Overview¶

Dispatcher¶

Dispatcher is the core component of the framework. Subclassing this enables all of the event functionality.

Usage:

from pydispatch import Dispatcher

class MyEmitter(Dispatcher):
    _events_ = ['on_state', 'new_data']
    def do_some_stuff(self):
        # do stuff that makes new data
        data = self.get_some_data()
        self.emit('new_data', data=data)

# An observer - could inherit from Dispatcher or any other class
class MyListener(object):
    def on_new_data(self, *args, **kwargs):
        data = kwargs.get('data')
        print('I got data: {}'.format(data))
    def on_emitter_state(self, *args, **kwargs):
        print('emitter state changed')

emitter = MyEmitter()
listener = MyListener()

emitter.bind(on_state=listener.on_emitter_state)
emitter.bind(new_data=listener.on_new_data)

emitter.do_some_stuff()
# >>> I got data: ...

emitter.emit('on_state')
# >>> emitter state changed

The bind method above could also be combined:

emitter.bind(on_state=listener.on_emitter_state,
             new_data=listener.on_new_data)

Events can also be created after object creation:

emitter.register_event('need_data')

# Multiple events can also be created:
emitter.register_event('value_changed', 'something_happened')

Stop listening by calling unbind:

emitter.unbind(listener.on_emitter_state)

# Or to unbind all events, just supply the instance object:
emitter.unbind(listener)

Event propagation will stop if any callback returns False. Any other return value is ignored.

There are no restrictions on event names. The idea is to keep things as simple and non-restrictive as possible. When calling emit, and positional or keyword arguments supplied will be passed along to listeners.

Note

The Dispatcher class does not use __init__ for any of its functionality. This is again to keep things simple and get the framework out of your way. It uses __new__ to handle instance creation. If your subclasses use __new__ for something, the call to super() is required, but you should probably check the code to determine how it fits with your own.

Properties¶

Property objects can be defined on subclasses of Dispatcher to create instance attributes that emit events when their values change. Binding and unbinding works exactly the same as with events. The callback signature is slightly different however. The first two arguments will be:

The instance object that generated the event
The Property value

Usage:

from pydispatch import Dispatcher, Property

class MyEmitter(Dispatcher):
    name = Property()
    value = Property()

class MyListener(object):
    def on_name(self, instance, value, **kwargs):
        print('emitter name is {}'.format(value))
    def on_value(self, instance, value, **kwargs):
        print('emitter value is {}'.format(value))

emitter = MyEmitter()
listener = MyListener()

emitter.bind(name=listener.on_name, value=listener.on_value)

emitter.name = 'foo'
# >>> emitter name is foo
emitter.value = 42
# >>> emitter value is 42

If the attribute is set to the same value, an event is not dispatched:

emitter.value = 42
# No event
emitter.value = 43
# >>> emitter value is 43

Container Properties¶

dict and list objects are implemented as subclasses of Property:

They will emit events when their contents change. Nesting is also supported, so even the contents of a list or dict anywhere inside of the structure can trigger an event.

Usage:

from pydispatch import Dispatcher
from pydispatch.properties import ListProperty, DictProperty

class MyEmitter(Dispatcher):
    values = ListProperty()
    data = DictProperty()

emitter = MyEmitter()

emitter.values.append('foo')
print(emitter.values)
# >>> ['foo']

emitter.values.extend(['bar', 'baz'])
print(emitter.values)
# >>> ['foo', 'bar', 'baz']

emitter.data = {'foo':'bar'}
# or
emitter.data['foo'] = 'bar'
print(emitter.data)
# >>> {'foo':'bar'}

emitter.data['fruit'] = {'apple':'red'}
emitter.data['fruit']['banana'] = 'yellow'
# event would be dispatched to listeners