1 files changed, 82 insertions, 0 deletions
diff --git a/browser/actors/LightweightThemeChild.sys.mjs b/browser/actors/LightweightThemeChild.sys.mjs
new file mode 100644
index 0000000000..97be44511c
--- /dev/null
+++ b/browser/actors/LightweightThemeChild.sys.mjs
@@ -0,0 +1,82 @@
+/* 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/. */
+
+/**
+ * LightweightThemeChild forwards theme data to in-content pages.
+ */
+export class LightweightThemeChild extends JSWindowActorChild {
+  constructor() {
+    super();
+    this._initted = false;
+    Services.cpmm.sharedData.addEventListener("change", this);
+  }
+
+  didDestroy() {
+    Services.cpmm.sharedData.removeEventListener("change", this);
+  }
+
+  _getChromeOuterWindowID() {
+    try {
+      // Getting the browserChild throws an exception when it is null.
+      let browserChild = this.docShell.browserChild;
+      if (browserChild) {
+        return browserChild.chromeOuterWindowID;
+      }
+    } catch (ex) {}
+
+    if (
+      Services.appinfo.processType === Services.appinfo.PROCESS_TYPE_DEFAULT
+    ) {
+      return this.browsingContext.topChromeWindow.docShell.outerWindowID;
+    }
+
+    // Return a false-y outerWindowID if we can't manage to get a proper one.
+    // Note that no outerWindowID will ever have this ID.
+    return 0;
+  }
+
+  /**
+   * Handles "change" events on the child sharedData map, and notifies
+   * our content page if its theme data was among the changed keys.
+   */
+  handleEvent(event) {
+    switch (event.type) {
+      // Make sure to update the theme data on first page show.
+      case "pageshow":
+      case "DOMContentLoaded":
+        if (!this._initted && this._getChromeOuterWindowID()) {
+          this._initted = true;
+          this.update();
+        }
+        break;
+
+      case "change":
+        if (
+          event.changedKeys.includes(`theme/${this._getChromeOuterWindowID()}`)
+        ) {
+          this.update();
+        }
+        break;
+    }
+  }
+
+  /**
+   * Forward the theme data to the page.
+   */
+  update() {
+    const event = Cu.cloneInto(
+      {
+        detail: {
+          data: Services.cpmm.sharedData.get(
+            `theme/${this._getChromeOuterWindowID()}`
+          ),
+        },
+      },
+      this.contentWindow
+    );
+    this.contentWindow.dispatchEvent(
+      new this.contentWindow.CustomEvent("LightweightTheme:Set", event)
+    );
+  }
+}