You'll want to obtain a Gio.File
handle for your hello
file, and then monitor it:
let helloFile = Gio.File.new_for_path('/home/user/temp/hello');
let monitor = helloFile.monitor(Gio.FileMonitorFlags.NONE, null);
monitor.connect('changed', function (file, otherFile, eventType) {
// change your UI here
});