From b16b3e523d38e21076f8a9891cd4baf5958e918d Mon Sep 17 00:00:00 2001
From: Jeremy Tuloup <jeremy.tuloup@gmail.com>
Date: Fri, 4 Dec 2020 19:04:02 +0100
Subject: [PATCH] Add checkpoint indicator

---
 packages/application-extension/src/index.ts   | 50 ++++++++++++++++++-
 packages/application-extension/style/base.css | 10 ++++
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/packages/application-extension/src/index.ts b/packages/application-extension/src/index.ts
index 0f57f08b2..2aff95010 100644
--- a/packages/application-extension/src/index.ts
+++ b/packages/application-extension/src/index.ts
@@ -14,7 +14,7 @@ import {
   DOMUtils
 } from '@jupyterlab/apputils';
 
-import { PageConfig } from '@jupyterlab/coreutils';
+import { PageConfig, Time } from '@jupyterlab/coreutils';
 
 import { IDocumentManager, renameDialog } from '@jupyterlab/docmanager';
 
@@ -44,6 +44,53 @@ namespace CommandIDs {
   export const open = 'docmanager:open';
 }
 
+/**
+ * A plugin for the checkpoint indicator
+ */
+const checkpoints: JupyterFrontEndPlugin<void> = {
+  id: '@jupyterlab-classic/application-extension:checkpoints',
+  autoStart: true,
+  requires: [IDocumentManager],
+  optional: [IClassicShell],
+  activate: (
+    app: JupyterFrontEnd,
+    docManager: IDocumentManager,
+    classicShell: IClassicShell
+  ) => {
+    const { shell } = app;
+    const widget = new Widget();
+    widget.id = DOMUtils.createDomID();
+    widget.addClass('jp-ClassicCheckpoint');
+    app.shell.add(widget, 'top', { rank: 100 });
+
+    const onChange = async () => {
+      const current = shell.currentWidget;
+      if (!current) {
+        return;
+      }
+      const context = docManager.contextForWidget(current);
+
+      context?.fileChanged.disconnect(onChange);
+      context?.fileChanged.connect(onChange);
+
+      const checkpoints = await context?.listCheckpoints();
+      if (!checkpoints) {
+        return;
+      }
+      const checkpoint = checkpoints[checkpoints.length - 1];
+      widget.node.textContent = `Last Checkpoint: ${Time.formatHuman(
+        new Date(checkpoint.last_modified)
+      )}`;
+    };
+
+    if (classicShell) {
+      classicShell.currentChanged.connect(onChange);
+    } else {
+      onChange();
+    }
+  }
+};
+
 /**
  * The kernel logo plugin.
  */
@@ -310,6 +357,7 @@ const tree: JupyterFrontEndPlugin<void> = {
  * Export the plugins as default.
  */
 const plugins: JupyterFrontEndPlugin<any>[] = [
+  checkpoints,
   kernelLogo,
   logo,
   main,
diff --git a/packages/application-extension/style/base.css b/packages/application-extension/style/base.css
index 049399f11..6ece8eca1 100644
--- a/packages/application-extension/style/base.css
+++ b/packages/application-extension/style/base.css
@@ -36,3 +36,13 @@
 #jp-title h1:hover {
   background: var(--jp-layout-color2);
 }
+
+.jp-ClassicCheckpoint {
+  font-size: 14px;
+  margin-left: 5px;
+  margin-right: 5px;
+  font-weight: normal;
+  color: var(--jp-ui-font-color0);
+  font-family: var(--jp-ui-font-family);
+  line-height: calc(1.5 * var(--jp-private-title-panel-height));
+}