRCed and
wxWorkshop create dialogs based exclusively on sizers, practically
forcing the user to create platform independent layouts without
compromises.
The idea behind sizers
----------------------
The layout algorithm used by sizers in wxPython is closely related to
layout systems in other GUI toolkits, such as Java's AWT, the GTK
toolkit or the Qt toolkit. It is based upon the idea of individual
subwindows reporting their minimal required size and their ability to
get stretched if the size of the parent window has changed. This will
most often mean that the programmer does not set the start-up size of
a dialog, the dialog will rather be assigned a sizer and this sizer
will be queried about the recommended size. This sizer in turn will
query its children (which can be normal windows, empty space or other
sizers) so that a hierarchy of sizers can be constructed. Note that
wx.Sizer does not derive from wx.Window and thus does not interfere
with tab ordering and requires very few resources compared to a real
window on screen.
What makes sizers so well fitted for use in wxPython is the fact that
every control reports its own minimal size and the algorithm can
handle differences in font sizes or different window (dialog item)
sizes on different platforms without problems. For example, if the
standard font as well as the overall design of Linux/GTK widgets
requires more space than on Windows, the initial dialog size will
automatically be bigger on Linux/GTK than on Windows.
There are currently five different kinds of sizers available in
wxPython. Each represents either a certain way to lay out dialog items
in a dialog or it fulfils a special task such as wrapping a static box
around a dialog item (or another sizer). These sizers will be
discussed one by one in the text below.
Common features
---------------
All sizers are containers, that is, they are used to lay out one
dialog item (or several dialog items), which they contain. Such items
are sometimes referred to as the children of the sizer. Independent
of how the individual sizers lay out their children, all children have
certain features in common:
A minimal size
~~~~~~~~~~~~~~
This minimal size is usually identical to the initial size of the
controls and may either be set explicitly in the size field of the
control constructor or may be calculated by wxPython, typically by
setting the height and/or the width of the item to -1. Note that only
some controls can calculate their size (such as a checkbox) whereas
others (such as a listbox) don't have any natural width or height and
thus require an explicit size. Some controls can calculate their
height, but not their width (e.g. a single line text control):
[Need graphics]
A border
~~~~~~~~
The border is just empty space and is used to separate dialog items in
a dialog. This border can either be all around, or at any combination
of sides such as only above and below the control. The thickness of
this border must be set explicitly, typically 5 points. The following
samples show dialogs with only one dialog item (a button) and a border
of 0, 5, and 10 pixels around the button:
[Need graphics]
An alignment
~~~~~~~~~~~~
Often, a dialog item is given more space than its minimal size plus
its border. Depending on what flags are used for the respective dialog
item, the dialog item can be made to fill out the available space
entirely, i.e. it will grow to a size larger than the minimal size, or
it will be moved to either the centre of the available space or to
either side of the space. The following sample shows a listbox and
three buttons in a horizontal box sizer; one button is centred, one is
aligned at the top, one is aligned at the bottom:
[Need graphics]
A stretch factor
~~~~~~~~~~~~~~~~
If a sizer contains more than one child and it is offered more space
than its children and their borders need, the question arises how to
distribute the surplus space among the children. For this purpose, a
stretch factor may be assigned to each child, where the default value
of 0 indicates that the child will not get more space than its
requested minimum size. A value of more than zero is interpreted in
relation to the sum of all stretch factors in the children of the
respective sizer, i.e. if two children get a stretch factor of 1, they
will get half the extra space each independent of whether one control
has a minimal sizer inferior to the other or not. The following
sample shows a dialog with three buttons, the first one has a stretch
factor of 1 and thus gets stretched, whereas the other two buttons
have a stretch factor of zero and keep their initial width:
[Need graphics]
Within wxDesigner, this stretch factor gets set from the Option menu.
