Implement variable resolver.

tests/var_scope.rs

@@ -1,4 +1,4 @@
-use rhai::{Engine, EvalAltResult, Scope, INT};
+use rhai::{Engine, EvalAltResult, Position, Scope, INT};
 
 #[test]
 fn test_var_scope() -> Result<(), Box<EvalAltResult>> {
@@ -52,3 +52,41 @@ fn test_scope_eval() -> Result<(), Box<EvalAltResult>> {
 
     Ok(())
 }
+
+#[test]
+fn test_var_resolver() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+
+    let mut scope = Scope::new();
+    scope.push("innocent", 1 as INT);
+    scope.push("chameleon", 123 as INT);
+    scope.push("DO_NOT_USE", 999 as INT);
+
+    engine.on_var(|name, _, scope, _| {
+        match name {
+            "MYSTIC_NUMBER" => Ok(Some((42 as INT).into())),
+            // Override a variable - make it not found even if it exists!
+            "DO_NOT_USE" => {
+                Err(EvalAltResult::ErrorVariableNotFound(name.to_string(), Position::none()).into())
+            }
+            // Silently maps 'chameleon' into 'innocent'.
+            "chameleon" => scope.get_value("innocent").map(Some).ok_or_else(|| {
+                EvalAltResult::ErrorVariableNotFound(name.to_string(), Position::none()).into()
+            }),
+            // Return Ok(None) to continue with the normal variable resolution process.
+            _ => Ok(None),
+        }
+    });
+
+    assert_eq!(
+        engine.eval_with_scope::<INT>(&mut scope, "MYSTIC_NUMBER")?,
+        42
+    );
+    assert_eq!(engine.eval_with_scope::<INT>(&mut scope, "chameleon")?, 1);
+    assert!(
+        matches!(*engine.eval_with_scope::<INT>(&mut scope, "DO_NOT_USE").expect_err("should error"),
+        EvalAltResult::ErrorVariableNotFound(n, _) if n == "DO_NOT_USE")
+    );
+
+    Ok(())
+}