I'm developing an app and my current task is to implement localization for several languages. Now that I've translated most of it, I need to create a button that I would nest in the app bar that would drop down and offer the different languages.
To that extent, I've followed this tutorial : https://www.youtube.com/watch?v=yX0nNHz1sFo&list=PLyHn8N5MSsgEfPAxCytQDPATDlHwpP5rE&index=3.
Now, since I plan on reusing this button for different pages, I created its own class, so far for good. But when comes time to add the onChanged property that would call the function responsible for language switching, it doesn't work. In fact, I can't call any function at all.
Here's the code snippet:
class LanguageButton extends StatelessWidget {
void changeLanguage(Language lang) {
print(lang.languageCode); //placeHolder
}
Widget build(BuildContext context) {
return DropdownButton(
underline: SizedBox(),
onChanged: (Language lang) {
changeLanguage(lang);
},
icon: Icon(
Icons.language,
color: Colors.white,
),
items: Language.languageList()
.map<DropdownMenuItem<Language>>((lang) => DropdownMenuItem(
value: lang,
child: Row(children: [
Text(lang.flag),
Text(lang.language),
])))
.toList(),
);
}
}
Which returns the error The argument type 'void Function(Language)' can't be assigned to the parameter type 'void Function(Language?)?'..
I've tried replacing by onChanged: ()=>print('hello') but it's still not working with a similar error message.
Any help is appreciated!
The onChanged method of a DropdownButton (see the documentation) is a ValueChanged<T?>?. That means it can itself be null (for example if you don't set it at all), but if you set it, you need to set it to a method with the signature void Function(T?). Your method does not confirm to that signature, because it does not allow nulls.
Change yourmethod to:
void changeLanguage(Language? lang) {
and either pass it directly:
onChanged: changeLanguage,
or make the anonymous method have a nullable parameter, too:
onChanged: (Language? lang) {
changeLanguage(lang);
},
Related
i want to change displayed data in Flutter? I wrote a function changeDataForTest (only a function for testing the event), which should change the data displayed in Text.
But if I click on this, it isn't changed. The value of the displayed string only changes, if i add (context as Element).reassemble(); after calling the method. Is this the normal way to go, or is there a smoother way to solve my problem?
dynamic changeDataForTest(neuerWert) {
this.data = neuerWert;
}
Column(
children: [
Center(
child: Text(
this.data + this.wiegehts,
),
),
FlatButton(
textColor: Color(0xFF6200EE),
onPressed: () {
changeDataForTest('neuerWert');
(context as Element).reassemble();
},
)
],
)
Thanks
Lukas
If you're using only a small widget, you could use a StatefulWidget using the method:
setState(() {
// change your variable
})
If your widget is complex and has lots of different possible variables, I'll not recommend using setState as this method calls the build method every time is being used.
One simple and fast option, is to use ValueNotifier:
final myVariable = ValueNotifier(false); // where you can replace 'false' with any Object
and then, using it this way:
ValueListenableBuilder(
valueListenable: myVariable,
builder: (context, value, child) {
return Text(value); // or any other use of Widgets
},
);
myVariable.value = true; // if you're looking for to change the current value
finally, if you logic is truly complex and you need to scale, I'll recommend to use a StateManagement library like:
Provider
Riverpod
BloC
Others
You can find those libraries and examples over: https://pub.dev
When I select a value from my dropdown the hint text does not change:
String fontSizeValue;
new DropdownButton<String>(
items: new List<double>.generate(72, (i) => i + 2.0).map((double value) {
return new DropdownMenuItem<String>(
value: value.toString(),
child: new Text(value.toString()),
);
}).toList(),
onChanged: (String _) {
setState(() {
fontSize = double.parse(_);
fontSizeValue = _;
print(fontSizeValue);
});
},
value: fontSizeValue,
hint: Text('Select'),
)),
],
),
),
);
Any idea how I can get the selected value to show instead of "select"? Thanks
You did not post enough code, but I'll take a wild guess anyway, because there are a lot of questions with this problem:
Your variable String fontSizeValue; is defined locally, probably in the build function.
