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<H1 ALIGN="RIGHT"><A NAME="basics">2 pyFLTK Basics</A></H1>
<P>This chapter teaches you the basics of writing Python scripts
that use pyFLTK.</P>
<H2>Writing Your First pyFLTK Script</H2>
<P>All scripts must import fltk: <TT>from fltk import *</TT>.
Listing 1 shows a simple "Hello,
World!" program that uses pyFLTK to display the window.</P>
<UL>
<P><I>Listing 1 - "hello.py"</I>
<PRE>
from fltk import *
import sys
window = Fl_Window(300,180)
box = Fl_Box(20,40,260,100,"Hello, World!")
box.box(FL_UP_BOX)
box.labelsize(36)
box.labelfont(FL_BOLD+FL_ITALIC)
box.labeltype(FL_SHADOW_LABEL)
window.end()
window.show(sys.argv)
Fl.run()
</PRE></UL>
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<P>After running python hello.py, the program first creates a
window:</P>
<UL><PRE>
window = Fl_Window(300,180)
</PRE></UL>
<P>and a box with the "Hello, World!" string in it:</P>
<UL><PRE>
box = Fl_Box(20,40,260,100,"Hello, World!")
</PRE></UL>
<P>Next, we set the type of box and the size, font, and style of the label:</P>
<UL><PRE>
box.box(FL_UP_BOX)
box.labelsize(36)
box.labelfont(FL_BOLD+FL_ITALIC)
box.labeltype(FL_SHADOW_LABEL)
</PRE></UL>
<P>Finally, we show the window and enter the FLTK event loop:</P>
<UL><PRE>
window.end()
window.show(sys.argv)
Fl.run()
</PRE></UL>
<P>The resulting program will display the window in Figure 2-1.
You can quit the program by closing the window or pressing the
<KBD>ESC</KBD>ape key.</P>
<P ALIGN="CENTER"><IMG src="Hello.jpg" alt="Hello, World! Window"><BR>
<I>Figure 2-1: The Hello, World! Window</I></P>
<H3>Creating the Widgets</H3>
<P>The widgets are created using a normal Python constructor. For
most widgets the arguments to the constructor are:</P>
<UL><PRE>
Fl_Widget(x, y, width, height, label)
</PRE></UL>
<P>The <TT>x</TT> and <TT>y</TT> parameters determine where the
widget or window is placed on the screen. In pyFLTK the top left
corner of the window or screen is the origin (i.e. x = 0, y =
0) and the units are in pixels.</P>
<P>The <TT>width</TT> and <TT>height</TT> parameters determine
the size of the widget or window in pixels. The maximum widget
size is typically governed by the underlying window system or
hardware.</P>
<P><tt>label</tt> is a character string to label
the widget with or <tt>None</tt>. If not specified the label
defaults to <tt>None</tt>. The label string must keep an external reference, because pyFLTK does not increase its reference count.</P>
<H3>Get/Set Methods</H3>
<P><tt>box.box(FL_UP_BOX)</tt> sets the type of box the
Fl_Box draws, changing it from the default of
<tt>FL_NO_BOX</tt>, which means that no box is drawn. In our
"Hello, World!" example we use <TT>FL_UP_BOX</TT>,
which means that a raised button border will be drawn around
the widget. You can learn more about boxtypes in
<A href="CH3_Common.html#boxtypes">Chapter 3</A>.</P>
<P>You could examine the boxtype in by doing
<tt>box.box()</tt>. pyFLTK uses method name overloading to make
short names for get/set methods. </P>
<H3>Redrawing After Changing Attributes</H3>
<P>Almost all of the set/get pairs are very fast, short
functions and thus very efficient. However, <i>the "set" methods
do not call <TT>redraw()</TT></i> - you have to call it
yourself. This greatly reduces code size and execution time. The
only common exceptions are <tt>value()</tt> which calls
<TT>redraw()</TT> and <tt>label()</tt> which calls
<TT>redraw_label()</TT> if necessary.</P>
<H3>Labels</H3>
<P>All widgets support labels. In the case of window widgets,
the label is used for the label in the title bar. Our example
program calls the <A href=fltk.html#Fl_Widget-labelfont>
<TT>labelfont</TT></A>,
<A href=fltk.html#Fl_Widget-labelsize><TT> labelsize</TT></A>,
and <A href=fltk.html#Fl_Widget-labeltype><TT>labeltype</TT></A>
