title: How can I customise the flexibleSpace property in SliverAppBar in Flutter?
body: <p>Instead of using FlexibleSpaceBar in the flexibleSpace property of SliverAppBar I want to use a custom widget tree, when expanded, but upon scrolling, I want to display a custom text, and not the widget tree.</p>
  
  <p>I have created a custom widget tree which shall be assigned to the flexibleSpace property, but I don't know how to display custom text on scrolling, and hide the widget tree.</p>
  
  <pre><code>SliverAppBar(
            expandedHeight: 180.0,
            backgroundColor: const Color(0xFF9e0118),
            iconTheme: IconThemeData(color: Colors.white),
            floating: true,
            pinned: true,
            flexibleSpace: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: &lt;Widget&gt;[
                Container(
                    margin: EdgeInsets.only(top: 16.0),
                    padding: EdgeInsets.only(left: 32.0, right: 32.0),
                    child: Text(
                      'Some text',
                      textAlign: TextAlign.center,
                      style: TextStyle(
                          color: Colors.white,
                          fontFamily: 'PlayfairDisplay',
                          fontStyle: FontStyle.italic,
                          fontSize: 16.0),
                    )),
                Container(
                    margin: EdgeInsets.only(top: 16.0),
                    padding: EdgeInsets.only(left: 32.0, right: 32.0),
                    child: Text(
                      'some text',
                      textAlign: TextAlign.center,
                      style: TextStyle(
                          color: Colors.white,
                          fontFamily: 'PlayfairDisplay',
                          fontSize: 16.0),
                    )),
              ],
            ),
          ),
  </code></pre>