Deduplicate code found in both egui_glium and egui_glow (#819)

* Move window building to egui-winit

* Move icon loading to egui-winit

* `use glow::HasContext;` -> `use glow::HasContext as _;`

* Move FileStorage into epi behind a feature flag

* De-duplicate screen_size_in_pixels and native_pixels_per_point

* Move creation of FileStorage to epi

* Handle epi app output (window size changes etc) in egui-winit

* Move app and memory persistence and autosave logic to egui-winit

* fix check.sh

* Make the epi backend opt-in for egui_glium and egui_glow

* Fix persistence

* Add integration name to epi::IntegrationInfo and the demo

* Clean up Cargo.toml files and fix making egui_glium optional

* fix typo

* Make egui_glium compile without the `epi` feature

epi/src/file_storage.rs

@@ -0,0 +1,116 @@
+use std::{
+    collections::HashMap,
+    path::{Path, PathBuf},
+};
+
+// ----------------------------------------------------------------------------
+
+/// A key-value store backed by a [RON](https://github.com/ron-rs/ron) file on disk.
+/// Used to restore egui state, glium window position/size and app state.
+pub struct FileStorage {
+    ron_filepath: PathBuf,
+    kv: HashMap<String, String>,
+    dirty: bool,
+}
+
+impl FileStorage {
+    /// Store the state in this .ron file.
+    pub fn from_ron_filepath(ron_filepath: impl Into<PathBuf>) -> Self {
+        let ron_filepath: PathBuf = ron_filepath.into();
+        Self {
+            kv: read_ron(&ron_filepath).unwrap_or_default(),
+            ron_filepath,
+            dirty: false,
+        }
+    }
+
+    /// Find a good place to put the files that the OS likes.
+    pub fn from_app_name(app_name: &str) -> Option<Self> {
+        if let Some(proj_dirs) = directories_next::ProjectDirs::from("", "", app_name) {
+            let data_dir = proj_dirs.data_dir().to_path_buf();
+            if let Err(err) = std::fs::create_dir_all(&data_dir) {
+                eprintln!(
+                    "Saving disabled: Failed to create app path at {:?}: {}",
+                    data_dir, err
+                );
+                None
+            } else {
+                Some(Self::from_ron_filepath(data_dir.join("app.ron")))
+            }
+        } else {
+            eprintln!("Saving disabled: Failed to find path to data_dir.");
+            None
+        }
+    }
+}
+
+impl crate::Storage for FileStorage {
+    fn get_string(&self, key: &str) -> Option<String> {
+        self.kv.get(key).cloned()
+    }
+
+    fn set_string(&mut self, key: &str, value: String) {
+        if self.kv.get(key) != Some(&value) {
+            self.kv.insert(key.to_owned(), value);
+            self.dirty = true;
+        }
+    }
+
+    fn flush(&mut self) {
+        if self.dirty {
+            // eprintln!("Persisted to {}", self.path.display());
+            let file = std::fs::File::create(&self.ron_filepath).unwrap();
+            let config = Default::default();
+            ron::ser::to_writer_pretty(file, &self.kv, config).unwrap();
+            self.dirty = false;
+        }
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+fn read_ron<T>(ron_path: impl AsRef<Path>) -> Option<T>
+where
+    T: serde::de::DeserializeOwned,
+{
+    match std::fs::File::open(ron_path) {
+        Ok(file) => {
+            let reader = std::io::BufReader::new(file);
+            match ron::de::from_reader(reader) {
+                Ok(value) => Some(value),
+                Err(err) => {
+                    eprintln!("ERROR: Failed to parse RON: {}", err);
+                    None
+                }
+            }
+        }
+        Err(_err) => {
+            // File probably doesn't exist. That's fine.
+            None
+        }
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+/// Alternative to `FileStorage`
+pub fn read_memory(ctx: &egui::Context, memory_file_path: impl AsRef<std::path::Path>) {
+    let memory: Option<egui::Memory> = read_ron(memory_file_path);
+    if let Some(memory) = memory {
+        *ctx.memory() = memory;
+    }
+}
+
+/// Alternative to `FileStorage`
+///
+/// # Errors
+/// When failing to serialize or create the file.
+pub fn write_memory(
+    ctx: &egui::Context,
+    memory_file_path: impl AsRef<std::path::Path>,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let file = std::fs::File::create(memory_file_path)?;
+    let ron_config = Default::default();
+    ron::ser::to_writer_pretty(file, &*ctx.memory(), ron_config)?;
+    Ok(())
+}

epi/src/lib.rs

@@ -78,6 +78,10 @@
 #![allow(clippy::manual_range_contains)]
 #![warn(missing_docs)] // Let's keep `epi` well-documented.
 
+/// File storage which can be used by native backends.
+#[cfg(feature = "file_storage")]
+pub mod file_storage;
+
 pub use egui; // Re-export for user convenience
 
 // ----------------------------------------------------------------------------
@@ -296,6 +300,9 @@ pub struct WebInfo {
 /// Information about the integration passed to the use app each frame.
 #[derive(Clone, Debug)]
 pub struct IntegrationInfo {
+    /// The name of the integration, e.g. `egui_web`, `egui_glium`, `egui_glow`
+    pub name: &'static str,
+
     /// If the app is running in a Web context, this returns information about the environment.
     pub web_info: Option<WebInfo>,