I try to make each step will be in another dart file. (Steps contains a lot of code, it's hard to maintain it in single file)
What is the idea?
Create Scaffold with stepper (parent widget)
Parent widget creates an instance of steps, each instance modifies the same model (object of class)
Parent widget may call isValid() method on each step. (if it's valid, we can switch to next step)
Requirements:
Steps may be StatefulWidget or StatelessWidget (it may be hard to achieve so StatefulWidget will be enough)
Each step should contain bool isValid() method.
What I have? - Almost nothing
abstract class ValidatedStep {
bool isValid();
}
And see below Step, we implement this abstract class into Step1, but this method should ask state if it's valid. From StatefulWidget we do not have access to state.
class Step1 extends StatefulWidget implements ValidatedStep {
//its ok to create it here?
final _Step1State state = _Step1State();
#override
_Step1State createState() {
//we could (and should?) create state here, but variables in this widget should be final
return state;
}
#override
bool isValid() {
return state.isValid();
}
}
class _Step1State extends State<Step1> implements ValidatedStep {
#override
Widget build(BuildContext context) {
return Container();
}
#override
bool isValid() {
return true;
}
}
You can use map.. Here is a modified example of my code.. Perhaps you can modify to add function to validate the steps.
You can simply put the different Widget from different files in the list.
class StepperScreen extends StatelessWidget {
final List<SomeClass> list;
StepperScreen({#required this.list});
#override
Widget build(BuildContext context) {
return SafeArea(
child: Scaffold(
appBar: AppBar(
title: Text(
'Stepper Screen',
),
),
body: Stepper(
steps: getStep(list),
),
),
);
}
}
List<Step> getStep(List<SomeClass> list) {
return list
.map(
(e) => Step(
title: Text(
e.name,
style: TextStyle(fontSize: 20),
),
content: e.child,
),
)
.toList();
}
Of course, your SomeClass should have the properties:
class SomeClass {
String name;
Widget child;
SomeClass({#required this.name, #required this.child});
}
A bit late to the party, but instead of asking the step if it isValid() pass some callback e.g. onReadyChange(bool) down to the step and call it from within the step according to your business logic.
Related
I'm building a simple app with lots of nested widgets/classes from different specialised files
list of files:
main.dart -> the menu file used to start the activity
"Activity()"
group_widgets.dart -> the file that contains the custom widget
"CustomWidget()"
file_a.dart -> the file that uses the custom widgets
inside the "Activity()"
other.dart -> other files that needs to manage data changed in CustomWidget()
inside main.dart:
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => const Activity(),
));
},
inside group_widgets.dart:
class CustomWidget extends StatefulWidget {
const CustomWidget({Key? key}) : super(key: key);
#override
State<CustomWidget> createState() => _CustomWidgetState();
}
class _CustomWidgetState extends State<CustomWidget> {
var _boolean = false;
bool switchBoolean(bool state) => !state;
#override
Widget build(BuildContext context) {
return ElevatedButton(
onPressed: () => {
setState(() {
_boolean = switchBoolean(_boolean);
})
},
child: Container(
color: _boolean == true ? Colors.green : Colors.red,
),
);
}
}
inside file_a.dart
class Activity extends StatefulWidget {
const Activity({Key? key}) : super(key: key);
#override
State<Activity> createState() => _ActivityState();
}
class _ActivityState extends State<Activity> {
#override
Widget build(BuildContext context) {
bool boolean = true;
return Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: const [
CustomWidget(),
Text('Here where to show the variable from CustomWidget'
'and prove I can retrieve it')
],
),
),
);
}
}
inside other.dart
if ( booleanFromCustomWidget == true) {
Something ...
}
What is the best practice to achieve it?
I've read a lot here but nothing seems to well fit my needing.
Just comment if my request is not as clear as it seems to me))
Please correct me if I am wrong, but if you want to access data from parent widgets from inside their descendants (children or even nested children) you can either pass them down via parameter arguments:
Child(int age, String name);
And then accept it in the new file, where the Child widget lives, via its constructor:
class Child {
String name;
int age;
// Constructor
Child(String passedName, int passedAge) {
this.name = passedName;
this.age = passedAge;
}
}
Inside the parent.dart you then have to import the children.dart to use it.
Or use a popular package like the provider package: https://pub.dev/packages/provider
This allows you to store data containers, which you can access basically anywhere in your code. Feel free to google it & watch some tutorials to get started, as it is the preferred approach to avoid passing data to widget which really do not care about the passed parameters.
