melib/jmap: impl watch() with polling

Concerns #22

melib/src/backends/jmap.rs

@@ -341,7 +341,32 @@ impl MailBackend for JmapType {
     }
 
     fn watch(&self) -> ResultFuture<()> {
-        Err(MeliError::from("JMAP watch for updates is unimplemented"))
+        let connection = self.connection.clone();
+        let store = self.store.clone();
+        Ok(Box::pin(async move {
+            {
+                let mut conn = connection.lock().await;
+                conn.connect().await?;
+            }
+            loop {
+                {
+                    let mailbox_hashes = {
+                        store
+                            .mailboxes
+                            .read()
+                            .unwrap()
+                            .keys()
+                            .cloned()
+                            .collect::<SmallVec<[MailboxHash; 16]>>()
+                    };
+                    let conn = connection.lock().await;
+                    for mailbox_hash in mailbox_hashes {
+                        conn.email_changes(mailbox_hash).await?;
+                    }
+                }
+                crate::connections::sleep(std::time::Duration::from_secs(60)).await;
+            }
+        }))
     }
 
     fn mailboxes(&self) -> ResultFuture<HashMap<MailboxHash, Mailbox>> {