Function retain_with_key 

pub fn retain_with_key<K, F, T>(key: K, init: F) -> State<T>
where
    K: Hash,
    F: FnOnce() -> T,
    T: Send + Sync + 'static,

Retain a value across recomposition (build) passes with an explicit key, even if unused.

Unlike remember_with_key, state created with this function will not be recycled when the component stops calling it. This is useful for state that should persist across navigation, such as scroll positions or form inputs.

The init closure is executed only once — when the key is first encountered. On subsequent updates with the same key, the stored value is returned and init is not called.

Use Cases

• Preserving scroll position when navigating away and returning to a page
• Retaining form input values across route changes
• Caching expensive computation results that should survive component unmounts

Interior mutability

This function returns a State<T> handle that internally uses an Arc<RwLock<T>>. Use with, with_mut, get, or set to read or update the value without handling synchronization primitives directly.

Comparison with remember_with_key

Use remember_with_key for ephemeral component state that should be cleaned up when the component is no longer rendered. Use retain_with_key for persistent state that must survive even when a subtree is not rebuilt for some time.

Panics

This function must be called during a component’s build/render phase. Calling it during the measure or input handling phases will panic.