Develop Reference

Flutter StreamProvider fails to update Widget state. How do I go around this?

I'm using a StreamProvider to capture changes that occur in Firestore and display them on the user homepage. When the user initially signs in or registers, they're directed to this homepage and expect to see their details, for which I use the StreamProvider.
Keeping in mind that the first value fetched will be null, I've added a Loading Text Widget. However, the widget does not seem to change unless I hot-refresh the page and the Stateful Widget gets rebuilt again.
What seems to be happening is that the Stateful Widget I'm using as my HomePage gets "built" only once due to which even when the stream gets populated with non-null values, the loading widget does not disappear because its parent widget doesn't get rebuilt.
This is the basic HomeScreen widget around which I'm wrapping my StreamProvider.
#Override
Widget build(BuildContext context) {
return StreamProvider<UserDataModel>.value(
initialData: null,
value: DatabaseServices(userDetails: _userDetails).userDataDocument,
updateShouldNotify: (previous, current) => true,
child: Scaffold(
backgroundColor: Colors.white,
body: HomeBody(),
)
);
}
This is the relevant snippet from the HomeBody widget
class HomeBody extends StatefulWidget {
#override
_HomeBodyState createState() => _HomeBodyState();
}
class _HomeBodyState extends State<HomeBody> {
#override
Widget build(BuildContext context) {
final AuthenticationServices _authServices = AuthenticationServices();
final UserDataModel _data = Provider.of<UserDataModel>(context);
print(_data);
print((_data != null) ? _data.name : 'NULL');
/* Some code */
Row(
children: [
Padding(
padding: const EdgeInsets.fromLTRB(25, 0, 0, 5),
child: Text(_data != null ? _data.name : 'LOADING...', style: dashboardName),
),
],
),
Row(
children: [
Padding(
padding: const EdgeInsets.fromLTRB(25, 2, 0, 5),
child: Text(_data != null ? _data.scholarId : 'LOADING...', style: dashboardName),
),
],
),
I also noticed that the print statements I've used for debugging get executed only once printing those initial null values, until, I hot-refresh the page and the non-null values are shown.
Is there a way to go around this? I've been stuck for a couple of days now without making any significant advances and any sort of guidance would be appreciated.

Alert Dialog stays open through back button push

I'm having a little error I can't seem to figure out, and I believe it's due to my Alert Dialog implementation. I'm using the flutter Provider package, and opening my Dialog like this:
_openSearchHistory(
BuildContext context, TextEditingController searchController) {
final searchModel = Provider.of<SearchModel>(context, listen: false);
showDialog(
context: context,
builder: (_) => ChangeNotifierProvider<SearchModel>.value(
value: searchModel,
child: DialogSearchHistory(
searchHistory: searchModel.searchHistory,
searchController: searchController,
),
));
}
The problem is when on android and the user presses the back button, the Dialog does not close, but the page behind the Dialog pops back a page. I have a close button on the Dialog that successfully closes the dialog, but I want users to be able to use the back button for a better experience. The Dialog does close if the user clicks outside of it as well. I've tried wrapping the Dialog with WillPopScope, but it does not get called on a back button press.
Could anyone shed some light on what I'm doing wrong here?
I have tried wrapping the Widget in the calling method with WillPopScope
showDialog(
context: context,
builder: (_) => ChangeNotifierProvider<SearchModel>.value(
value: searchModel,
child: WillPopScope(
onWillPop: () {
Navigator.of(context).pop();
return Future.value(false);
},
child: DialogSearchHistory(
searchHistory: searchModel.searchHistory,
searchController: searchController,
),
),
));
As well as wrapping the Alert Dialog itself in the build function
#override
Widget build(BuildContext context) {
return WillPopScope(
onWillPop: () {
Navigator.of(context).pop();
return Future.value(false);
},
child: AlertDialog(
backgroundColor: kCardColor,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(20.0)),
),...
Neither have worked. The page behind the Dialog pops back, but the Dialog stays put. onWillPop is never hit by the debugger

