Source code for edifice.state

Often in an application, certain state is shared across multiple Components, and changes
to the state might be requested at multiple endpoints.
In the Edifice data flow, where state flows exclusively from parent to children,
this requires setting one parent as the owner of that state,
and passing this state to all its descendants.
Changes to this state requires requesting the parent to change its state,
which will trigger the render method call for all its descendents.
This is potentially wasteful,
since many of the intermediate Components are mere "message-passers"
that only pass the state to their children and does not use it directly.

StateValue and StateManager provide an alternative model of state storage,
with the principal advantages that

    - There is no need to pass both a "getter" and a "setter" for every state
      down the Component Tree. Indeed, if you choose to create a global StateValue
      or a global StateManager object, there is no need to pass any state down
      the Component Tree.
    - Changes to the value will trigger automatic re-render, just like calling
      set_state. However, only the nodes that actually use the state will be

A StateValue is a value (any Python object) that Edifice will keep track of.
The principal methods of a StateValue are set and subscribe.
Subscribe is the method you use to get the underlying value of the state value.
It should be called only in the render function or after the component mounts.
The method returns the underlying value, and also subscribes the component
to all updates of the underlying value, so that changes to the value
will cause the component to re-render. For example::

    def render(self)::
        # Assume USER is a module-level variable
        user = USER.subscribe(self)
        # Assume balance is passed from a parent
        balance = self.props.balance.subscribe(self)
        return Label(f"{user}: {balance}")

This component will rerender whenever user or balance changes.

Set is the method you use to set the value of a StateValue.
It will trigger re-renders of all subscribed components::

    def on_click(self):

Like all Edifice render triggers (the render_changes context, set_state),
the StateValue set method is robust to exceptions.
If any exception is thrown while re-rendering,
all changes are unwound, including the StateValue,
allowing you to properly handle the exception with guarantees of consistency.

A StateManager is very similar in concept to a StateValue;
you can think of it as a key-value store for multiple values.
StateManagers allow you to store related state together and update them in batch.
Components subscribe to individual keys in the StateManager, and a StateValue
tied to that key is returned. This can be used by the Component directly,
or passed to children, who would not need to know about the underlying StateManager.

from collections import defaultdict, OrderedDict
import functools
import itertools
import typing as tp

from ._component import Component

def _add_subscription(previous, new_comp):
    # Adds a subscription in topological sort order,
    # so that ancestors will appear before descendants.
    if new_comp in previous:
        return previous
    new_ancestors = set()
    node = new_comp
    while node is not None:
        node = node._edifice_internal_parent

    keys = []
    inserted = False
    for p in previous:
        if not inserted and new_comp in previous[p]:
            # new_comp is ancestor of p
            inserted = True
    if not inserted:
    previous[new_comp] = new_ancestors

    return OrderedDict((k, previous[k]) for k in keys)

[docs]class StateValue(object): """Container to store a value and rerender on value change. A StateValue stores an underlying Python object. Components can subscribe to the StateValue. StateValues are modified by the set method, which will trigger re-renders for all subscribed components. Args: initial_value: the initial value for the StateValue """ def __init__(self, initial_value: tp.Any): self._value = initial_value self._subscriptions = OrderedDict() def _set_subscriptions(self, new_subscriptions): # This helper method is overridden by StateManager self._subscriptions = new_subscriptions
[docs] def subscribe(self, component: Component) -> tp.Any: """Subscribes a component to this value's updates and returns the current value. Call this method in the Component render method (or after Component mounts). Args: component: Edifice Component Returns: Current value. """ self._set_subscriptions(_add_subscription(self._subscriptions, component)) return self._value
@property def value(self) -> tp.Any: """Returns the current value. **This will not subscribe your component to this value. Changes in the value will not cause your component to rerender!!!** Most of the time you probably want to use subscribe. Returns: Current value. """ return self._value
[docs] def set(self, value: tp.Any): """Sets the current value and trigger rerender. Re-renders will only be triggered for subscribed components. If an exception occurs while re-rendering, all changes are unwound and the StateValue retains the old value. Args: value: value to set to. Returns: None """ old_value = self._value try: self._value = value by_app = defaultdict(list) for comp in self._subscriptions: by_app[comp._controller].append(comp) for app, components in by_app.items(): app._request_rerender(components, {}) except Exception as e: self._value = old_value raise e
[docs]class StateManager(object): """A key value store where changes to values will trigger a rerender. Components can subscribe to keys in the store. The values are modified by the update method, which will update all provided keys and trigger re-renders for for all subscribed components for those keys. """ def __init__(self, initial_values: tp.Optional[tp.Mapping[tp.Text, tp.Any]] = None): self._values = initial_values or {} self._subscriptions_for_key = defaultdict(OrderedDict) def _set_subscriptions(self, key, new_subscriptions): self._subscriptions_for_key[key] = new_subscriptions
[docs] def subscribe(self, component: Component, key: tp.Text) -> tp.Any: """Subscribes a component to the given key. This returns a StateValue, which can be dereferenced (via state_value.value) and set (via state_value.set. This triggers a re-render for all subscribed components). All state values produced by this method share the same subscription list. The state value can also be passed to child components, which can subscribe to it without knowing about the underlying StateManager. Args: component: component to subscribe key: key to subscribe to. Returns: A StateValue """ self._set_subscriptions(key, _add_subscription(self._subscriptions_for_key[key], component)) state_value = StateValue(self._values[key]) state_value._subscriptions = self._subscriptions_for_key[key] state_value._set_subscriptions = lambda new_subscriptions: self._set_subscriptions(key, new_subscriptions) state_value.set = lambda value: self.set(key, value) return state_value
[docs] def copy(self): """Returns a (shallow) copy of this StateManager. The new StateManager is a copy and not a reference of the original, and do not share any subscriptions. """ return StateManager(self._values.copy())
[docs] def as_dict(self): """Returns a (shallow) copy of this StateManager as a dictionary. """ return self._values.copy()
def __getitem__(self, key: tp.Text) -> tp.Any: return self._values[key]
[docs] def set(self, key, value): self.update({key: value})
[docs] def keys(self): return self._values.keys()
[docs] def update(self, d: tp.Mapping[tp.Text, tp.Any]): """Updates the key value store. Re-renders will only be triggered for subscribed components. If an exception occurs while re-rendering, all changes are unwound and the state of the dictionary will be as before the update. Args: d: a dictionary mapping key to the value to update to. """ old_values = {} del_values = [] by_app = defaultdict(list) try: for key, val in d.items(): if key in self._values: old_values[key] = self._values[key] else: del_values.append(key) self._values[key] = val for comp in self._subscriptions_for_key[key]: by_app[comp._controller].append(comp) for app, components in by_app.items(): app._request_rerender(components, {}) except Exception as e: for key, val in old_values.items(): self._values[key] = val for key in del_values: del self._values[key] raise e