methods.</P>
<P>The <TT>labelfont</TT> method sets the typeface and style
that is used for the label, which for this example we are using
<TT>FL_BOLD</TT> and <TT>FL_ITALIC</TT>. You can also specify
typefaces directly. </P> <P>The <TT>labelsize</TT> method sets
the height of the font in pixels. </P> <P>The <TT>labeltype</TT>
method sets the type of label. FLTK supports normal, embossed,
and shadowed labels internally, and more types can be added as
desired.</P>
<P>A complete list of all label options can be found in
<A href="CH3_Common.html#labels">Chapter 3</A>.</P>
<H3>Showing the Window</H3>
<P>The <TT>show()</TT> method shows the widget or window. For windows
you can also provide the command-line arguments to allow users to
customize the appearance, size, and position of your windows.</P>
<H3>The Main Event Loop</H3>
<P>All pyFLTK applications (and most GUI applications in general)
are based on a simple event processing model. User actions such
as mouse movement, button clicks, and keyboard activity generate
events that are sent to an application. The application may then
ignore the events or respond to the user, typically by redrawing
a button in the "down" position, adding the text to an input
field, and so forth.</P>
<P>pyFLTK also supports idle, timer, and file pseudo-events that
cause a function to be called when they occur. Idle functions
are called when no user input is present and no timers or files
need to be handled - in short, when the application is not doing
anything. Idle callbacks are often used to update a 3D display
or do other background processing.</P>
<P>Timer functions are called after a specific amount of time
has expired. They can be used to pop up a progress dialog after
a certain amount of time or do other things that need to happen
at more-or-less regular intervals. FLTK timers are not 100%
accurate, so they should not be used to measure time intervals,
for example.</P>
<!-- P>File functions are called when data is ready to read or
write, or when an error condition occurs on a file. They are
most often used to monitor network connections (sockets) for
data-driven displays.</P -->
<P>PYFLTK applications must periodically check
(<TT>Fl.check()</TT>) or wait (<TT>Fl.wait()</TT>) for events
or use the <A href="fltk.html#Fl-run"><TT>Fl.run()</TT></A>
method to enter a standard event processing loop. Calling
<TT>Fl.run()</TT> is equivalent to the following code:</P>
<UL><PRE>
while Fl.wait() > 0:
pass
</PRE></UL>
<P><TT>Fl.run()</TT> does not return until all of the windows
under FLTK control are closed by the user or your program.</P>
<H2>Naming</H2>
<P>All public symbols in FLTK start with the characters 'F' and 'L':</P>
<UL>
<LI>Functions are either <TT>Fl.foo()</TT> or
<TT>fl_foo()</TT>.</LI>
<LI>Class and type names are capitalized:
<TT>Fl_Foo</TT>.</LI>
<LI><A href="fltk.html#enumerations">Constants and
enumerations</A> are uppercase: <TT>FL_FOO</TT>.</LI>
</UL>
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<H2>Import</H2>
<P>The proper way to import pyFLTK is:</P>
<UL><PRE>
import fltk
</PRE></UL>
or
<UL><PRE>
from fltk import *
</PRE></UL>
<P>
<H2><A NAME="interactive">Interactive usage</A></H2>
<P>
(Courtesy of Michiel de Hoon)<br>
Interactive behavior is particularly useful for rapid development of
GUIs, for beginning users of pyFltk, as well as for scientific
visualization.<br>
As an example, the following will work:<br>
<PRE>
> python
>>> from fltk import *
>>> window = Fl_Window(300,300)
>>> window.end()
>>> window.show()
# Window pops up here
>>> window.label("My new title")
# Window label changes immediately
>>> window.color(FL_RED)
>>> window.redraw()
# Window color changes to red immediately
</PRE>
... and so on, all without calling Fl.run().
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