Add ContextProvider example

examples/dynamic/main.rs

@@ -0,0 +1,45 @@
+use serde_json::Value;
+use std::{borrow::Cow, env};
+use tera::{Context, ContextProvider, Tera};
+
+#[derive(Clone)]
+struct MyContext {
+    upper_layer: Context, // for overrides
+}
+
+impl MyContext {
+    pub fn new() -> Self {
+        Self { upper_layer: Context::new() }
+    }
+}
+
+impl ContextProvider for MyContext {
+    fn try_insert<T: serde::Serialize + ?Sized, S: Into<String>>(
+        &mut self,
+        key: S,
+        val: &T,
+    ) -> tera::Result<()> {
+        self.upper_layer.try_insert(key, val)
+    }
+
+    fn find_value(&self, key: &str) -> Option<Cow<Value>> {
+        if let Some(val) = self.upper_layer.find_value(key) {
+            return Some(val);
+        }
+
+        env::var(key.to_uppercase()).map(Value::String).map(Cow::Owned).ok()
+    }
+
+    fn into_json(self) -> Value {
+        let Value::Object(map) = self.upper_layer.into_json() else { unreachable!() };
+        Value::Object(map)
+    }
+}
+
+fn main() {
+    let ctx = MyContext::new();
+
+    let output = Tera::one_off("Hello {{ user }}!", &ctx, false).unwrap();
+
+    println!("{output}");
+}