Add Support for Custom Storage Backends in eframe via a New NativeOptions Field

[UnknownSuperficialNight]

Problem: Currently, eframe only supports saving app state using RON files. This limitation restricts developers who wish to integrate with different storage systems like SQLite or handle deserialization themselves for alternative data formats.

Solution: Introduce a customization option that allows users to provide a custom storage mechanism. This could be achieved by accepting a closure or function during initialization, enabling dynamic storage creation (e.g., SQLite, PostgreSQL).

egui/crates/eframe/src/native/glow_integration.rs

Lines 193 to 209 in 0db56dc

	fn init_run_state(
	&mut self,
	event_loop: &ActiveEventLoop,
	) -> Result<&mut GlowWinitRunning<'app>> {
	profiling::function_scope!();
	let storage = if let Some(file) = &self.native_options.persistence_path {
	epi_integration::create_storage_with_file(file)
	} else {
	epi_integration::create_storage(
	self.native_options
	.viewport
	.app_id
	.as_ref()
	.unwrap_or(&self.app_name),
	)
	};

egui/crates/eframe/src/native/epi_integration.rs

Lines 130 to 147 in 0db56dc

	/// For loading/saving app state and/or egui memory to disk.
	pub fn create_storage(_app_name: &str) -> Option<Box<dyn epi::Storage>> {
	#[cfg(feature = "persistence")]
	if let Some(storage) = super::file_storage::FileStorage::from_app_id(_app_name) {
	return Some(Box::new(storage));
	}
	None
	}
	#[allow(clippy::unnecessary_wraps)]
	pub fn create_storage_with_file(_file: impl Into<PathBuf>) -> Option<Box<dyn epi::Storage>> {
	#[cfg(feature = "persistence")]
	return Some(Box::new(
	super::file_storage::FileStorage::from_ron_filepath(_file),
	));
	#[cfg(not(feature = "persistence"))]
	None
	}

Proposed Implementation: Modify eframe to accept a Box<dyn Fn() -> Box<dyn crate::epi::Storage>> as an optional parameter in the initialization process. This closure would allow developers to return their own custom storage implementation instead of the default FileStorage.

Examples:

pub struct NativeOptions {
    // ... other fields ...
    pub persistence_path: Option<PathBuf>,
    /// Override the default storage creation with your own custom storage implementation
    pub storage_creator: Option<Box<dyn Fn() -> Option<Box<dyn crate::epi::Storage>>>>,
}

fn init_run_state(
    &mut self,
    event_loop: &ActiveEventLoop,
) -> Result<&mut GlowWinitRunning<'app>> {
    profiling::function_scope!();

    let storage = if let Some(storage_creator) = &self.native_options.storage_creator {
        storage_creator()
    } else if let Some(file) = &self.native_options.persistence_path {
        epi_integration::create_storage_with_file(file)
    } else {
        epi_integration::create_storage(
            self.native_options
                .viewport
                .app_id
                .as_ref()
                .unwrap_or(&self.app_name),
        )
    };

    // ... rest of function
}

[Evrey]

Same problem. Currently i’m just ignoring the provided Storage ref when saving state, pointing at my own, but i’d like to not have that RON file exist in the first place and install my own Storage properly. I have an SQLite-based settings DB and have in it a table for all the eframe stuff.