title: How can I increment a value from firebase?
body: <p>I'm trying to elaborate a function where when the icon is pressed the firebase number would increment by 1, when the icon was pressed again it would increment by -1 and it would show de total of likes returning as a "Text().toString()". I'm trying to use "FieldValue.increment" but unfortunately the number has not being increased on firebase. What am I doing wrong? thanks in advance for your help!</p>
  
  <pre><code>class LanchonetesContact extends StatefulWidget {
  final DocumentSnapshot lanchonetes;
  
  LanchonetesContact(this.lanchonetes);
  
  @override
   _LanchonetesContactState createState() =&gt; 
   _LanchonetesContactState();
  }
  
  class _LanchonetesContactState extends State&lt;LanchonetesContact&gt; {
  
  bool liked = false;
  
   _pressed() {
   setState(() {
    liked = !liked;
        });
    }
  
   @override
   Widget build(BuildContext context) {
     return Scaffold(
      appBar: AppBar(
        leading: IconButton(
            icon: Icon(Icons.arrow_back),
            onPressed: () {
            Navigator.pop(context);
            }),
      title: Text("Lanchonetes"),
      centerTitle: true,
      backgroundColor: Colors.purple[900],
      actions: &lt;Widget&gt;[
        IconButton(
            icon: Icon(
   liked ? Icons.favorite : Icons.favorite_border, 
   color: Colors.white,) ,
            onPressed: () {
              _pressed();
  if(liked){
  Firestore.instance.collection('lanchonetes')
  .document("")
  .updateData({"likes": FieldValue.increment(1)});
  
  }else {
  
  Firestore.instance.collection('lanchonetes')
  .document("")
  .updateData({"likes": FieldValue.increment(-1)});
              }
            }
        )],
      ),
   Padding(
        padding: EdgeInsets.only(top: 3.0),
        child: Card(
            elevation: 1.0,
            child: GestureDetector(
              child: Container(
                height: 100.0,
                width: 500.0,
                color: Colors.white,
                child: Padding(
                  padding: EdgeInsets.symmetric(
                      vertical: 30.0, horizontal: 15.0),
                  child: Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: &lt;Widget&gt;[
                      Icon(Icons.favorite, color: Colors.black,),
                    Text(widget.lanchonetes.data["likes"].toString()),
                    ],
                  ),
                ),
              ),
              onTap: () {
  
              },
            )),
      ),
  </code></pre>