title: checkbox in a listview inside a 'Dialog' not working in flutter
body: <p>I'm triggering a dialog box on a button click in which I want to print some rows containing data fetched from <strong>firestore</strong>. The <strong>'Dialog'</strong> box has a <strong>Listview</strong> widget where I'm looping through my document snapshot to print rows along with checkboxes in front of them. Basically I want to perform an action on all the items selected using the checkboxes . But when I tap, the checkboxes are not working. </p>
  
  <p>Please help! I'm new to flutter. Here's my complete code of the dialog box.</p>
  
  <pre><code>  final snapshot = await Firestore.instance
          .collection('students')
          .where('vehicle_code', isEqualTo: _userId)
          .getDocuments(); 
   await showDialog&lt;void&gt;(
        context: context,
        builder: (BuildContext context) {
          bool isChecked = false;
        return Dialog(
  
        shape:
          RoundedRectangleBorder(borderRadius: new BorderRadius.circular(15)),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: &lt;Widget&gt;[
  
              Container(
                padding: EdgeInsets.all(12),
                alignment: Alignment.center,
                decoration: BoxDecoration(
                    color: Colors.grey[300],
                    borderRadius: new BorderRadius.only(
                    topLeft: const Radius.circular(15),
                    topRight: const Radius.circular(15)),
                ),
                child: Text(
                  "Select Students",
                  style: TextStyle(
                      color: Colors.black,
                      fontSize: 18,
                      fontWeight: FontWeight.w600),
                ),
              ),
                Container(
                  width: 300,
                  height: 400,
                  child: ListView(
                    //mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                    children: &lt;Widget&gt;[
  
                      for(var item in snapshot.documents )
                        //Text(item['first_name'])
                        Row(
                          children: [
                            SizedBox(
                              width: 100.0,
                              height: 100.0,
                              child: Image.asset('assets/newlogo.png'),
                            ),
                            Text(item['first_name']),
                            Checkbox(
                              value: isChecked,
                              onChanged: (bool value) {
                                print(isChecked);
                                setState(() {
                                  isChecked = value;
                                });
                              },
                            ),
  
                          ],
                        ),
  
                    ],
                  ),
  
                ),
  
  
        Row(
                mainAxisAlignment: MainAxisAlignment.center,
                crossAxisAlignment: CrossAxisAlignment.end,
                children: &lt;Widget&gt;[
                  RaisedButton(
                    color: Colors.blue,
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                    child: Text(
                      'Okay',
                      style: TextStyle(fontSize: 18.0, color: Colors.white),
                    ),
                  ),
                  SizedBox(
                    width: 20,
                  ),
                  RaisedButton(
                    color: Colors.red,
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                    child: Text(
                      'Cancel!',
                      style: TextStyle(fontSize: 18.0, color: Colors.white),
                    ),
                  )
                ],
              ),
  
        ],
      ),
      );
      },
      );
  </code></pre>