Note: You can transfer the idea to output the String data like in your example code above.
you can use a state manager like provider, or bloc
At the top level, you set up the data services
I am getting this error in Dart:
"Missing concrete implementation of "state.build""
The first method is the following:
class MyHomePage extends StatefulWidget {
// String titleInput;
// String amountInput;
#override
MyHomePageState createState() => MyHomePageState();
}
class MyHomePageState extends State<MyHomePage> {
final List<Transaction> _userTransactions = [
// Transaction(
// id: "t1",
// title: "New Shoes",
// amount: 69.99,
// date: DateTime.now(),
// ),
// Transaction(
// id: "t2",
// title: "Weekly Groceries",
// amount: 16.53,
// date: DateTime.now(),
// ),
];
Does anyone knows what this error means and how to solve it?
Thank you.
You need to add a build method to the State of your widget, this method describes the part of the user interface represented by your widget, e.g.,
(add this to the MyHomePageState)
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child:
Container(
height: 200,
width: 100,
color: Colors.yellow,
),
),
);
}
Check the curly brace that closes the class where the Widget build method is found. You might just have closed it in the wrong place.
This was the case for me.
So make sure your build method is within the curly braces that enclose the class
class _LandingPageState extends State<LandingPage> {
Widget build(BuildContext context) {
...
}
All the Stateful widgets and Stateless widgets should have build method.
#override
Widget build(BuildContext context) {
return Container(
...
);
}
If you want to use it without build do not extend the class with State, use it like
class YourClassName {
}
Goto the definitation of State<T> class and see what are the abstract methods. You will find build() method as the only abstract method i.e. a method without body. So when yu are inheriting from State<MyHomePage>, you must override the build() and give a body; basically you will create your Widgets inside the build() method.
So to fix the error add the below code to your class:
class MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return Scaffold( // Your Widget
...
);
}
}
Just check
#override
Widget build( Buildcontext context)
{
return Container();
}
Check spelling of build
reason : As u extend your class with stateless or statefull widget , U use overiding method to over ride the pre defined method which is already written in parent class which is
state less/full class
I am unable to access a public static boolean from a different class, eg. I have a boolean isFull in my StudyjiosListviewScreen class as shown:
class StudyjiosListviewScreen extends StatefulWidget {
#override
_StudyjiosListviewScreenState createState() => _StudyjiosListviewScreenState();
}
class _StudyjiosListviewScreenState extends State<StudyjiosListviewScreen> {
static bool isFull = false;
...
I want to use this boolean isFull in another class JoinStudyjio.
I created an instance of the StudyjiosListviewScreen class in the JoinStudyjio class like this:
StudyjiosListviewScreen listviewScreen = StudyjiosListviewScreen();
But when I try to use the boolean isFull like this:
if (listviewScreen.isFull) {
...
I get an error. I have already imported the file for the StudyjiosListviewScreen class inside the file for the JoinStudyjio class.
This is because StudyjiosListviewScreen and _StudyjiosListviewScreenState are 2 different classes.
The static variable isFull which you are trying to access is of the later one and you are trying to access it by creating an instance of the first one. If it had been a static variable of the class StudyjiosListviewScreen, you could have accessed it without even creating an instance of that class like this StudyjiosListviewScreen.isFull
If I understood your issue correctly, and following the suggestion I made in my comment, here is a code example of sharing a variable and a method to change it's value, down to two classes from a parent class:
class VariableSharing62951032 extends StatefulWidget {
#override
_VariableSharing62951032State createState() => _VariableSharing62951032State();
}
class _VariableSharing62951032State extends State<VariableSharing62951032> {
bool isFull = false;
#override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
ClassA62951032(isFull: isFull, swapIsFull: swapIsFull,),
ClassB62951032(isFull: isFull, swapIsFull: swapIsFull,),
],
);
}
void swapIsFull(){
setState(() {
isFull = !isFull;
});
}
}
class ClassA62951032 extends StatefulWidget {
final bool isFull;
final Function swapIsFull;
ClassA62951032({
this.isFull,
this.swapIsFull
});
#override
_ClassA62951032State createState() => _ClassA62951032State();
}
class _ClassA62951032State extends State<ClassA62951032> {
#override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
Text('Class A'),
Text(widget.isFull.toString()),
RaisedButton(
child: Text('Swap isFull'),
onPressed: () => widget.swapIsFull(),
),
],
);
}
}
class ClassB62951032 extends StatefulWidget {
final bool isFull;
final Function swapIsFull;
ClassB62951032({
this.isFull,
this.swapIsFull
});
#override
_ClassB62951032State createState() => _ClassB62951032State();
}
class _ClassB62951032State extends State<ClassB62951032> {
#override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
Text('Class B'),
Text(widget.isFull.toString()),
RaisedButton(
child: Text('Swap isFull'),
onPressed: () => widget.swapIsFull(),
),
],
);
}
}
Sharing variables and methods between classes it's a huge deal in Flutter.