Late to the party but setting useRootNavigator to false inside showDialog worked for me.
showDialog(
context: context,
useRootNavigator: false,
builder: (context) {
return AlertDialog();
},
);

If the application has multiple Navigator objects, it may be necessary to call Navigator.of(context, rootNavigator: true).pop(result) to close the dialog rather than just Navigator.pop(context, result).
I hope this will close the alert dialog If you have multiple navigator objects. Give it a try
onWillPop: (){
Navigator.of(context, rootNavigator: true).pop();
return Future.value(false);
},
OR
Try Navigator.pop(context);, this will call internally Navigator.of(context).pop() method.
Official Implementation of Navigator.pop() method.
static void pop<T extends Object>(BuildContext context, [ T result ]) {
Navigator.of(context).pop<T>(result);
}
Doc - https://api.flutter.dev/flutter/widgets/Navigator/pop.html

Hi there i face similar issue and the below method worked for me as Vinoth vino tell wrap your page in WillPopScope widget and try to add pop function there like
body: WillPopScope(
onWillPop: (){
Navigator.of(context).pop();
return Future.value(false);
},
child:...your code

Flutter setState() method calling

As I am a new dev in Flutter it’s very confusing me to when should I call setState() ?, If I call this entire application is reloading (redrawing view) in build(). I want to update one TextView widget value in tree widgets structure

Here is example. On click on fab you recreate only _MyTextWidget
StreamController<int> _controller = StreamController<int>.broadcast();
int _seconds = 1;
Widget build(BuildContext context) {
return Scaffold(
body: Container(
color: Colors.cyan.withOpacity(0.3),
width: 300.0,
height: 200.0,
child: _MyTextWidget(_controller.stream)),
),
floatingActionButton: FloatingActionButton(
onPressed: () {
_controller.add(_seconds++);
},
child: Icon(Icons.add),
),
);
}
...
class _MyTextWidget extends StatefulWidget {
_MyTextWidget(this.stream);
final Stream<int> stream;
#override
State<StatefulWidget> createState() => _MyTextWidgetState();
}
class _MyTextWidgetState extends State<_MyTextWidget> {
int secondsToDisplay = 0;
#override
Widget build(BuildContext context) {
return StreamBuilder(
stream: widget.stream,
builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
return snapshot.hasData ? Text(snapshot.data.toString()) : Text('nodata');
});
}
}

To make it simple, SetState() {} invalidates the widget in which it is called and forces the widget to rebuild itself by calling build(). That means that every child widgets are being rebuilt.
There are other methods you can use to pass data to a widget down a tree and make it rebuilt itself (and all its chidlren) than using SetState () {}. Those are really helpfull, especially if the widget you want to rebuilt is far away from yours in the widget tree.
One of them is the example provided by #andrey-turkovsky that uses a combination of StreamBuilder and a Stream. The StreamBuidler is a widget that rebuilt itself when there is an interaction in a Stream. Based on that, the idea is to wrap your TextView in a StreamBuilder, and use the stream to sent the data you want your TextView to display.

Can someone explain to me what the Builder Class does in Flutter?

The documentation is very confusing and vague. Here is what it states:
Builder class
A platonic widget that calls a closure to obtain its child widget.
Here are my questions:
What do they mean by "platonic"?
What do they mean by "closure"?
What exactly is the purpose of this class?

