Real reactivity shines with derived state: a value defined as a function of another, that updates itself whenever its source changes. A Computed subscribes to its source, recomputes on every change, and, like an Observable, only notifies its own subscribers when its result changes.
_on_source_change(source_value): recompute self.fn(source_value); if the result differs from the current computed value, store it and notifyclass Subject:
def __init__(self):
self._observers = []
def subscribe(self, observer):
self._observers.append(observer)
def unsubscribe():
if observer in self._observers:
self._observers.remove(observer)
return unsubscribe
def notify(self, *args):
for observer in self._observers:
observer(*args)
class Observable:
def __init__(self, value=None):
self._value = value
self._subject = Subject()
def get(self):
return self._value
def subscribe(self, observer):
return self._subject.subscribe(observer)
def set(self, value):
if value == self._value:
return
self._value = value
self._subject.notify(value)
class Computed:
def __init__(self, source, fn):
self.fn = fn
self._value = fn(source.get())
self._subject = Subject()
source.subscribe(self._on_source_change)
def get(self):
return self._value
def subscribe(self, observer):
return self._subject.subscribe(observer)
def _on_source_change(self, source_value):
"""Recompute from the source; notify only if the computed value changed."""
# TODO: feed source_value through self.fn to get the fresh result, then weigh
# it against the computed value currently in self._value: identical means
# stay silent. Otherwise store the fresh result and have self._subject
# notify this Computed's own observers with it.
pass
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Real reactivity shines with derived state: a value defined as a function of another, that updates itself whenever its source changes. A Computed subscribes to its source, recomputes on every change, and, like an Observable, only notifies its own subscribers when its result changes.
_on_source_change(source_value): recompute self.fn(source_value); if the result differs from the current computed value, store it and notifyPress Run Tests or ⌘↩ to check your solution.