You have to define it in a wider scope, so it will retain it's value after another call to build that will happen when you call setState. Probably as a class member of your State class.
I am new to flutter, and am trying to code mobile game. So far I have been able to manage through previous questions on forums, youtube tutorials and example apps. However I have hit a wall that I cannot solve.
Many of the features in my UI are supposed to change based on user behavior but do not update, I am using this DropdownButton as my example but it is a problem I am having elsewhere in the code as well. When the user makes a selection from the DropdownButton it should update the value of the button, but instead only the hint is displayed. I have been able to determine through print statements that the selection is registered.
Based on what I have learned so far I suspect that the issue entails my widget's statefulness or lack thereof. I found a few similar questions which suggested using a StatefulBuilder, however when I tried to implement it the code had errors or else duplicated my entire ListTile group. I also saw suggestions about creating a stateful widget instead but the instructions were too vague for me to follow and implement without errors. I am including snippets of the code below, I am hesitant to paste the whole thing because the app is nearly 2000 lines at this point. Please let me know if any further information or code is needed.
this is the problematic code, see below for the code in context.
DropdownButton(
value: skillChoice,
items: listDrop,
hint: Text("Choose Skill"),
onChanged: (value) {
setState(() {
skillChoice = value;
});
},
),
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Hunters Guild',
theme: ThemeData(
primaryColor: Colors.grey,
),
home: Main(),
);
}
}
final List<Combatant> _combatants = List<Combatant>();
//combatant is a class that holds stats like HP which may change during a battle
//they also have a Fighter and a Monster and one of those will be null and the other useful
//depending if faction = "good" or "evil
//I am aware that this may not be the most elegant implementation but that is a problem for another day.
class MainState extends State<Main> {
final List<Fighter> _squad = List<Fighter>();
//Fighter is a class which stores info like HP, name, skills, etc for game character,
//this List is populated in the page previous to _fight
//where the user picks which characters they are using in a given battle
void _fight(Quest theQuest){
for(Fighter guy in _squad){
_combatants.add(Combatant("good", guy, null));
}
_combatants.add(Combatant("evil", null, theQuest.boss));
//quests are a class which store a boss Monster, later on I will add other things like rewards and prerequisites
final tiles = _combatants.map((Combatant comba){ //this structure is from the build your first app codelab
List<DropdownMenuItem<String>> listDrop = [];
String skillChoice = null;
if (comba.faction == "good"){
for (Skill skill in comba.guy.skills){
listDrop.add((DropdownMenuItem(child: Text(skill.name), value: skill.name)));
}
}
else if (comba.faction == "evil"){
for (Skill skill in comba.boss.skills){
listDrop.add((DropdownMenuItem(child: Text(skill.name), value: skill.name)));
}
}
//^this code populates each ListTile (one for each combatant) with a drop down button of all their combat skills
return ListTile(
leading: comba.port,
title: Text(comba.info),
onTap: () {
if(comba.faction == "good"){
_fighterFightInfo(comba.guy, theQuest.boss); //separate page with more detailed info, not finished
}
else{
_monsterFightInfo(theQuest.boss); //same but for monsters
}
},
trailing: Row(
mainAxisSize: MainAxisSize.min,
children: [
DropdownButton( //HERE IS WHERE THE ERROR LIES
value: skillChoice,
items: listDrop,
hint: Text("Choose Skill"),
onChanged: (value) {
setState(() {
skillChoice = value;
});
},
),
IconButton(icon: Icon(Icons.check), onPressed: (){
//eventually this button will be used to execute the user's skill choice
})
],
),
subtitle: Text(comba.hp.toString()),
);
},
);
Navigator.of(context).push(
MaterialPageRoute<void>(
builder: (BuildContext context) {
//I have tried moving most of the above code into here,
//and also implementing StatelessBuilders here and other places in the below code to no effect
final divided = ListTile.divideTiles(
context: context,
tiles: tiles,
).toList();
return Scaffold(
appBar: AppBar(
title: Text("Slay "+theQuest.boss.name+" "+theQuest.boss.level.toString()+" "+theQuest.boss.type.name, style: TextStyle(fontSize: 14),),
leading: IconButton(icon: Icon(Icons.arrow_back), onPressed: () {
_squadSelect(theQuest);
}),
),
body: ListView(children: divided),
);
},
),
);
}
//there are a lot more methods here which I haven't included
#override
Widget build(BuildContext context) {
//I can show this if you need it but there's a lot of UI building on the main page
//Also might not be the most efficiently implemented
//what you need to know is that there are buttons which call _fight and go to that page
}
class Main extends StatefulWidget {
#override
MainState createState() => MainState();
}
Thank you for any help or advice you can offer.