After long hours of extensive hair-pulling research on the internet, I collected small snippets and combined them to put a cohesive and clear explanation of what the Builder Class does.
Terminology:
According to the official flutter documentation, the Builder Class is defined as:
A platonic widget that calls a closure to obtain its child widget.
Platonic means the simplest possible thing of that kind. The term closure is just another name for a lambda function.
Purpose:
This is going to be a lengthy explanation, but please bare with me:
In the Flutter framework, every widget has a build method that accepts a BuildContext parameter:
Widget build (
BuildContext context
) {
... }
We have to remember that the context object is passed to the widget's build function automatically by the framework. Since the framework takes care of that automatically, there is no reason for any widget to have a constructor or function (aside from build) that would need to accept a context parameter.
Hence, if you were trying to pass a specific context object to a child, you won't be able to. You cannot call build() and pass your own context object manually. I mean, you can, but you would be calling the build function twice:
Your manual call.
The automatic call by the framework.
So, how can we pass a specific context object? This is where the Builder class comes in. The purpose of the Builder class is simply to build and return child widgets. How is that different from any other widget? Aha! The Builder class allows you to pass a specific context object down to its children. The Builder class is basically your own build function that you setup.
Why would I need to pass a specific context object? Lets take a look at an example:
Lets say that we want to add a new SnackBar widget to its new Scaffold parent widget that is being returned:
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Container(),
/// Scaffold doesn't exist in this context here
/// because the context thats passed into 'build'
/// refers to the Widget 'above' this one in the tree,
/// and the Scaffold doesn't exist above this exact build method
///
/// This will throw an error:
/// 'Scaffold.of() called with a context that does not contain a Scaffold.'
floatingActionButton: new FloatingActionButton(onPressed: () {
Scaffold.of(context).showSnackBar(
new SnackBar(
content: new Text('SnackBar'),
),
);
}));
}
This code above does not work. The Scaffold.of(context) function will not find the Scaffold because:
The Scaffold widget has not been created yet.
The context object that was passed to the build function refers to the parent widget, which is not a Scaffold widget.
So, how do we give the child SnackBar widget access to the parent Scaffold widget? We use a Builder class to pass the context of the Scaffold widget:
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Container(),
/// Builders let you pass context
/// from your *current* build method
/// Directly to children returned in this build method
///
/// The 'builder' property accepts a callback
/// which can be treated exactly as a 'build' method on any
/// widget
floatingActionButton: new Builder(builder: (BuildContext context) {
return new FloatingActionButton(onPressed: () {
Scaffold.of(context).showSnackBar(
new SnackBar(
backgroundColor: Colors.blue,
content: new Text('SnackBar'),
),
);
});
}),
);
}
Remember, the Builder class constructor:
Builder({Key key, #required WidgetBuilder builder })
creates a widget by delegating its build to the callback function passed through its constructor.
So, in the code:
new Builder(builder: (BuildContext context){ ... });
we provided a closure that:
Contains a BuildContext context parameter
Builds and returns child widget(s) based on that context passed.
Basically, you provided your own build function. The BuildContext context parameter in this closure is the Scaffold's context! Baboom!
That is basically it. The Flutter documentation does not provide a thorough explanation of this at all. I feel like I would have an easier time deciphering ancient hieroglyphs than decoding the Flutter documentation.
SUMMARY: For anyone still having a hard time grasping this concept, let me explain in a more concise form. The Builder function simply allows you to attain and use the context object of the widget that the Builder widget is in. In the example above, it is the new Scaffold() widget. Remember, the only context object available to use is that of the parent widget (above Scaffold) since the current widget (Scaffold) has not been created yet. I hope that helps those were still scratching their heads.

It basically converts a function that builds a widget into a widget.
Wo where you need to pass a widget but only have a function that returns a widget, you can use the Builder widget.
bool bar;
Widget createFooOrBarWidget() {
if(bar) {
return BarWidget();
}
return FooWidget();
}
Widget build(BuildContext context) =>
Container(child: Builder((context) => createFooOrBarWidget()));
you could also use
Widget build(BuildContext context) =>
Container(child: createFooOrBarWidget());
but the former delays the creation of the Foo or Bar widget until build is actually called.