First of all, you are passing it in the wrong way. That variable is saved in your state widget, which is defined as private.
So, or you define it as public and than you pass a key associated with your state, or you change complitelly approach. I don't like passing keys and it is not good for production, so I will give you a better example using providers:
add provider library to your pubspec.yaml:
provider: ^4.3.1 // Or latest version
Create a class where you can save that value:
class valuesHelper {
//In this class we are storing global, dynamic values
bool _isSeen;
valuesHelper() {
this._isSeen = false;
}
void setValue(bool value) {
this._isSeen = value;
}
bool getValue(){
return this._isSeen;
}
}
Now wrap your main with the provider and pass the valuesHelper();
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return Provider(
create: (_) => valuesHelper(),
child: MaterialApp(
home: MyHomePage(),
),
);
}
}
Now call the Provider.of(context) wherever you want.
//Somwhere in your code when you have access to context:
ValueHelper helper = Provider.of<valueHelper>(context);
helper.setValue(true);
//Somwhereelse in your code when you have access to context:
ValueHelper helper = Provider.of<valueHelper>(context);
bool theValueIWant = helper.getValue();
If you have asynchronous stuff and huge state managment Blocs are even better and fancier, but for this kind of things Providers are more than enough.
I have a stateful widget LetterButton()
#override
Widget build(BuildContext context) {
return RaisedButton(
child: Text(widget.caption),
onPressed: onChanged,
color: colors[currentIndex],
padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
);
}
In my main.dart file I declare an array of LetterButtons
List<LetterButton> buttonArray;
which I initialize and fill during initState() via the method
void makeButtons() {
for (var letter in alphabet) {
buttonArray.add(
LetterButton(letter),
);
}
}
The buttons in the list are then displayed in the UI
Wrap(
children: buttonArray,
)
How can I change the value of currentIndex (an int in
class LetterButtonState extends State<LetterButton>) or otherwise change all the buttons to the same color from main.dart?
NOTE: I asked a similar question a few days ago, but the answer was a little above my current knowledge, as are responses I've seen to similar Q's here on SO. I have a little understanding of callbacks, and experimented a little with the provider package, but there's such a variety of answers and info available online that it's hard for me to even know what I don't know to be able to answer my question :-)
Create stateful widget with state as public access, so that you can access outside of the package and provide key to constructor. So that you can refer key and get can get state to change value. See the following example
class LetterButton extends StatefulWidget {
LetterButton({GlobalKey key}) : super(key: key);
#override
LetterButtonState createState() => LetterButtonState();
}
class LetterButtonState extends State<LetterButton> {
int value = 0;
//this public method is to update int value
setValue(int value) {
setState(() {
this.value = value;
});
}
#override
Widget build(BuildContext context) {
return Center(
child: Text(value.toString()),
);
}
}
//In Main.dart
GlobalKey<LetterButtonState> _buttonStateKey = GlobalKey();
//while creating widget
LetterButton(key:_buttonStateKey)
//in onTapCallback you can call to update value
_buttonStateKey.currentState?.setValue(10);
Just send your created function to new class by parameter and the new class should be Constarcter with Function lThen you can call the function from a new class.
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 setState in 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/StatefulWidget pattern 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.
I think I'm with user48956 on this. (Catchy name by the way).
Unfortunately, the Flutter authors seem to have suffixed their View class with the word 'State'. This has rather confused the whole Flutter state management discussions.
I think the purpose of the two classes is actually to make the painting more performant but it comes with a very heavy plumbing cost for us developers.
As to the naming convention:
The dirty flag approach allows the widget painter to optimise their painting without knowing about our state, thereby alleviation the need for two classes.
Also generateView() is kinda meaningful (unless of course, you start using these widgets to hold model-fragments (as per Package:provider).