tmpl_tutorial.html

Urwid is a Python library for making text console applications. It has many features including fluid

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<ul>
<li>An <a href="reference.html#Edit">Edit</a> widget is created with the
caption "What is your name?". A newline at the end of the caption makes
the user input start on the next row.
<li>A <a href="reference.html#Filler">Filler</a> widget is created to wrap
the edit widget. Its <a href="reference.html#Filler-render">render</a>
function is called to create the canvas. The render function is called with
the optional parameter "focus" set to True. This parameter allows the
wrapped Edit widget to render its cursor.
<li>Keys are processed one at a time. Most keys are sent to the Filler widget's
<a href="reference.html#Filler-keypress">keypress</a> function which will
call the Edit widget's <a href="reference.html#Edit-keypress">keypress</a>
function to handle the key.
<li>Once the ENTER key is pressed the wrapped object in the Filler widget
is changed to a reply text.
<li>Any keypress then causes the program to exit.
</ul>

The Edit widget has many capabilities. It lets you make corrections and move
the cursor around with the HOME, END and arrow keys. It is based on the Text
widget so it supports the same wrapping and alignment modes.

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This program asks for your name and responds "Nice to meet you, (your name)"
<i>while</i> you type your name.  F8 exits.

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<ul>
<li>In the __init__ function a list called self.items is created. It 
contains an Edit widget with the caption "What is your name?".
<li>A <a href="reference.html#ListBox">ListBox</a> widget called 
self.listbox is created that is passed the self.items list.
This ListBox widget will display and scroll through the widgets in that list.
ListBox widgets default to using the first item in the list as the focus.
<li>A <a href="reference.html#Frame">Frame</a> widget called self.top
is created that contains self.listbox and some header text. Frame widgets
wrap around a box widget and may have header and footer flow widgets.
The header and footer are always displayed. The contained box widget uses the
remaining space in between.
<li>When a key is pressed the reply text is inserted or updated in 
self.items.  This updated text will be displayed by self.listbox.
</ul>

When changing the contents of ListBox widgets remember to use in-place 
editing operations on the list, eg. "list = list + [something]" will not work,
use "list += [something]" instead.  The former code will create a new list
but the ListBox will still be displaying the old list.

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This program asks for your name and responds "Nice to meet you, (your name)."
It then asks again, and again. Old values may be changed and the responses
will be updated when you press ENTER. F8 exits.
<br><br>
Update the <a href="#frlb">previous program</a> with this code:
<pre class="code">%example[0]%</pre>

<ul>
<li>In this version only the ENTER key receives special attention. When the
user presses ENTER:
  <ul>
  <li>The widget in focus and its current position is retrieved by calling the
  <a href="reference.html#ListBox-get_focus">get_focus</a> function.
  <li>If the widget in focus does not have an edit_text attribute, then it
  is not one of the Edit widgets we are interested in. 
  One of the Text widgets might receive focus
  if it covers the entire visible area of the ListBox widget and there is
  no Edit widget to take focus. While this is unlikely, it should be handled
  or the program will fail when trying to call 
  <a href="reference.html#Edit-get_edit_text">get_edit_text</a>.
  <li>If the current position is at the end of the list, a response is 
  appended followed by a new question. If the current position is not at the
  end then a previous response is replaced with an updated one.
  <li>The cursor position is moved to the far left by calling 
  <a href="reference.html#Edit-set_edit_pos">set_edit_pos</a>.
  <li>Finally, the focus is moved down two positions to the next question 
  by calling
  <a href="reference.html#ListBox-set_focus">set_focus</a>.
  </ul>
<li>All other keys are passed to the top widget to handle. The ListBox widget
does most of the hard work:
  <ul>
  <li>UP and DOWN will change the focus and/or scroll the widgets in the list
  box.
  <li>PAGE UP and PAGE DOWN will try to move the focus one screen up or down.
  <li>The cursor's column is maintained as best as possible when moving
  from one Edit widget to another.
  </ul>
</ul>

