1 files changed, 65 insertions, 0 deletions
diff --git a/components/servo/javascript_evaluator.rs b/components/servo/javascript_evaluator.rs
new file mode 100644
index 00000000000..41cb5539b05
--- /dev/null
+++ b/components/servo/javascript_evaluator.rs
@@ -0,0 +1,65 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::collections::HashMap;
+
+use base::id::WebViewId;
+use constellation_traits::EmbedderToConstellationMessage;
+use embedder_traits::{JSValue, JavaScriptEvaluationError, JavaScriptEvaluationId};
+
+use crate::ConstellationProxy;
+
+struct PendingEvaluation {
+    callback: Box<dyn FnOnce(Result<JSValue, JavaScriptEvaluationError>)>,
+}
+
+pub(crate) struct JavaScriptEvaluator {
+    current_id: JavaScriptEvaluationId,
+    constellation_proxy: ConstellationProxy,
+    pending_evaluations: HashMap<JavaScriptEvaluationId, PendingEvaluation>,
+}
+
+impl JavaScriptEvaluator {
+    pub(crate) fn new(constellation_proxy: ConstellationProxy) -> Self {
+        Self {
+            current_id: JavaScriptEvaluationId(0),
+            constellation_proxy,
+            pending_evaluations: Default::default(),
+        }
+    }
+
+    fn generate_id(&mut self) -> JavaScriptEvaluationId {
+        let next_id = JavaScriptEvaluationId(self.current_id.0 + 1);
+        std::mem::replace(&mut self.current_id, next_id)
+    }
+
+    pub(crate) fn evaluate(
+        &mut self,
+        webview_id: WebViewId,
+        script: String,
+        callback: Box<dyn FnOnce(Result<JSValue, JavaScriptEvaluationError>)>,
+    ) {
+        let evaluation_id = self.generate_id();
+        self.constellation_proxy
+            .send(EmbedderToConstellationMessage::EvaluateJavaScript(
+                webview_id,
+                evaluation_id,
+                script,
+            ));
+        self.pending_evaluations
+            .insert(evaluation_id, PendingEvaluation { callback });
+    }
+
+    pub(crate) fn finish_evaluation(
+        &mut self,
+        evaluation_id: JavaScriptEvaluationId,
+        result: Result<JSValue, JavaScriptEvaluationError>,
+    ) {
+        (self
+            .pending_evaluations
+            .remove(&evaluation_id)
+            .expect("Received request to finish unknown JavaScript evaluation.")
+            .callback)(result)
+    }
+}