A simple hello_world application example

A project log for Linux Control System

*moved to pyLCI project*An expandable control system for Linux with apps that lets you control your Linux devices using a screen and buttons

aryaArya 08/17/2015 at 16:550 Comments

Now it's time to present you a simple "Hello world" app. It's supposed to be short and easy to understand, and as for now it's as simple as a web framework "Hello world". The app is located here and, well, works for me ;-)

First, you import the wcs module:

import wcs

Then, you send an application request to a window manager. WM returns an application object which has information about your application.

application = wcs.wm.create_new_application("Hello world")

This object contains a Window object, which stores your input and output interfaces you can use for controlling your screen and input device.

window = application.get_window("Hello window")

Now it's time to make an object class for your application. It needs to have a function with a blocking loop that can be interrupted on a call - a simple:

while self.running:

will do. It also needs to have a function to interrupt the loop (setting the self.running variable to False). This is our stop() method.

But, of course, it needs to have output functions:

self.output.display_data("Hello world", "ENTER to exit")

output.display_data takes an arbitrary number of strings and display them line by line on the screen provided. It also needs a function that'd end the loop on a keypress:

self.input.set_callback('KEY_ENTER', 'stop', self)

This function takes a key ecode, an object and a object's method name which to call on a keypress. An object passing is necessary because Pyro, an IPC solution that's used, cannot pass functions around but can only pass objects and built-in types. There's also a 'KEY_KPENTER' callback which does the same - it's just that Enter key ecodes for a numpad depend on whether Numlock is on or off.

helloworld = HelloWorld(input, output)

That's it, we're done with the class! we'll just make an instance of it and register the object with Pyro, a function that's conveniently hidden by a wcs module, which also takes care of the concurrency - the loop that runs the Pyro daemon is blocking:


Let's ask for a WM to switch context to our application - so that it immediately appears on the screen:


The last thing is running the blocking loop. WCS has a wrapper for this that gives a graceful shutdown feature. First argument to this wrapper is the function to be run - our run() method, second is the application.destroy function - it lets the WM know about the shutdown and cleans it all up nicely., application.shutdown)

So, here's our Hello, world!