BoxSizer
--------
BoxSizer can lay out its children either vertically or horizontally,
depending on what flag is being used in its constructor. When using a
vertical sizer, each child can be centered, aligned to the right or
aligned to the left. Correspondingly, when using a horizontal sizer,
each child can be centered, aligned at the bottom or aligned at the
top. The stretch factor described in the last paragraph is used for
the main orientation, i.e. when using a horizontal box sizer, the
stretch factor determines how much the child can be stretched
horizontally. The following sample shows the same dialog as in the
last sample, only the box sizer is a vertical box sizer now:
[Need graphics]
StaticBoxSizer
--------------
StaticBoxSixer is the same as a BoxSizer, but surrounded by a static
box. Here is a sample:
[Need graphics]
GridSizer
---------
GridSizer is a two-dimensional sizer. All children are given the same
size, which is the minimal size required by the biggest child, in this
case the text control in the left bottom border. Either the number of
columns or the number or rows is fixed and the grid sizer will grow in
the respectively other orientation if new children are added:
[Need graphics]
FlexGridSizer
-------------
Another two-dimensional sizer derived from GridSizer. The width of
each column and the height of each row are calculated individually
according the minimal requirements from the respectively biggest
child. Additionally, columns and rows can be declared to be
stretchable if the sizer is assigned a size different from that which
it requested. The following sample shows the same dialog as the one
above, but using a flex grid sizer:
[Need graphics]
NotebookSizer
-------------
NotebookSizer can be used with notebooks. It calculates the size of
each notebook page and sets the size of the notebook to the size of
the biggest page plus some extra space required for the notebook tabs
and decorations.
[Need graphics]
Programming with BoxSizer
-------------------------
The basic idea behind a BoxSizer is that windows will most often be
laid out in rather simple basic geometry, typically in a row or a
column or several hierarchies of either.
As an example, we will construct a dialog that will contain a text
field at the top and two buttons at the bottom. This can be seen as a
top-hierarchy column with the text at the top and buttons at the
bottom and a low-hierarchy row with an OK button to the left and a
Cancel button to the right. In many cases (particularly dialogs under
Unix and normal frames) the main window will be resizable by the user
and this change of size will have to get propagated to its children.
In our case, we want the text area to grow with the dialog, whereas
the button shall have a fixed size. In addition, there will be a thin
border around all controls to make the dialog look nice and - to make
matter worse - the buttons shall be centred as the width of the dialog
changes.
It is the unique feature of a box sizer, that it can grow in both
directions (height and width) but can distribute its growth in the
main direction (horizontal for a row) unevenly among its children. In
our example case, the vertical sizer is supposed to propagate all its
height changes to only the text area, not to the button area. This is
determined by the proportion parameter when adding a window (or
another sizer) to a sizer. It is interpreted as a weight factor,
i.e. it can be zero, indicating that the window may not be resized at
all, or above zero. If several windows have a value above zero, the
value is interpreted relative to the sum of all weight factors of the
sizer, so when adding two windows with a value of 1, they will both
get resized equally much and each half as much as the sizer owning
them.
Then what do we do when a column sizer changes its width? This
behaviour is controlled by flags (the second parameter of the Add()
function): zero or no flag indicates that the window will preserve it
is original size, wx.GROW flag (same as wx.EXPAND) forces the window
to grow with the sizer, and wx.SHAPED flag tells the window to change
it is size proportionally, preserving original aspect ratio. When
wx.GROW flag is not used, the item can be aligned within available
space. wx.ALIGN_LEFT, wx.ALIGN_TOP, wx.ALIGN_RIGHT, wx.ALIGN_BOTTOM,
wx.ALIGN_CENTER_HORIZONTAL and wx.ALIGN_CENTER_VERTICAL do what they
say. wx.ALIGN_CENTRE (same as wx.ALIGN_CENTER) is defined as
(``wx.ALIGN_CENTER_HORIZONTAL | wx.ALIGN_CENTER_VERTICAL``). Default
alignment is ``wx.ALIGN_LEFT | wx.ALIGN_TOP``.
As mentioned above, any window belonging to a sizer may have border,
and it can be specified which of the four sides may have this border,
using the wx.TOP, wx.LEFT, wx.RIGHT and wx.BOTTOM constants or wx.ALL
for all directions (and you may also use wx.NORTH, wx.WEST etc
instead). These flags can be used in combination with the alignment
flags above as the second parameter of the Add() method using the
binary or operator (``|``). The sizer of the border also must be made
known, and it is the third parameter in the Add() method. This means,
that the entire behaviour of a sizer and its children can be
controlled by the three parameters of the Add() method.
[Show code and graphic here.]
Programming with GridSizer
--------------------------
GridSizer is a sizer which lays out its children in a two-dimensional
table with all table fields having the same size, i.e. the width of
each field is the width of the widest child, the height of each field
is the height of the tallest child.
[Show code and graphic here.]
Programming with FlexGridSizer
------------------------------
FlexGridSizer is a sizer which lays out its children in a
two-dimensional table with all table fields in one row having the same
height and all fields in one column having the same width, but all
rows or all columns are not necessarily the same height or width as in
the GridSizer.
[Show code and graphic here.]
