1 files changed, 54 insertions, 0 deletions
diff --git a/devtools/client/debugger/src/utils/resource/memoize.js b/devtools/client/debugger/src/utils/resource/memoize.js
new file mode 100644
index 0000000000..b00501b4df
--- /dev/null
+++ b/devtools/client/debugger/src/utils/resource/memoize.js
@@ -0,0 +1,54 @@
+/* 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/>. */
+
+// @flow
+
+import type { ResourceBound } from "./core";
+import type { QueryMap } from "./base-query";
+import { shallowEqual } from "./compare";
+
+/**
+ * Wraps a 'mapper' function to create a shallow-equality memoized version
+ * of the mapped result. The returned function will return the same value
+ * even if the input object is different, as long as the identity is the same
+ * and the mapped result is shallow-equal to the most recent mapped value.
+ */
+export function memoizeResourceShallow<
+  R: ResourceBound,
+  Args,
+  Mapped,
+  T: QueryMap<R, Args, Mapped>
+>(map: T): T {
+  const cache = new WeakMap();
+
+  const fn = (input, identity, args) => {
+    let existingEntry = cache.get(identity);
+
+    if (!existingEntry || existingEntry.input !== input) {
+      const mapper = (map: any);
+      const output = mapper(input, identity, args);
+
+      if (existingEntry) {
+        // If the new output is shallow-equal to the old output, we reuse
+        // the previous object instead to preserve object equality.
+        const newOutput = shallowEqual(output, existingEntry.output)
+          ? existingEntry.output
+          : output;
+
+        existingEntry.output = newOutput;
+        existingEntry.input = input;
+      } else {
+        existingEntry = {
+          input,
+          output,
+        };
+        cache.set(identity, existingEntry);
+      }
+    }
+
+    return existingEntry.output;
+  };
+  fn.needsArgs = map.needsArgs;
+  return (fn: any);
+}