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The 
<a href="reference.html#ListBox">ListBox</a> 
is a box widget that contains flow widgets.  Its contents
are displayed stacked vertically, and the ListBox allows the user to scroll
through its content.  One of the flow widgets displayed
in the ListBox is the focus widget.  The ListBox passes key presses to the
focus widget to allow the user to interact with it. If the
focus widget does not handle a keypress then the ListBox may handle the
keypress by scrolling and/or selecting another widget to become the focus
widget.
<br><br>
The ListBox tries to do the most sensible thing when scrolling and
changing focus.  When the widgets displayed are all Text widgets or
other unselectable widgets then the ListBox will behave like a web browser
does when the user presses UP, DOWN, PAGE UP and PAGE DOWN: new text is
immediately scrolled in from the top or bottom.  The ListBox chooses one of the
visible widgets as its focus widget when scrolling. When scrolling up the
ListBox chooses the topmost widget as the focus, and when scrolling down 
the ListBox chooses the bottommost widget as the focus.
<br><br>
When all the widgets displayed are not selectable the user would typically
have no way to tell which widget is in focus, but if we wrap the widgets with
AttrWrap we can see what is happening while the focus changes:

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The ListBox remembers the location of the widget in focus as either an
"offset" or an "inset". An offset is the number of rows between the top
of the ListBox and the beginning of the focus widget. An offset of zero
corresponds to a widget with its top aligned with the top of the ListBox.
An inset is the fraction of rows of the focus widget
that are "above" the top of the ListBox and not visible. 
The ListBox uses this method of remembering the focus widget location
so that when the ListBox is resized the text displayed will stay 
roughly aligned with the top of the ListBox.
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When there are selectable widgets in the ListBox the focus will move
between the selectable widgets, skipping the unselectable widgets.
The ListBox will try to scroll all the rows of a selectable widget into
view so that the user can see the new focus widget in its entirety.
This behavior can be used to bring more than a single widget into view
by using composite widgets to combine a selectable widget with other
widgets that should be displayed at the same time.
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While the ListBox stores the location of its focus widget, it
does not directly store the actual focus widget or other contents of the
ListBox. The storage of a ListBox's content is delegated to a "List Walker" 
object. If a list of widgets is passed to the ListBox constructor then it
creates a 
<a href="reference.html#SimpleListWalker">SimpleListWalker</a> 
object to manage the list.
<br><br>
When the ListBox is 
<a href="reference.html#ListBox-render">rendering a canvas</a> or 
<a href="reference.html#ListBox-keypress">handling input</a> it will:
<ol>
<li>Call the 
<a href="reference.html#List_Walker_interface_definition-get_focus">get_focus</a> 
method of its list walker object. 
This method will return the focus widget and a position object.  
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-get_prev">get_prev</a> 
method of its List Walker object 
one or more times, initially passing 
the focus position and then passing the new position returned on each 
successive call.  This method will return the widget and position object
"above" the position passed.
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-get_next">get_next</a> 
method of its List Walker object 
one or more times, similarly, to collect widgets and position objects 
"below" the focus position.
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-set_focus">set_focus</a> 
method passing one of the position 
objects returned in the previous steps.
</ol>
This is the only way the ListBox accesses its contents, and it will not
store copies of any of the widgets or position objects beyond the current
rendering or input handling operation.
<br><br>
The SimpleListWalker stores a list of widgets, and uses integer indexes into
this list as its position objects.  It stores the focus
position as an integer, so if you insert a widget into the list above the
focus position then you need to remember to increment the focus position in
the SimpleListWalker object or the contents of the ListBox will shift.
<br><br>
A custom List Walker object may be passed to the ListBox constructor instead
of a plain list of widgets. List Walker objects must implement the 
<a href="reference.html#List_Walker_interface_definition">List Walker interface</a>.
<br><br>
The 
<a href="fib.py.html">fib.py</a> 
example program demonstrates a custom list walker that doesn't
store any widgets. It uses a tuple of two successive Fibonacci numbers
as its position objects and it generates Text widgets to display the numbers
on the fly. 
The result is a ListBox that can scroll through an unending list of widgets.
<br><br>
The 
<a href="edit.py.html">edit.py</a> 
example program demonstrates a custom list walker that
loads lines from a text file only as the user scrolls them into view.  
This allows even huge files to be opened almost instantly.
<br><br>
The 
<a href="browse.py.html">browse.py</a>
example program demonstrates a custom list walker that
uses a tuple of strings as position objects, one for the parent directory
and one for the file selected. The widgets are cached in a separate class
that is accessed using a dictionary indexed by parent directory names.
This allows the directories to be read only as required. The custom list
walker also allows directories to be hidden from view when they are
"collapsed".
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