Programming with NotebookSizer
------------------------------
NotebookSizer is a specialized sizer to make sizers work in connection
with using notebooks. This sizer is different from any other sizer as
you must not add any children to it - instead, it queries the notebook
class itself. The only thing this sizer does is to determine the size
of the biggest page of the notebook and report an adjusted minimal
size to a more toplevel sizer.
In order to query the size of notebook page, this page needs to have
its own sizer, otherwise the NotebookSizer will ignore it. Notebook
pages get their sizer by assigning one to them using SetSizer() and
setting the auto-layout option to True using SetAutoLayout(). Here is
one example showing how to add a notebook page that the notebook sizer
is aware of:
[Show code and graphic here.]
Programming with StaticBoxSizer
-------------------------------
StaticBoxSizer is a sizer derived from BoxSizer but adds a static box
around the sizer. Note that this static box has to be created
separately.
[Show code and graphic here.]
CreateButtonSizer
-----------------
As a convenience, wx.CreateButtonSizer(flags) can be used to create a
standard button sizer in which standard buttons are displayed. The
following flags can be passed to this function::
wxYES_NO // Add Yes/No subpanel
wxYES // return wxID_YES
wxNO // return wxID_NO
wxNO_DEFAULT // make the wxNO button the default, otherwise wxYES or wxOK button will be default
wxOK // return wxID_OK
wxCANCEL // return wxID_CANCEL
wxHELP // return wxID_HELP
wxFORWARD // return wxID_FORWARD
wxBACKWARD // return wxID_BACKWARD
wxSETUP // return wxID_SETUP
wxMORE // return wxID_MORE
Date and time classes overview
==============================
wxPython provides a set of powerful classes to work with dates and
times. Some of the supported features of the DateTime class are:
============= =======================================================
Wide range The range of supported dates goes from about 4714 B.C. to
some 480 million years in the future.
Precision Not using floating point calculations anywhere ensures that
the date calculations don't suffer from rounding
errors.
Many features Not only all usual calculations with dates are
supported, but also more exotic week and year day
calculations, work day testing, standard astronomical
functions, conversion to and from strings in either
strict or free format.
Efficiency Objects of DateTime are small (8 bytes) and working
with them is fast
============= =======================================================
All date/time classes at a glance
---------------------------------
There are 3 main classes: except DateTime itself which represents an
absolute moment in time, there are also two classes - TimeSpan and
DateSpan which represent the intervals of time.
There are also helper classes which are used together with DateTime:
DateTimeHolidayAuthority which is used to determine whether a given
date is a holiday or not and DateTimeWorkDays which is a derivation of
this class for which (only) Saturdays and Sundays are the holidays.
See more about these classes in the discussion of the holidays.
DateTime characteristics
------------------------
DateTime stores the time as a signed number of milliseconds since the
Epoch which is fixed, by convention, to Jan 1, 1970 - however this is
not visible to the class users (in particular, dates prior to the
Epoch are handled just as well (or as bad) as the dates after it).
But it does mean that the best resolution which can be achieved with
this class is 1 millisecond.
The size of DateTime object is 8 bytes because it is represented as a
64 bit integer. The resulting range of supported dates is thus
approximatively 580 million years, but due to the current limitations
in the Gregorian calendar support, only dates from Nov 24, 4714BC are
supported (this is subject to change if there is sufficient interest
in doing it).
Finally, the internal representation is time zone independent (always
in GMT) and the time zones only come into play when a date is broken
into year/month/day components. See more about timezones below.
Currently, the only supported calendar is Gregorian one (which is used
even for the dates prior to the historic introduction of this calendar
which was first done on Oct 15, 1582 but is, generally speaking,
country, and even region, dependent). Future versions will probably
have Julian calendar support as well and support for other calendars
(Maya, Hebrew, Chinese...) is not ruled out.
Difference between DateSpan and TimeSpan
----------------------------------------
While there is only one logical way to represent an absolute moment in
the time (and hence only one DateTime class), there are at least two
methods to describe a time interval.
First, there is the direct and self-explaining way implemented by
TimeSpan: it is just a difference in milliseconds between two moments
in time. Adding or subtracting such an interval to DateTime is always
well-defined and is a fast operation.
But in daily life other, calendar-dependent time interval
specifications are used. For example, 'one month later' is commonly
used. However, it is clear that this is not the same as TimeSpan of
60*60*24*31 seconds because 'one month later' Feb 15 is Mar 15 and not
Mar 17 or Mar 16 (depending on whether the year is leap or not).
