Full screen applications

quo can be used to create complex full screen terminal applications. Typically, an application consists of a layout (to describe the graphical part) and a set of key bindings.

The sections below describe the components required for full screen applications (or custom, non full screen applications), and how to assemble them together.

Note

Also remember that the examples directory of the quo repository contains plenty of examples. Each example is supposed to explain one idea. So, this as well should help you get started.

Don’t hesitate to open a GitHub issue if you feel that a certain example is missing.

A simple application

Almost every quo application is an instance of an Console object. The simplest full screen example would look like this:

from quo.console import Console

Console(full_screen=True).run()

This will display an application with no layout specified.

Note

If we wouldn’t set the full_screen option, the application would not run in the alternate screen buffer, and only consume the least amount of space required for the layout.

An application consists of several components. The most important are:

• I/O objects: the input and output device.

• The layout: this defines the graphical structure of the application. For instance, a text box on the left side, and a button on the right side.

• A style: this defines what colors and underline/bold/italic styles are used everywhere.

• A set of key bindings.

We will discuss all of these in more detail below.

I/O objects

Every Console instance requires an I/O object for input and output:

• An Input instance, which is an abstraction of the input stream (stdin).

• An Output instance, which is an abstraction of the output stream, and is called by the renderer.

Both are optional and normally not needed to pass explicitly. Usually, the default works fine.

There is a third I/O object which is also required by the application, but not passed inside. This is the event loop, an eventloop instance. This is basically a while-true loop that waits for user input, and when it receives something (like a key press), it will send that to the the appropriate handler, like for instance, a key binding.

When run() is called, the event loop will run until the application is done. An application will quit when exit() is called.

The layout

Layout is the layout for a quo Console. This also keeps track of which user control is focused or used in the application.

Parameters

• container - The “root” container for the layout.

• focused_element - Element to be focused initially. (Can be anything the `focus` function accepts.)

A layered layout architecture

There are several ways to create a layout, depending on how customizable you want things to be. In fact, there are several layers of abstraction.

• The most low-level way of creating a layout is by combining Container and UIControl objects.

  Examples of Container objects are VSplit (vertical split), HSplit (horizontal split) and FloatContainer. These containers arrange the layout and can split it in multiple regions. Each container can recursively contain multiple other containers. They can be combined in any way to define the “shape” of the layout.

  The Window object is a special kind of container that can contain a UIControl object. The UIControl object is responsible for the generation of the actual content. The Window object acts as an adaptor between the UIControl and other containers, but it’s also responsible for the scrolling and line wrapping of the content.

  Examples of UIControl objects are BufferControl for showing the content of an editable/scrollable buffer, and FormattedTextControl for displaying (formatted) text.

  Normally, it is never needed to create new UIControl or Container classes, but instead you would create the layout by composing instances of the existing built-ins.

• A higher level abstraction of building a layout is by using “widgets”. A widget is a reusable layout component that can contain multiple containers and controls. Widgets have a __pt_container__ function, which returns the root container for this widget. Quocontains several widgets like TextArea, Button, Frame, VerticalLine and so on.

• The highest level abstractions can be found in the dialog module. There we don’t have to think about the layout, controls and containers at all. This is the simplest way to use quo, but is only meant for specific use cases, like a prompt or a simple dialog window.

Containers and controls

The biggest difference between containers and controls is that containers arrange the layout by splitting the screen in many regions, while controls are responsible for generating the actual content.

Note

Under the hood, the difference is:

• containers use absolute coordinates, and paint on a Screen instance.

• user controls create a UIContent instance. This is a collection of lines that represent the actual content. A UIControl is not aware of the screen.

Abstract base class	Examples
Container	HSplit VSplit FloatContainer Window ScrollablePane
UIControl	BufferControl FormattedTextControl

The Window class itself is particular: it is a Container that can contain a UIControl. Thus, it’s the adaptor between the two. The Window class also takes care of scrolling the content and wrapping the lines if needed.

Finally, there is the Layout class which wraps the whole layout. This is responsible for keeping track of which window has the focus.