Its a bit difficult to understand what is going on here, so if possible add the problem areas in a new widget and post it here.
In any case, for now it seems your skillShare variable is local, so every time you are setting state it does not update. Try something like this:
class MainState extends State<Main> {
String skillShare = ""; //i.e make skillShare a global variable
final List<Fighter> _squad = List<Fighter>();
The following question had information which helped me find the answer. Different question but the solution applies. Thanks user:4576996 Sven. Basically I need to reformat from doing my constructing in the void _fight to a new stateful page. This may be useful for anyone else who is using the beginning tutorial as a framework to build their app.
flutter delete item from listview
I'm new to Flutter and am working through the intro course on Udacity. In one of the tasks, I was trying to follow the code and I can't make much sense of it. Here's the code from the solution of the project (I've cut and paste the parts that matter, and also legal disclaimer I do not own any of this code, it's from the sample Flutter Udacity project):
Widget build(BuildContext context) {
final input = Padding(
padding: _padding,
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
TextField(...),
_createDropdown(_fromValue.name, _updateFromConversion),
],
),
);
}
void _updateFromConversion(dynamic unitName) {
setState(() {
_fromValue = _getUnit(unitName);
});
if (_inputValue != null) {
_updateConversion();
}
}
Widget _createDropdown(String currentValue, ValueChanged<dynamic> onChanged) {
return Container(
margin: EdgeInsets.only(top: 16.0),
decoration: BoxDecoration(...),
padding: EdgeInsets.symmetric(vertical: 8.0),
child: Theme(...),
child: DropdownButtonHideUnderline(
child: ButtonTheme(
alignedDropdown: true,
child: DropdownButton(
value: currentValue,
items: _unitMenuItems,
onChanged: onChanged,
style: Theme.of(context).textTheme.title,
),
),
),
),
);
}
Here's where I'm stuck. _updateFromConversion requires an input parameter unitName. But when they call it, in _createDropdown, they don't pass any. So how does _updateFromConversion know what unitName is? Also, is _updateFromConversion executed before _createDropdown, or is it executed when the "onChanged" property of DropdownButton is set?
Second question: they're passing that function with return type void into _createDropdown, which is expecting ValueChanged. Shouldn't this throw an error?
If someone can explain the flow of this code and what I am missing I would greatly appreciate it.
Thank you!
You seem to be misunderstanding the assignment of a variable to a function for a function call.
Let me try to show it with example code.
void _updateFromConversion(dynamic unitName) {
print("Unit name: $unitName");
}
class SomeClass {
void Function(dynamic arg) myFunction;
}
void main() {
final c = SomeClass()..myFunction = _updateFromConversion;
print("Created c. Calling its function");
c.myFunction("foo");
print("Done");
}
When you run this code, you will see this printed:
Created c. Calling its function
Unit name: foo
Done
This shows that the _updateFromConversion function is not called when you create the SomeClass instance in SomeClass()..myFunction = _updateFromConversion;. This is only an assignment (it assigns to the field myFunction the value _updateFromConversion... yes, in Dart a function itself can be a value)!
You should know that because there's no () after the function name, and function invocation in Dart always must contain the list of arguments between () even if it's empty.
So, here's where the function is invoked:
c.myFunction("foo");
See? There's a list of arguments containing a single value, "foo". That's why the function then prints Unit name: foo, because the argument unitName takes the value "foo".
