1 files changed, 99 insertions, 0 deletions
diff --git a/devtools/client/memory/actions/task-cache.js b/devtools/client/memory/actions/task-cache.js
new file mode 100644
index 000000000..14e44d0c3
--- /dev/null
+++ b/devtools/client/memory/actions/task-cache.js
@@ -0,0 +1,99 @@
+/* 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");
+
+/**
+ * The `TaskCache` allows for re-using active tasks when spawning a second task
+ * would simply duplicate work and is unnecessary. It maps from a task's unique
+ * key to the promise of its result.
+ */
+const TaskCache = module.exports = class TaskCache {
+  constructor() {
+    this._cache = new Map();
+  }
+
+  /**
+   * Get the promise keyed by the given unique `key`, if one exists.
+   *
+   * @param {Any} key
+   * @returns {Promise<Any> | undefined}
+   */
+  get(key) {
+    return this._cache.get(key);
+  }
+
+  /**
+   * Put the task result promise in the cache and associate it with the given
+   * `key` which must not already have an entry in the cache.
+   *
+   * @param {Any} key
+   * @param {Promise<Any>} promise
+   */
+  put(key, promise) {
+    assert(!this._cache.has(key),
+           "We should not override extant entries");
+
+    this._cache.set(key, promise);
+  }
+
+  /**
+   * Remove the cache entry with the given key.
+   *
+   * @param {Any} key
+   */
+  remove(key) {
+    assert(this._cache.has(key),
+           `Should have an extant entry for key = ${key}`);
+
+    this._cache.delete(key);
+  }
+};
+
+/**
+ * Create a new action-orchestrating task that is automatically cached. The
+ * tasks themselves are responsible from removing themselves from the cache.
+ *
+ * @param {Function(...args) -> Any} getCacheKey
+ * @param {Generator(...args) -> Any} task
+ *
+ * @returns Cacheable, Action-Creating Task
+ */
+TaskCache.declareCacheableTask = function ({ getCacheKey, task }) {
+  const cache = new TaskCache();
+
+  return function (...args) {
+    return function* (dispatch, getState) {
+      const key = getCacheKey(...args);
+
+      const extantResult = cache.get(key);
+      if (extantResult) {
+        return extantResult;
+      }
+
+      // Ensure that we have our new entry in the cache *before* dispatching the
+      // task!
+      let resolve;
+      cache.put(key, new Promise(r => {
+        resolve = r;
+      }));
+
+      resolve(dispatch(function* () {
+        try {
+          args.push(() => cache.remove(key), dispatch, getState);
+          return yield* task(...args);
+        } catch (error) {
+          // Don't perma-cache errors.
+          if (cache.get(key)) {
+            cache.remove(key);
+          }
+          throw error;
+        }
+      }));
+
+      return yield cache.get(key);
+    };
+  };
+};