This is why there is another class for representing such intervals
called DateSpan. It handles these sort of operations in the most
natural way possible, but note that manipulating with intervals of
this kind is not always well-defined. Consider, for example, Jan 31 +
'1 month': this will give Feb 28 (or 29), i.e. the last day of
February and not the non-existent Feb 31. Of course, this is what is
usually wanted, but you still might be surprised to notice that now
subtracting back the same interval from Feb 28 will result in Jan 28
and not Jan 31 we started with!
So, unless you plan to implement some kind of natural language parsing
in the program, you should probably use TimeSpan instead of DateSpan
(which is also more efficient). However, DateSpan may be very useful
in situations when you do need to understand what 'in a month' means
(of course, it is just DateTime.Now() + DateSpan.Month()).
Date arithmetics
----------------
Many different operations may be performed with the dates, however not
all of them make sense. For example, multiplying a date by a number
is an invalid operation, even though multiplying either of the time
span classes by a number is perfectly valid.
Here is what can be done:
============== ======================================================
Addition a TimeSpan or DateSpan can be added to DateTime resulting in
a new DateTime object and also 2 objects of the same
span class can be added together giving another object
of the same class.
Subtraction the same types of operations as above are allowed and,
additionally, a difference between two DateTime
objects can be taken and this will yield TimeSpan.
Multiplication a TimeSpan or DateSpan object can be multiplied by an
integer number resulting in an object of the same
type.
Unary minus a TimeSpan or DateSpan object may finally be negated
giving an interval of the same magnitude but of
opposite time direction.
============== ======================================================
Time zone considerations
------------------------
Although the time is always stored internally in GMT, you will usually
work in the local time zone. Because of this, all DateTime
constructors and setters which take the broken down date assume that
these values are for the local time zone. Thus, DateTime(1,
DateTime.Jan, 1970) will not correspond to the DateTime Epoch unless
you happen to live in the UK.
All methods returning the date components (year, month, day, hour,
minute, second...) will also return the correct values for the local
time zone by default. So, generally, doing the natural things will
lead to natural and correct results.
If you only want to do this, you may safely skip the rest of this
section. However, if you want to work with different time zones, you
should read it to the end.
In this (rare) case, you are still limited to the local time zone when
constructing DateTime objects, i.e. there is no way to construct a
DateTime corresponding to the given date in, say, Pacific Standard
Time. To do it, you will need to call ToTimezone or MakeTimezone
methods to adjust the date for the target time zone. There are also
special versions of these functions ToGMT and MakeGMT for the most
common case - when the date should be constructed in GMT.
You also can just retrieve the value for some time zone without
converting the object to it first. For this you may pass TimeZone
argument to any of the methods which are affected by the time zone
(all methods getting date components and the date formatting ones, for
example). In particular, the Format() family of methods accepts a
TimeZone parameter and this allows to simply print time in any time
zone.
To see how to do it, the last issue to address is how to construct a
TimeZone object which must be passed to all these methods. First of
all, you may construct it manually by specifying the time zone offset
in seconds from GMT, but usually you will just use one of the symbolic
time zone names and let the conversion constructor do the
job. I.e. you would just write
wxDateTime dt(...whatever...);
printf("The time is %s in local time zone", dt.FormatTime().c_str());
printf("The time is %s in GMT", dt.FormatTime(wxDateTime::GMT).c_str());
Daylight saving time (DST)
--------------------------
DST (a.k.a. 'summer time') handling is always a delicate task which is
better left to the operating system which is supposed to be configured
by the administrator to behave correctly. Unfortunately, when doing
calculations with date outside of the range supported by the standard
library, we are forced to deal with these issues ourselves.
Several functions are provided to calculate the beginning and end of
DST in the given year and to determine whether it is in effect at the
given moment or not, but they should not be considered as absolutely
correct because, first of all, they only work more or less correctly
for only a handful of countries (any information about other ones
appreciated!) and even for them the rules may perfectly well change in
the future.
The time zone handling methods use these functions too, so they are
subject to the same limitations.
DateTime and Holidays
---------------------
[TODO]
Classes by category
===================
Not done yet.
Contributors
============
Individuals who contributed to this documentation (in order by last
name):
* Robin Dunn
* Patrick K. O'Brien
* Robert Roebling
* Julian Smart
* Vadim Zeitlin
License
=======
This document began as a translation of the wxWindows documentation.
As such, it adheres to the same license, which is provided here:
.. include:: LICENSE.txt
:literal:
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