diff --git a/src/index/map_key.rs b/src/index/map_key.rs
new file mode 100644
index 0000000..73cdce8
--- /dev/null
+++ b/src/index/map_key.rs
@@ -0,0 +1,123 @@
+pub use {
+    std::{
+        sync::{Arc, RwLock},
+        boxed::Box
+    },
+    crate::{
+        core::{
+            View,
+            Observer,
+            ObserverExt,
+            ObserverBroadcast,
+            ViewPort,
+            InnerViewPort,
+            OuterViewPort
+        },
+        index::{IndexView}
+    }
+};
+
+impl<SrcKey: 'static, Item: 'static + Default> OuterViewPort<dyn IndexView<SrcKey, Item = Item>> {
+    pub fn map_key<
+        DstKey: 'static,
+        F1: Fn(&SrcKey) -> DstKey + Send + Sync + 'static,
+        F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync + 'static,
+    >(
+        &self,
+        f1: F1,
+        f2: F2
+    ) -> OuterViewPort<dyn IndexView<DstKey, Item = Item>> {
+        let port = ViewPort::new();
+        let map = MapIndexKey::new(port.inner(), f1, f2);
+        self.add_observer(map.clone());
+        port.into_outer()
+    }
+}
+
+pub struct MapIndexKey<DstKey, SrcKey, SrcView, F1, F2>
+where SrcView: IndexView<SrcKey> + ?Sized,
+      F1: Fn(&SrcKey) -> DstKey + Send + Sync,
+      F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync,
+{
+    src_view: Option<Arc<SrcView>>,
+    f1: F1,
+    f2: F2,
+    cast: Arc<RwLock<ObserverBroadcast<dyn IndexView<DstKey, Item = SrcView::Item>>>>
+}
+
+impl<DstKey, SrcKey, SrcView, F1, F2> MapIndexKey<DstKey, SrcKey, SrcView, F1, F2>
+where DstKey: 'static,
+      SrcKey: 'static,
+      SrcView: IndexView<SrcKey> + ?Sized + 'static,
+      SrcView::Item: Default + 'static,
+      F1: Fn(&SrcKey) -> DstKey + Send + Sync + 'static,
+      F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync + 'static,
+{
+    fn new(
+        port: InnerViewPort<dyn IndexView<DstKey, Item = SrcView::Item>>,
+        f1: F1,
+        f2: F2
+    ) -> Arc<RwLock<Self>> {
+        let map = Arc::new(RwLock::new(
+            MapIndexKey {
+                src_view: None,
+                f1,
+                f2,
+                cast: port.get_broadcast()
+            }
+        ));
+
+        port.set_view(Some(map.clone()));
+        map
+    }
+}
+
+impl<DstKey, SrcKey, SrcView, F1, F2> View for MapIndexKey<DstKey, SrcKey, SrcView, F1, F2>
+where SrcView: IndexView<SrcKey> + ?Sized,
+      F1: Fn(&SrcKey) -> DstKey + Send + Sync,
+      F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync,
+{
+    type Msg = DstKey;
+}
+
+impl<DstKey, SrcKey, SrcView, F1, F2> IndexView<DstKey> for MapIndexKey<DstKey, SrcKey, SrcView, F1, F2>
+where SrcView: IndexView<SrcKey> + ?Sized,
+      SrcView::Item: Default,
+      F1: Fn(&SrcKey) -> DstKey + Send + Sync,
+      F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync,
+{
+    type Item = SrcView::Item;
+
+    fn get(&self, key: &DstKey) -> Self::Item {
+        if let (Some(v), Some(k)) = (self.src_view.as_ref(), (self.f2)(key)) {
+            v.get(&k)
+        } else {
+            Self::Item::default()
+        }
+    }
+
+    fn area(&self) -> Option<Vec<DstKey>> {
+        Some(self.src_view.as_ref()?.area()?.iter().map(&self.f1).collect())
+    }
+}
+
+impl<DstKey, SrcKey, SrcView, F1, F2> Observer<SrcView> for MapIndexKey<DstKey, SrcKey, SrcView, F1, F2>
+where SrcView: IndexView<SrcKey> + ?Sized,
+      SrcView::Item: Default,
+      F1: Fn(&SrcKey) -> DstKey + Send + Sync,
+      F2: Fn(&DstKey) -> Option<SrcKey> + Send + Sync,
+{
+    fn reset(&mut self, view: Option<Arc<SrcView>>) {
+        let old_area = self.area();
+        self.src_view = view;
+        let new_area = self.area();
+
+        if let Some(area) = old_area { self.cast.notify_each(area); }
+        if let Some(area) = new_area { self.cast.notify_each(area); }
+    }
+
+    fn notify(&self, msg: &SrcKey) {
+        self.cast.notify(&(self.f1)(msg));
+    }
+}
+