Add a global_module example for a singleton configuration module dese…

…rialized to a predefined structures

examples/global_module/config/default.toml

@@ -0,0 +1,5 @@
+verbose=1
+
+[cred]
+user="robert"
+key="123456789"

examples/global_module/cred.rs

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+use super::settings::Settings;
+
+pub fn list_cred() {
+    match Settings::user() {
+        Ok(u) => println!("My name is: {u}"),
+        Err(e) => println!("{e}")
+    }
+
+    match Settings::key() {
+        Ok(k) => println!("My key is: {k}"),
+        Err(e) => println!("{e}")
+    }
+}

examples/global_module/main.rs

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+mod settings;
+mod cred;
+
+use crate::settings::Settings;
+use crate::cred::list_cred;
+
+fn main() {
+    // init the config module
+    Settings::init(Some("examples/global_module/config/default.toml"));
+
+    // now your config may be used anywhere in the code where you are able to
+    // use "crate::settings" or "super::settings".
+    let verbosity = Settings::verbosity();
+
+    if verbosity > 0 {
+        println!("Hello world");
+    }
+
+    list_cred();
+
+    return;
+}

examples/global_module/settings.rs

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+use config::{Config, File};
+use lazy_static::lazy_static;
+use serde_derive::Deserialize;
+use std::sync::RwLock;
+
+lazy_static! {
+    static ref SETTINGS: RwLock<Settings> = RwLock::new(Settings::new());
+}
+
+
+#[derive(Default, Clone, Deserialize)]
+struct Cred {
+    user: String,
+    key: String,
+}
+
+#[derive(Default, Clone, Deserialize)]
+pub struct Settings {
+    verbose: Option<u8>,
+    cred: Option<Cred>,
+}
+
+
+fn build_config(file: &str) -> Settings {
+    let s = Config::builder()
+        // Configuration file
+        .add_source(File::with_name(file).required(false))
+        .build()
+        .expect("Config build failed");
+
+    // Deserialize (and thus freeze) the entire configuration
+    s.try_deserialize().unwrap()
+}
+
+impl Settings {
+    fn new() -> Self {
+        let settings: Settings = Default::default();
+        settings
+    }
+
+    pub fn init(cfgfile: Option<&str>) {
+        let file = match cfgfile {
+            Some(x) => x,
+            None => "config.toml"
+        };
+
+        let mut new_settings = SETTINGS.write().unwrap();
+        *new_settings = build_config(file);
+    }
+
+    pub fn user() -> Result<String, String> {
+        match &SETTINGS.read().unwrap().cred {
+            Some(c) => Ok(c.user.clone()),
+            None => Err(format!("Credential config is missing"))
+        }
+    }
+
+    pub fn key() -> Result<String, String> {
+        match &SETTINGS.read().unwrap().cred {
+            Some(c) => Ok(c.key.clone()),
+            None => Err(format!("Credential config is missing"))
+        }
+    }
+
+    pub fn verbosity() -> u8 {
+        match SETTINGS.read().unwrap().verbose {
+            Some(v) => v,
+            None => 0
+        }
+    }
+}
+