title: Flutter how do scroll controllers work,how do they get executed
body: <p>Flutter : i am new to flutter , i wanted to know how do they work behind the scenes(what actually happens when we attach a scroll controller to  a scrollable widget.) . i read the docs but its hard for me to understand.</p>
  <pre><code>                  controller: ** _controller **,
                    scrollDirection: Axis.vertical,
                    child: Wrap(
                      // direction: Axis.horizontal,
                      crossAxisAlignment: WrapCrossAlignment.center,
                      spacing: 4.0,
                      runSpacing: 4.0,
                      children: *MY WIDGET*,
                    ),```
  </code></pre>