Here is an example of a layout that displays the content of the default buffer on the left, and displays "Hello world" on the right. In between it shows a vertical line:

from quo.buffer import Buffer
from quo.console import Console
from quo.layout import BufferControl, FormattedTextControl, Layout, VSplit, Window

buffer1 = Buffer()  # Editable buffer.

root_container = VSplit([
    # One window that holds the BufferControl with the default buffer on the left.
    Window(BufferControl(buffer=buffer1)),

    # A vertical line in the middle. We explicitly specify the width, to
    # make sure that the layout engine will not try to divide the whole
    # width by three for all these windows. The window will simply fill its
    # content by repeating this character.
    Window(width=1, char='|'),

    # Display the text 'Hello world' on the right.
    Window(FormattedTextControl('Hello world')),
])

layout = Layout(root_container)

Console(layout=layout, full_screen=True).run()
# You won't be able to Exit this app unless you add a key binder

Notice that if you execute this right now, there is no way to quit this application yet. This is something we explain in the next section below.

More complex layouts can be achieved by nesting multiple VSplit, HSplit and FloatContainer objects.

If you want to make some part of the layout only visible when a certain condition is satisfied, use a ConditionalContainer.

Finally, there is ScrollablePane, a container class that can be used to create long forms or nested layouts that are scrollable as a whole.

Focusing windows

Focusing something can be done by calling the focus() method. This method is very flexible and accepts a Window, a Buffer, a UIControl and more.

In the following example, we use get_app() for getting the active application.

from quo.console import get_app

# This window was created earlier.
w = Window()

# ...

# Now focus it.
get_app().layout.focus(w)

Changing the focus is something which is typically done in a key binding, so read on to see how to define key bindings.

Key bindings

In order to react to user actions, we need to create a Bind object and pass that to our Console.

There are two kinds of key bindings:

• Global key bindings, which are always active.

• Key bindings that belong to a certain UIControl and are only active when this control is focused. Both BufferControl FormattedTextControl take a bind argument.

Global key bindings

Key bindings can be passed to the application as follows:

from quo.console import Console
from quo.keys import Bind

bind = Bind()

Console(bind=bind).run()

To register a new keyboard shortcut, we can use the add() method as a decorator of the key handler:

from quo.keys import Bind

bind = Bind()

@bind.add('ctrl-q')
def exit_(event):
    """
    Pressing Ctrl-Q will exit the user interface.

    Setting a return value means: quit the event loop that drives the user
    interface and return this value from the `Suite.run()` call.
    """
    event.app.exit()

Console(bind=bind, full_screen=True).run()

The callback function is named exit_ for clarity, but it could have been named _ (underscore) as well, because we won’t refer to this name.

Read more about key bindings …

Modal containers

The following container objects take a modal argument VSplit, HSplit, and FloatContainer.

Setting modal=True makes what is called a modal container. Normally, a child container would inherit its parent key bindings. This does not apply to modal containers.

Consider a modal container (e.g. VSplit) is child of another container, its parent. Any key bindings from the parent are not taken into account if the modal container (child) has the focus.

This is useful in a complex layout, where many controls have their own key bindings, but you only want to enable the key bindings for a certain region of the layout.

The global key bindings are always active.

More about the Window class

As said earlier, a Window is a Container that wraps a UIControl, like a BufferControl or FormattedTextControl.

Note

Basically, windows are the leafs in the tree structure that represent the UI.

A Window provides a “view” on the UIControl, which provides lines of content. The window is in the first place responsible for the line wrapping and scrolling of the content, but there are much more options.

• Adding left or right margins. These are used for displaying scroll bars or line numbers.

• There are the cursorline and cursorcolumn options. These allow highlighting the line or column of the cursor position.

• Alignment of the content. The content can be left aligned, right aligned or centered.

• Finally, the background can be filled with a default character.

More about buffers and BufferControl

Input processors

A Processor is used to postprocess the content of a BufferControl before it’s displayed. It can for instance highlight matching brackets or change the visualisation of tabs and so on.

A Processor operates on individual lines. Basically, it takes a (formatted) line and produces a new (formatted) line.

Some build-in processors:

Processor	Usage:
HighlightSearchProcessor	Highlight the current search results.
HighlightSelectionProcessor	Highlight the selection.
PasswordProcessor	Display input as asterisks. (* characters).
BracketsMismatchProcessor	Highlight open/close mismatches for brackets.
BeforeInput	Insert some text before.
AfterInput	Insert some text after.
AppendAutoSuggestion	Append auto suggestion text.
ShowLeadingWhiteSpaceProcessor	Visualise leading whitespace.
ShowTrailingWhiteSpaceProcessor	Visualise trailing whitespace.
TabsProcessor	Visualise tabs as n spaces, or some symbols.

A BufferControl takes only one processor as input, but it is possible to “merge” multiple processors into one with the merge_processors() function. [1]