Simple Definition
This Builder widget is, like its name is implying; used to create a child widget with a "new context".
Technical Definition
This Builder has a builder property and this property accept WidgetBuilder typedef (WidgetBuilder typedef is a Signature for a function that creates(return) a widget with new context)
If you want to know about that WidgetBuilder typedef please use this link ➡https://api.flutter.dev/flutter/widgets/WidgetBuilder.html
usage:
1. when Scaffold widget and Scaffold.of method are in same build method.
[ At that time scaffold.of method cannot find closet Scaffold widget ,because both are
in
same context ,by creating new context inside build method you can solve this, that why
we use Builder Widget to create widget with New BuildContext. ]
Below code shows real usage of Builder widget ,when you want use Scaffold.of and Scaffold widget in same build method
(Look these comments carefully-it will help you to understand the context)
Widget build(BuildContext context) { // context - 1
return Scaffold(
appBar: AppBar(
title: Text('Demo')
),
body: Builder(
// Create an inner BuildContext so that the onPressed methods
// can refer to the Scaffold with Scaffold.of().
builder: (BuildContext context) { // context - 2
return Center(
child: RaisedButton(
child: Text('SHOW A SNACKBAR'),
onPressed: () {
Scaffold.of(context).showSnackBar(SnackBar( // here context is (context- 2)
content: Text('Have a snack!'),
));
},
),
);
},
),
);
}
2. When Theme.of method and Theme widget in a same build method.
[here also purpose same as above 1]
Below code shows real usage of Builder widget ,when you want use Theme.of and Theme widget in same build method
#override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData.light(),
body: Builder(
// Create an inner BuildContext so that we can refer to
// the Theme with Theme.of().
builder: (BuildContext context) {
return Center(
child: Text(
'Example',
style: Theme.of(context).textTheme.title,
),
);
},
),
);
}
EXTRA POINT
buider property (WidgetBuilder typedef) we can see on many instances
Down below that code part shows, 'how MaterialPageRoute ' use builder property to get widget for that route
Navigator.push(context, MaterialPageRoute<void>(
builder: (BuildContext context) { //here
return Scaffold(
appBar: AppBar(title: Text('My Page')),
body: Center(
child: FlatButton(
child: Text('POP'),
onPressed: () {
Navigator.pop(context);
},
),
),
);
},
));

Flutter Drawer lazy load when extracted to custom widget

I have a drawer on my main page which became bloated with content and logic. For example, I need to fetch an image, get some data from Internet and so on. After extracting Drawer in new MyCustomDrawerWidget which extends StatefulWidget, its state has build function which looks like this:
#override
Widget build(BuildContext context) {
return Drawer(
child: ...
);
}
My HomePageState has build function which looks like this:
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Home page'),
),
drawer: MyCustomDrawerWidget(),
body: ...
);
}
After refactoring the code like this, I noticed how drawer does not get initialized when page loads. It would initialize just after I open the drawer, which is bad because user needs to wait for data to be loaded.
Is there a way to set eager loading on drawer to get initialized along with the page?

I had the same problem. The Scaffold only attaches the drawer when necessary.
The solution: Move the data loading logic into the parent widget. You can still encapsulate it in a separate model class (e.g. DrawerModel), but this class must be instantiated when the parent widget is initialized. Keep it in the State of the parent. Pass DrawerModel to your drawer widget.
You have to be a bit careful when using streams and futures, because those usually can not be resubscribed. I would recommend you to use a StreamBuilder in the drawer widget that is connected to a BehaviorSubject in your DrawerModel.
That's very much the BLoC pattern. Do you need a code example?

Problem with having Drawer in MainPage:
Content, more logic, fetch an image, get some data from the Internet and so on.
Problem with having Drawer in MyCustomDrawerWidget:
Loading time, bad user experience
I would suggest getting the hybrid approach to solve the problem.
Trigger the network calls in MainPage
Send future to the MyCustomDrawerWidget (If the user opens drawer before network call completes, we can show loader based on future value)
Move the logic and contents related to drawer into MyCustomDrawerWidget.
class MyCustomDrawerWidget extends StatelessWidget {
var _future;
MyCustomDrawerWidget(this._future);
#override
Widget build(BuildContext context) {
return new Drawer(
child: FutureBuilder(
builder: (context, snapshot) {
if (snapshot.hasData) {
var data = snapshot.data;
return new Text(data);
} else {
return new Text("loading");
}
},
future: _future,
),
);
}
}
And in MainPage
#override
Widget build(BuildContext context) {
val future = Future.delayed(Duration(seconds: 10), () => "new value") //network call
return Scaffold(
appBar: AppBar(
title: Text('Home page'),
),
drawer: MyCustomDrawerWidget(future),
body: ...
);
}
Hope this helps.