title: how to get last value in a Stream<String> using rxdart?
body: <p>I have a stream that holds string values and i want to get the last value in that string. what's the best way to do it?</p>
  
  <pre><code>  Stream&lt;String&gt; get searchTextStream {
      return _searchController.stream.transform(
        StreamTransformer&lt;String, String&gt;.fromHandlers(
          handleData: (value, sink) {
            if (value.isEmpty || value.length &lt; 4) {
              sink.addError('please enter some text..');
            } else {
              sink.add(value);
            }
          },
        ),
      );
    }
  </code></pre>