My docs and Flutter videos, the explanation of the design of the StatefulWidget (+(Widget)State) is that it:
- promotes a declarative design (good)
- formalizes the process by which Flutter to efficiently decide which components need to be re-rendered (also good)
From the example:
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {...}
}
However:
- since we have to explicitly remember call
setStatein order to invalidate the state, is this really a declarative design? - Flutter doesn't automatically detect changes in the State object and decide to call build (although it could have), and so it doesn't really formalize/automate/make-safe the invalidation of view components. Since we have to explicitly call
setState, what's the benefit of the Flutter's(Widget)State/StatefulWidgetpattern over, let's say:
class MyHomePage extends StatefulWidget // Define dirty method
{
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
int _counter = 0;
_incrementCounter() {
_counter++;
this.dirty(); // Require the view to be rebuilt. Arranges generateView to be called.
}
@override
Widget generateView(BuildContext context) {return ... rendering description containing updated counter ... ;}
}
... which would place the same burden of marking the UI dirty on the programmer, is no less decalrative, and avoids additional abstraction that obfuscates the intention of the program.
What have I missed? What's the benefit of separating of StatefulWidget from (Widget)State in Flutter?
[Before people chime in with MVC comments, note that the Flutter model rather explicitly only manages only the widget's state and its tightly coupled to the UI's Widget through the build method - there is no separation of concern here and it doesn't have a lot to say about larger application state that's not attached to a view.]
[Also, moderators, these not the same questions: Why does Flutter State object require a Widget?, What is the relation between stateful and stateless widgets in Flutter?. My question is one about what's the benefit of the present design, not how this design works.]
Update: @Rémi Rousselet -- Here's a declarative example with only a new state class needing to be declared. With some work, you could even get rid of that (though it may not be better).
This way of declaring interaction with need didn't require (the user) declaring two new circularly type-referencing class, and the widget that is changing in response to state is decoupled from the state (its constructed a pure function of the state and does not need to allocate the state).
This way of doing things doesn't survive hot-reload. (sad face). I suspect this is more of an issue with hot-reload, but if there's a way to make it work it would be great,
import 'dart:collection';
import 'package:flutter/material.dart';
////////////////////////////////
// Define some application state
class MyAppState with ChangeSubscribeable<MyAppState> {
/***
* TODO. Automate notifyListeners on setter.
* Binds changes to the widget
*/
int _counter;
get counter => _counter;
set counter(int c) {
_counter = c;
notifyListeners(); // <<<<<< ! Calls ... .setState to invalidate widget
}
increment() {
counter = _counter + 1;
}
MyAppState({int counter: 0}) {
_counter = counter;
}
}
void main() => runApp(MyApp5());
class MyApp5 extends StatelessWidget {
@override
Widget build(BuildContext context) {
// Declare the mutable state.
// Note because the state is not coupled to any particular widget
// its possible to easily share the state between concerned.
// StateListeningWidgets register for, and are notified on changes to
// the state.
var state = new MyAppState(counter: 5);
return MaterialApp(
title: 'Flutter Demo',
home: Scaffold(
appBar: AppBar(
title: Text('Flutter Demo'),
),
body: Center(
child: Column(
children: [
// When the button is click, increment the state
RaisedButton(
onPressed: () => {
state.increment(),
print("Clicked. New state: ${state.counter}")
},
child: Text('Click me'),
),
// Listens for changes in state.
StateListeningWidget(
state,
// Construct the actual widget based on the current state
// A pure function of the state.
// However, is seems closures are not hot-reload.
(context, s) => new Text("Counter4 : ${s.counter}"),
),
],
))),
);
}
}
// //////////////////////
// Implementation
// This one is the onChange callback should accept the state.
//typedef OnChangeFunc<ARG0> = void Function(ARG0);
typedef OnChangeFunc = void Function();
mixin ChangeSubscribeable<STATE> {
final _listener2Notifier =
new LinkedHashMap<Object, OnChangeFunc>(); // VoidFunc1<STATE>>();
List<OnChangeFunc> get _listeners => List.from(_listener2Notifier.values);
void onChange(listenerKey, OnChangeFunc onChange) {
// onChange(listenerKey, VoidFunc1<STATE> onChange) {
assert(!_listener2Notifier.containsKey(listenerKey));
_listener2Notifier[listenerKey] = onChange;
print("Num listeners: ${_listener2Notifier.length}");
}
void removeOnChange(listenerKey) {
if (_listener2Notifier.containsKey(listenerKey)) {
_listener2Notifier.remove(listenerKey);
}
}
void notifyListeners() {
// _listener2Notifier.forEach((key, value)=>value(state));
// Safer, in-case state-update triggers add/remove onChange:
// Call listener
_listeners.forEach((value) => value());
}
}
typedef StateToWidgetFunction<WIDGET extends Widget,
STATE extends ChangeSubscribeable>
= WIDGET Function(BuildContext, STATE);
void noOp() {}
class _WidgetFromStateImpl<WIDGET extends Widget,
STATE extends ChangeSubscribeable> extends State<StatefulWidget> {
STATE _state;
// TODO. Make Widget return type more specific.
StateToWidgetFunction<WIDGET, STATE> stateToWidgetFunc;
_WidgetFromStateImpl(this.stateToWidgetFunc, this._state) {
updateState(){setState(() {});}
this._state.onChange(this, updateState);
}
@override
Widget build(BuildContext context) => stateToWidgetFunc(context, this._state);
@override
dispose() {
_state.removeOnChange(this);
super.dispose();
}
}
class StateListeningWidget<WIDGET extends Widget,
STATE extends ChangeSubscribeable> extends StatefulWidget {
STATE _watched_state;
StateToWidgetFunction<WIDGET, STATE> stateToWidgetFunc;
StateListeningWidget(this._watched_state, this.stateToWidgetFunc) {}
@override
State<StatefulWidget> createState() {
return new _WidgetFromStateImpl<WIDGET, STATE>(
stateToWidgetFunc, _watched_state);
}
}
I've been directed at the ChangeProvider pattern: https://github.com/flutter/samples/blob/master/provider_counter/lib/main.dart
class MyHomePage extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('Flutter Demo Home Page'),),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('You have pushed the button this many times:'),
Consumer<Counter>( // <<< Pure. Hidden magic mutable parameter
builder: (context, counter, child) => Text(
'${counter.value}',
style: Theme.of(context).textTheme.display1,
),),],),),
floatingActionButton: FloatingActionButton(
onPressed: () =>
// <<< Also a hidden magic parameter
Provider.of<Counter>(context, listen: false).increment(),
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
... but this also suffers problems:
its not clear to reader of what the state requirements are or how to provide them -- the interface (at least in this github example HomePage) example does not require Counter as a formal parameter. Here we have
new HomePage()that has configuration that is not provided in its parameters - this type of access suffers similar problems to global variables.access to state is by class type, not object reference - so its not clear (or at least straightforward) what to do if you want two objects of the same type (e.g. shippingAddress, billingAddress) that are peers in the model. To resolve this, the state model likely needs to be refactored.
