added scroll reducer

src/services/scroll.reducer.ts

@@ -0,0 +1,40 @@
+const reducer = (state, action) => {
+  switch (action.type) {
+    case "UPDATE_TOTAL_TO_SCROLL": {
+      const totalToScroll = action.payload;
+      const result = { ...state };
+      if (state.lastTotalToScroll !== totalToScroll) {
+        result.lastTotalToScroll = state.totalToScroll;
+        result.totalToScroll = totalToScroll;
+      }
+      return result;
+    }
+
+    case "UPDATE_LAST_SCROLL_POSITION": {
+      return {
+        ...state,
+        lastScrollPosition: action.payload
+      };
+    }
+
+    case "UPDATE_TRIGGERED_FLAGS": {
+      const { payload: { scroll, down } } = action;
+      const triggered = {
+        ...state.triggered
+      };
+      if (down) {
+        triggered.down = scroll;
+      } else {
+        triggered.up = scroll;
+      }
+      return {
+        ...state,
+        triggered
+      };
+    }
+
+    default: {
+      return state;
+    }
+  }
+};