1 files changed, 49 insertions, 0 deletions
diff --git a/devtools/client/memory/reducers/view.js b/devtools/client/memory/reducers/view.js
new file mode 100644
index 000000000..5d31e0c9b
--- /dev/null
+++ b/devtools/client/memory/reducers/view.js
@@ -0,0 +1,49 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+const { assert } = require("devtools/shared/DevToolsUtils");
+const { actions, viewState } = require("../constants");
+
+const handlers = Object.create(null);
+
+handlers[actions.POP_VIEW] = function (view, _) {
+  assert(view.previous, "Had better have a previous view state when POP_VIEW");
+  return Object.freeze({
+    state: view.previous.state,
+    previous: null,
+  });
+};
+
+handlers[actions.CHANGE_VIEW] = function (view, action) {
+  const { newViewState, oldDiffing, oldSelected } = action;
+  assert(newViewState);
+
+  if (newViewState === viewState.INDIVIDUALS) {
+    assert(oldDiffing || oldSelected);
+    return Object.freeze({
+      state: newViewState,
+      previous: Object.freeze({
+        state: view.state,
+        selected: oldSelected,
+        diffing: oldDiffing,
+      }),
+    });
+  }
+
+  return Object.freeze({
+    state: newViewState,
+    previous: null,
+  });
+};
+
+const DEFAULT_VIEW = {
+  state: viewState.TREE_MAP,
+  previous: null,
+};
+
+module.exports = function (view = DEFAULT_VIEW, action) {
+  const handler = handlers[action.type];
+  return handler ? handler(view, action) : view;
+};