Observables

An Observable notifies Observers of state changes. An Observer is attached to an Observable to be notified that state has changed. harami implements the Observable-Observer pattern using a single class Observable[T], and Observers are simple Callable[[T],None] (ie. functions or methods, or objects with conforming __call__ implementations.)

Initialization

class Observable

Observable init does not accept any parameters.

Properties

Observable.state

The most-recent state of the Observable.

Type:

Any

Methods

Observable.attach(observer)

Attach an Observer.

Parameters:

observer (Observer) – A callable that accepts a single parameter (the state being observed.)

Tip

Attempting to attach an Observer more than once to a single Observable results in only a single attachment. Conversely, an Observer can be attached to multiples Observables.

Observable.detach(observer)

Detach an Observer.

Parameters:

observer (Observer) – An observer that was previously attached to the Observable, to be detached.

notify(state)

Notifies attached Observers of a state change.

Parameters:

state (T) – The state change observed.

Example:

from harami import Observable

observer = lambda state: print(f'Observing {state}')
subject:Observable[str] = Observable()
subject.attach(observer)
subject.notify('foo')
subject.detach(observer)
subject.notify('bar')

# Outputs: "Observing foo" (but not "Observing bar")

Example (alternative syntax):

from harami import Observable

observer = lambda state: print(f'Observing {state}')
subject:Observable[str] = Observable()
subject += observer
subject('foo')
subject.state = 'bar'
subject -= observer
subject.notify('baz')

# Outputs: "Observing foo" and "Observing bar", but not "Observing baz"