build function triggered on Textfield tap - flutter

Question

The build() function is triggered when TextField comes to focus on tap. If you run the following app, notice the TextField at bottom. If TextField is tapped, it causes build() to run and entire screen is rebuilt. Anyway to prevent build() from running when this field comes to focus?
Generally speaking, What causes the build function to run and how to restructure a program that prevents the entire screen from repainting even if the build is triggered?
class TestPad extends StatefulWidget {
_TestPadSate createState() => new _TestPadSate();
final Bloc _bloc = new Bloc();
}
class _TestPadSate extends State<TestPad>{
static final txtAreaController = TextEditingController();
#override
void initState() {
super.initState();
widget._bloc.fetchVal();
}
#override
Widget build(BuildContext context) {
return Material(
child: Scaffold(
appBar: new AppBar(),
body: StreamBuilder(
stream: widget._bloc.getVal,
builder: (context, snapshot) {
if(snapshot.connectionState == ConnectionState.waiting){
return buildProgressIndicator(true, width: 50, height: 50);
}
return Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Expanded(
child: new Container(
margin: EdgeInsets.only(
),
//height: MediaQuery.of(context).size.height - 210,
decoration: BoxDecoration(
color: Colors.white,
border: Border(top: BorderSide(color: Colors.grey))),
child: ListView(
reverse: true,
shrinkWrap: true,
children: List(),
),
)
),
Container(
margin: EdgeInsets.only(
top: 2.0,
),
decoration: new BoxDecoration(
color: Colors.white,
),
height: 72,
child: new Column(
children: <Widget>[
textAreaSection(context),
],
)),
],
);
}
)
)
);
}
Widget _postBtn() {
return IconButton(
icon: Icon(Icons.send, size: 22,),
);
}
Widget textAreaSection(context) {
return Row(
mainAxisSize: MainAxisSize.min,
//mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.start,
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Expanded(
child: textFormFieldScroller(),
),
_postBtn()
],
);
}
Widget textFormFieldScroller() {
return SingleChildScrollView(
physics: ClampingScrollPhysics(),
child: Form(
child: TextFormField(
controller: txtAreaController,
keyboardType: TextInputType.text,
style: new TextStyle(
//height: 0.5,
color: Colors.grey),
autofocus: false,
//initialValue: "What's on your mind?",
// maxLines: null,
decoration: new InputDecoration(
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
hintText: "What's on your mind?",
hintStyle: new TextStyle(
color: Colors.grey[600],
),
fillColor: Colors.white,
filled: true,
border: InputBorder.none,
),
)
),
);
}
Widget buildProgressIndicator(showIndicator,{double width, double height}) {
return new Padding(
padding: const EdgeInsets.all(8.0),
child: new Center(
child: new Opacity(
opacity: showIndicator ? 1.0 : 0.0,
child: Container(
width: width??10.0,
height: height??10.0,
child: new CircularProgressIndicator(
)),
),
),
);
}
}
class Bloc {
final _fetcher = BehaviorSubject<String>();
Observable<String> get getVal => _fetcher.stream;
fetchVal() async{
await Future.delayed(Duration(seconds: 2), (){
_fetcher.sink.add("test");
});
}
dispose() {
_fetcher.close();
}
}

Answer 1

setState() causes a rebuild. I believe that the Flutter team don't consider rebuilding the screen a problem because it is very fast. However, the thing to do is to separate the collection of data (eg. Select) from the rebuild. IE. Structure your program so that if a re-select of data is done, then it is separate from the build. If and when a re-Select of data is required and the screen needs to be repainted then do a setState().