Trait State 

pub trait State: Sized + 'static {
    type Widget: StatefulWidget<State = Self>;

    // Required method
    fn build(&self, cx: &StateContext<'_, '_, Self>) -> Box<dyn Widget>;

    // Provided methods
    fn init_state(&mut self, _cx: &StateContext<'_, '_, Self>) { ... }
    fn did_update_widget(
        &mut self,
        _old: &Self::Widget,
        _cx: &StateContext<'_, '_, Self>,
    ) { ... }
    fn dispose(&mut self) { ... }
    fn save(&self) -> Option<String> { ... }
    fn restore(&mut self, _saved: &str) { ... }
}

What a StatefulWidget keeps, and how it draws itself from it.

Required Associated Types

type Widget: StatefulWidget<State = Self>

The widget this is the state of.

Required Methods

fn build(&self, cx: &StateContext<'_, '_, Self>) -> Box<dyn Widget>

The widget this state stands for. Called whenever the tree is rebuilt.

It only reads the state. What a handler changes goes through StateContext::callback or StateContext::handle; changing the state from inside build itself panics, because a build that changes what it builds from never settles.

Provided Methods

fn init_state(&mut self, _cx: &StateContext<'_, '_, Self>)

Called once, right after the state is made. Start what has to run for as long as the widget is there — a listener, a timer — here, and stop it in State::dispose.

fn did_update_widget( &mut self, _old: &Self::Widget, _cx: &StateContext<'_, '_, Self>, )

Called when the tree is rebuilt and the widget above changed, or was made again. old is the configuration it replaces; the new one is StateContext::widget.

fn dispose(&mut self)

Called once, when the widget leaves the tree. The state is never used again.

fn save(&self) -> Option<String>

What this state keeps across a restart under frus run (R, or r): the new build of the application starts with it, given back through State::restore. Any text will do — a number, a few fields joined, a JSON document.

None, the default, keeps nothing: the state starts afresh, as on a first launch. Nothing is saved outside frus run.

fn save(&self) -> Option<String> {
    Some(self.count.to_string())
}

fn restore(&mut self, saved: &str) {
    self.count = saved.parse().unwrap_or(0);
}

fn restore(&mut self, _saved: &str)

Takes back what State::save returned before a restart. Called once, right after StatefulWidget::create_state and before State::init_state, and only for the component at the same place, of the same type. What no longer fits — the code changed in between — can simply be ignored.

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors