Struct State

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pub struct State<T: Clone + 'static> { /* private fields */ }

Expand description

Cheap copyable read-only view of a state cell.

Implementations§

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impl<T: Clone + 'static> State<T>

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pub fn is_alive(&self) -> bool

Returns true while the state exists. It ends when its owner releases it or its runtime is dropped.

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pub fn try_with<R>(&self, f: impl FnOnce(&T) -> R) -> Option<R>

Reads the value through f without subscribing the current scope. Returns None if the state was released or has no readable value.

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pub fn try_value(&self) -> Option<T>

Returns a copy of the value without subscribing the current scope, or None if the state was released or has no readable value.

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pub fn with<R>(&self, f: impl FnOnce(&T) -> R) -> R

Reads a copy of the value through f and subscribes the current scope. f may write this state; see Self::read for a read that borrows instead of copying.

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pub fn read<R>(&self, f: impl FnOnce(&T) -> R) -> R

Reads the value in place through f and subscribes the current scope. Nothing is cloned, so a read of a large value costs nothing beyond f; in return f borrows the stored value and must not write this state.

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pub fn value(&self) -> T

Returns a copy of the value and subscribes the current scope, which recomposes when the value changes. Panics if the state was released.

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pub fn get(&self) -> T

Same as value.

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pub fn get_non_reactive(&self) -> T

Reads the value without subscribing the current scope, for a composition that changes what it shows by the value only through a layer or a draw that reads it again.

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pub fn has_subscribers(&self) -> bool

Returns true if any scope or observer is subscribed to this state.

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pub fn on_subscriber(&self, callback: impl Fn() + 'static)

Runs callback each time this state gains its first subscriber, and right away when it already has one.

The state owns callback for as long as the state lives, so nothing has to be kept alive on the caller’s side.

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pub fn subscription_hold(&self) -> StateSubscriptionHold

Counts this state as subscribed for as long as the returned guard lives, without observing it from any scope.

Snapshot observers subscribe reads made under composition or the draw phase, and producers gated on State::has_subscribers — an infinite transition, for one — stop themselves when the last such subscriber leaves. A consumer that reads the value from a polling context (a frame effect stepping it into a derived state, a background task) is invisible to that accounting and would starve the producer it depends on. Holding this guard keeps the producer alive; dropping it releases the count.