using fontdb to load system fonts

cjk_font_fallback.rs

use fontdb::{Database, Family, Query, Source};

use eframe::egui;
use std::{fs, io};

fn cjk_font_query() -> Query<'static> {
    Query {
        families: &[
            Family::Name("MS UI Gothic"),
            Family::Name("Arial Unicode MS"),
            Family::Name("Noto Sans"),
            Family::SansSerif,
        ],
        ..Query::default()
    }
}

fn to_font_data(src: Source) -> io::Result<egui::FontData> {
    match src {
        Source::File(path) => {
            let data = fs::read(path)?;
            Ok(egui::FontData::from_owned(data))
        }
        Source::Binary(bin) | Source::SharedFile(_, bin) => {
            let data = bin.as_ref().as_ref().to_vec();

            Ok(egui::FontData::from_owned(data))
        }
    }
}

fn main() {
    let mut font_database = Database::new();
    font_database.load_system_fonts();

    let font_defs = font_database.query(&cjk_font_query()).and_then(|id| {
        let name = font_database.face(id).map(|i| i.post_script_name.clone())?;

        let (src, n) = font_database.face_source(id)?;

        Some((name, src, n))
    });

    let definitions = if let Some((name, src, n)) = font_defs {
        let mut fontdata = to_font_data(src).unwrap();
        fontdata.index = n;

        let mut fd = egui::FontDefinitions::default();
        fd.font_data.insert(name.clone(), fontdata);
        fd.families
            .get_mut(&egui::FontFamily::Proportional)
            .unwrap()
            .push(name.clone());

        Some(fd)
    } else {
        None
    };

    let native_options = eframe::NativeOptions::default();
    let _ = eframe::run_native(
        "font fallback",
        native_options,
        Box::new(|cc| Box::new(CJKDemo::new(cc, definitions))),
    );
}

#[derive(Default)]
struct CJKDemo {}

impl CJKDemo {
    fn new(cc: &eframe::CreationContext<'_>, definitions: Option<egui::FontDefinitions>) -> Self {
        if let Some(defs) = definitions {
            cc.egui_ctx.set_fonts(defs);
        }

        Self::default()
    }
}

impl eframe::App for CJKDemo {
    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("Hello World 漢字?");
            ui.heading("مقالة اليوم المختارة");
        });
    }
}