A Django / jQuery Mobile application for the Raspberry PI

by Giacomo Graziosi     embedded   linux   django  

Posted on Aug 22, 2014

This is a late post about a skeleton project I published some time ago: django-rpi-jqm-sample. It features a sample application composed of two parts: the first one is a standard Django application, the second one is a stand-alone daemon. The daemon should be started before the Django application, you can find the details in the above Youtube video.

The two components communicate with each other using the Django’s cache framework. Every time the jQuery Mobile form is submitted the data is pushed to the cache:

def form_valid(self, form):
    cache.set('servo', form.cleaned_data['servo'])
    cache.set('led1', form.cleaned_data['led1'])
    cache.set('led2', form.cleaned_data['led2'])
    return super(Demo1View, self).form_valid(form)

The same happens the other way around, every time the form is initialized it gets its initial data from the cache:

def get_initial(self):
    servo = cache.get('servo', 0)
    led1 = cache.get('led1')
    led2 = cache.get('led2')
    initial = {'servo': servo, 'led1': led1, 'led2': led2}
    return initial

The daemon will do continous polling on the cache data and take proper actions when changes are detected:

while True:
    cache_tmp['servo'] = cache.get('servo', "0")
    if cache_cur['servo'] != cache_tmp['servo']:
        servo_control.set_servo(25, int(1200/60*float(cache_tmp['servo']))+900)
        cache_cur['servo'] = cache_tmp['servo']

Of course the cache framework is not supposed to be used like this, a proper message queueing solution like RabbitMQ is usually the way to go for this kind of problems, it just seemed a little bit overkill in this particular context so I opted for the cache framework hack .