TL;DR
This is a function invocation:
c.myFunction("foo");
This is NOT:
c.myFunction;
Yashvin,
In dart function can be passed as parameter to other functions. This is usually used to pass callbacks for instance.
In example you provided, function _updateFromConversion is passed as parameter onChanged to another function _createDropdown.
In that function it will be assigned to onChanged listener of DropdownButton button.
Every time value of DropdownButton changes, this function will be invoked and will be passed selected value of DropdownButton.
I am new to Flutter (and Dart) and when trying to build a form to edit an object I searched online for examples and tutorials, and I saw both of these used.
What is the difference between the two? Which one should I use?
If you making a Form where you require save, reset, or validate
operations- use TextFormField. Else For Simple user input capture
TextField is sufficient.
TextFormField, which integrates with the Form widget.
This is a convenience widget that wraps a TextField widget in a FormField.
A Form ancestor is not required. The Form simply makes it easier to save, reset, or validate multiple fields at once.
To use without a Form, pass a GlobalKey to the constructor and use GlobalKey.currentState to save or reset the form field.
sample:
TextFormField(
autovalidateMode: AutovalidateMode.always
decoration: const InputDecoration(
icon: Icon(Icons.person),
hintText: 'What do people call you?',
labelText: 'Name *',
),
onSaved: (String value) {
// This optional block of code can be used to run
// code when the user saves the form.
},
validator: (String value) {
return value.contains('#') ? 'Do not use the # char.' : null;
},
)
TextField, which is the underlying text field without the Form integration.
The text field calls the onChanged callback whenever the user changes the text in the field. If the user indicates that they are done typing in the field (e.g., by pressing a button on the soft keyboard), the text field calls the onSubmitted callback.
Short answer
If you don't know what you need, then use a TextField. This is the most basic Flutter widget for getting text input from a user. It's the one you should master first.
TextField
Using a TextField is an easy way to allow user input.
TextField(
decoration: InputDecoration(
hintText: 'Name'
),
);
To get the text that the user entered, you can either get notified every time there is a change like this:
TextField(
decoration: InputDecoration(
hintText: 'Name'
),
onChanged: (text) {
// do something with text
},
),
Or you can use a TextEditingController, as described here. This will give you access to the text state.
TextFormField
If you find yourself needing to validate user text input before you save it, you might consider using a TextFormField. Imagine something like this:
There are lots of validation checks that you might want to do on a username and password.
Of course, you could still just use a couple TextFields, but TextFormField has extra builtin functionality that will make your life easier. Generally, you will only use a TextFormField when you are using it inside of a Form widget (though that isn't a strict requirement).
Here is a stripped down example from the documentation:
class MyCustomForm extends StatefulWidget {
#override
MyCustomFormState createState() {
return MyCustomFormState();
}
}
class MyCustomFormState extends State<MyCustomForm> {
final _formKey = GlobalKey<FormState>();
#override
Widget build(BuildContext context) {
return Form(
key: _formKey,
child: Column(
children: <Widget>[
TextFormField(
validator: (value) {
// validation logic
},
),
RaisedButton(
child: Text('Submit'),
onPressed: () {
if (_formKey.currentState.validate()) {
// text in form is valid
}
},
),
],
),
);
}
}
See also
Realistic Forms in Flutter — Part 1
Forms with Flutter
TextField is a simple text field. (you don't care about user input)
TextFormField is a text field to be used in a form (you care about user input).
If you don't need to validate TextField.
If you need to validate user input, use TextFormField with validator.
TextFormField vs. TextField
TextFormField returns a TextField, but wraps the TextField with extra functionality you can use through a Form and also without (such as reset, validation, save, etc.).
Generally, you want to use TextFormField unless writing boiler-plate code is your thing.
How does TextFormField work? What can it do?
TextFormField extends FormField class, (a StatefulWidget).
FormField objects do a special thing when they are instantiated: they look up the widget tree for a Form and register themselves with that Form.
After registration, these FormField widgets can be changed by that parent Form.
Form is also a StatefulWidget. It has a FormState object.
