title: how to center a circle into a card in flutter ?
body: <p>I tried to create un geofencing layer in an image ( google map url)</p>
  
  <p>I have a card with a child image , and I add a circle stack over the map image and I use a slider to change the size of the zone. the problem is that I don't succeded to center the map in my container. The only solution currently found is to use margin to go down the circle but when I change the margin limit the top of the circle and not the center, so my center is shifting...</p>
  
  <p>here is my code example: </p>
  
  <pre><code>               new Card (
  
                  child :new Stack(
                    children: &lt;Widget&gt;[
  
                  new Container(
  
                    width: 200.0,
                    height: 200.0,
                  ),
  
                      new Container(
                      alignment: new FractionalOffset(0.0, 0.0),   
                        decoration: new BoxDecoration(
                          border: new Border.all(
                            color: Colors.blue.withOpacity(0.5),
                            width: val,  // it's my slider variable, to change the size of the circle
                          ),
                          shape: BoxShape.circle,
                        ),
                      ),
  
                    ],
                  ),
                 ),
  </code></pre>