Ability to manually save state

[duckinator]

Is your feature request related to a problem? Please describe.

If I know a particular action changes something significant enough to warrant saving the state to disk, I want to be able to request a save (similar to how you can use ctx.request_repaint() and related functions if you know a repaint will be needed).

Describe the solution you'd like

The idea that comes to mind is adding a ctx.request_save() or similar, which triggers the same behavior as if the autosave duration was reached.

Describe alternatives you've considered

I tried storing the creation context passed to the eframe::App, but that caused ownership problems.

I tried storing a reference to the eframe::Storage instead, and also ran into ownership problems.

I can't even figure out a messy way to do it with existing functionality. (EDIT: I found a way to implement it. See #5243 (comment).)

[duckinator]

I've found an approach that seems to work, basically by making auto_save_interval return 0 milliseconds if a save was requested, and 30 seconds otherwise. Setting self.needs_save = true means the next time maybe_autosave is called, it should save.

I think it'd make sense for this to potentially be integrated into egui/eframe, though.

#[derive(Default)]
struct MainApp {
    // ...
    tasks: TaskList,
    needs_save: bool,
}

impl eframe::App for MainApp {
    // ...

    fn auto_save_interval(&self) -> std::time::Duration {
        if self.needs_save {
            std::time::Duration::from_secs(0)
        } else {
            std::time::Duration::from_secs(30)
        }
    }

    fn save(&mut self, storage: &mut dyn eframe::Storage) {
        eframe::set_value(storage, eframe::APP_KEY, &self.tasks);
        self.needs_save = false;
    }
}