FormState can get & set data on any/allFormFields registered to it.
For example to clear the entire form we can call reset() on FormState and FormState will iterate through all child FormFields registered, resetting each FormField to its initialValue (null by default).
Validation is another common use case for putting several FormField like TextFormField inside Form/FormState. This allows multiple fields to be validated with a single validate() call on the Form.
How? FormState has a validate() method that iterates through each registered FormField and calls that FormField's validate() method. Much more convenient calling validate() once on Form than you manually keeping track of all TextField and validating each one separately with custom code.
Details
How does a TextFormField register itself with a Form?
FormField (base class of TextFormField, etc.) make a call within their build() method:
Form.of(context)?._register(this);
In English this means:
Search up my context hierarchy until we find a Form widget (if any) and call that form's register method on myself.
The ? is in case there is no Form widget parent. The _register call will only be run if there is a Form & FormState somewhere above.
How does Form.of(context)?._register(this) work?
Form & FormState sneakily use InheritedWidget.
In FormState.build() you'll see this code:
return WillPopScope(
onWillPop: widget.onWillPop,
child: _FormScope( // ← sneaky
formState: this,
generation: _generation,
child: widget.child,
),
);
Looking at _FormScope we see:
class _FormScope extends InheritedWidget
When a parent widget is an InheritedWidget, any child can find that parent using a special "find me a parent of this exact Type" method.
Here's how that "find me" method is used/exposed inside Form as a static method we can call from anywhere:
static FormState of(BuildContext context) {
final _FormScope scope = context.dependOnInheritedWidgetOfExactType<_FormScope>();
return scope?._formState;
}
The naming of that method dependOnInheritedWidgetOfExactType is a bit tricky. It'd be more readable as findInheritedWidgetOfExactType, but it does more than just find. (We can trigger rebuilds of Form through children that have registered as dependOn this FormState).
Summary
TextFormField is the deluxe package, air-conditioned, Bluetooth-connected 8-speaker stereo version of TextField. It includes a lot of common functionality you would use when accepting user-entered information.
I think this might be the most concise and simple explanation about the differences between the two.
From the material library:
TextField: A material design text field.
TextFormField: A FormField that contains a TextField.
Similarly, you can wrap FormField around any cupertino input component such as CupertinoTextField
Below is an example about a custom CheckboxFormField, which is a FormField that wraps around the material design component Checkbox:
// A custom CheckboxFormField, which is similar to the built-in TextFormField
bool agreedToTerms = false;
FormField(
initialValue: false,
validator: (value) {
if (value == false) {
return 'You must agree to the terms of service.';
}
return null;
},
builder: (FormFieldState formFieldState) {
return Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Row(
children: [
Checkbox(
value: agreedToTerms,
onChanged: (value) {
// When the value of the checkbox changes,
// update the FormFieldState so the form is
// re-validated.
formFieldState.didChange(value);
setState(() {
agreedToTerms = value;
});
},
),
Text(
'I agree to the terms of service.',
style: Theme.of(context).textTheme.subtitle1,
),
],
),
if (!formFieldState.isValid)
Text(
formFieldState.errorText ?? "",
style: Theme.of(context)
.textTheme
.caption
.copyWith(color: Theme.of(context).errorColor),
),
],
);
},
),
Rule of thumb: If your box only have a single input field, just use the raw material input like TextField (FormField is a bit overkill in this case though). If your box has many input fields, you need to wrap each one of them in a FormField, and then integrate all of them to the Form widget to reap the benefits of validating and saving all form fields at once.
Extra tip: If u have a TextField wrapped in a FormField that doesn't allow the user to enter any text such as a CupertinoPickerFormField or a SimpleDialogFormField that offers the user a choice between several options (which is basically a material SimpleDialog widget wrapped in a FormField), just simply use the hintText param of InputDecoration without using TextEditingController to manipulate text. Make the hint text have the same color as the normal input text with hintStyle: const TextStyle(color: Color(0xdd000000)).
This video from Flutter Europe will help you master forms in Flutter in no time.