I'm trying to find the name using the id in modelList from the example below.
class Example{
List<Abc> modelList = [
Abc(1, "John"),
Abc(2, "Christine"),
Abc(3, "Steven"),
Abc(4, "Others"),
];
myFun(){
int idToFind = 4;
String foundString = // Some iterable function??
}
}
class Abc{
int id;
String name;
Abc(this.id, this.name);
}
String foundString = modelList.firstWhere((abc) => abc.id == idToFind).name;
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void main(){
Student file1 = Student.empty;
Student file2 = Student.empty;
file1.name = 'ABC';
file2.name = 'DEF';
print(file1.name);
print(file2.name);
}
class Student{
String name;
Student({
required this.name,
});
static Student empty = Student(name: '');
}
Output Value
DEF
DEF
Expected Value
ABC
DEF
This happens, because you are using the same static instance of Student, since the static field is shared across all instances of Student.
So your variables file1 and file2 are referencing the same single instance of Student.
You may want to use a factory constructor instead:
https://dart.dev/guides/language/language-tour#factory-constructors
void main() {
Student file1 = Student.empty();
Student file2 = Student.empty();
file1.name = 'ABC';
file2.name = 'DEF';
print(file1.name);
print(file2.name);
}
class Student {
String name;
Student({
required this.name,
});
factory Student.empty() {
return Student(name: '');
}
}
so i recently started learning dart and I've found something kinda interesting.
why do we use constructors and getters/setters when we can achieve same results without them? (atleast when used for basic things).
class v1{
var name;
int age;
v1(this.name, this.age);
info(){
print("my name is $name and i am $age");
}
}
class v2{
var name = "bash";
int age = 100;
info(){
print("my name is $name and i am $age");
}
}
class v3{
var namee;
int agee;
String get name => namee;
int get age => agee;
set name(String name) => this.namee = name;
set age(int age) => this.agee = age;
info(){
print("my name is $name and i am $age");
}
}
void main(){
var x = v1("bash", 100);
x.info(); //my name is bash am i am 100
var z = v2();
var Z = v2();
Z.name = "vert";
Z.age = 20;
z.info(); //my name is bash and i am 100
Z.info(); //my name is vert and i am 100
var y = v3();
y.name = "rizz";
y.age = 40;
y.info(); //my name is rizz and i am 40
}
Here's a more correct version of your class:
class User {
final bool _isMale;
String _name;
int _age;
User(this._isMale, this._name, this._age);
bool isMale => _isMale;
String get name => _name;
int get age => _age;
set name(String name) {
// Sometimes you may want to update other properties here.
// For example:
// _isUpdated = true;
_name = name;
}
set age(int age) {
_age = age;
}
void info() {
print("my name is $name and i am $age");
}
}
Constructors are useful when you want to assign initial values to the class fields. They are essential if you need to assign final fields, as they are assignable only on class initialization (see _isMale field).
Setters are useful when you want to update other fields along with the field that's being modified.
Getters protect the internal state from being modified outside. In this example, nobody can change _isMale field.
You don't need to use getters and setters unless you have to.
You use getters and setters if you need to store the data in a private field, or if you want to modify it when saving or returning the value.
class Abc {
String _field;
String _otherField;
String anotherField; // No getters and setters required for this.
String get field => _field;
set field(String field) => _field = field;
String get otherField => "The value of otherField is: " + _otherField;
set otherField(String otherField) => _otherField = "[String] " + otherField;
}
As for constructors, you use them to initialize the object with custom values. When you need to work with immutable objects (which use final variables), you'll have to use constructors to set their initial value. You can also modify the incoming value according to your need before storing it,
class Def {
final field; // Dart generates getters for this field, but it's value can't be modified once the object is instantiated.
final _otherField; // No getters for this.
Def(String field, String otherField) {
this.field = "[String] $field"
this._otherField = "[String] $otherField"
}
String describeMe() {
return "[Def]: field: $field, _otherField: $_otherField"
}
}
i have one List (growable) with an item (actually item 0:
items is of class Team
items[_id = 1, _team = "Team01", _note = "blabla"]
and I want to transfer it into another list with a different structure:
participants is of class User
participants[id = 1, name = "participant1"]
skipping the note and translating _id into id and so on.So at last the result would give me
participants[id = 1, name = "team01"]
(sorry for the writing, I describe it out of the debugger)
i tried something like this, but doesnt work with value:
List<TestTeam> participants;
for (var value in items) {
participants.add(new TestTeam(value.id, value.team));
}
my class Team is defined like this:
class Team {
int _id;
String _team;
String _note;
Team(this._team, this._note);
Team.map(dynamic obj) {
this._id = obj['id'];
this._team = obj['team'];
this._note = obj['note'];
}
int get id => _id;
String get team => _team;
String get note => _note;
Map<String, dynamic> toMap() {
var map = new Map<String, dynamic>();
if (_id != null) {
map['id'] = _id;
}
map['team'] = _team;
map['note'] = _note;
return map;
}
Team.fromMap(Map<String, dynamic> map) {
this._id = map['id'];
this._team = map['team'];
this._note = map['note'];
}
}
You should implement below way
void main() {
List<Team> teams=[];
List<User> participants=[];
for (var i = 0; i < 4; i++) {
teams.add(Team(i,'Team_$i','Note_$i'));
}
for (var value in teams){
participants.add(User(value.id,value.team));
}
for (var value in teams){
print(value.toString());
}
for (var value in participants){
print(value.toString());
}
}
class Team{
int id;
String team;
String note;
Team(this.id,this.team,this.note);
toString()=> 'Team Map :{id:$id,team:$team,note:$note}';
}
class User{
int id;
String team;
User(this.id,this.team);
toString()=> 'User Map :{id:$id,team:$team}';
}
Output
Team Map :{id:0,team:Team_0,note:Note_0}
Team Map :{id:1,team:Team_1,note:Note_1}
Team Map :{id:2,team:Team_2,note:Note_2}
Team Map :{id:3,team:Team_3,note:Note_3}
User Map :{id:0,team:Team_0}
User Map :{id:1,team:Team_1}
User Map :{id:2,team:Team_2}
User Map :{id:3,team:Team_3}
var categoryList = [hotDrinks , coldDrinks];
List<Item> hotDrinks = [Item(name:"hot chocolate" , contents: "milk-chocolate")] ;
List<Item> coldDrinks = [Item(name: "orange juice" , contents: "orange")] ;
print(categoryList[i].tostring())
but it just print an instance of each item like [instance of 'item']
Override your Items toString method, example add the following method to the Item class
String toString() {
return this.name;
}
This prints out 'hot chocolate'. Hope it helps.
class Item{
String name;
String contents;
Item({this.name, this.contents});
String get nameStr => name;
}
main() {
List<Item> hotDrinks = [Item(name:"hot chocolate" , contents: "milk-chocolate")] ;
List<Item> coldDrinks = [Item(name: "orange juice" , contents: "orange")] ;
var categoryList = [hotDrinks , coldDrinks];
int i = 0;
print(categoryList[i][0].nameStr);
}
i want to do use the model's properties such as:
Animal.id as a param to a function or use some extension method to be able to "id". similarly, i'd like to use Animal.title in that way to get "title" as a returned value. how could i do this with my class to get a string for any given property name?
int _id;
String _title;
Animal(this._id, this._title);
int get id => _id;
String get title => _title;
}
the usage case is being able to query without having autocomplete on my model's property names in a string for sql querying:
List<Map> results = await db.query("Animal",
columns: Set. ["id", "title"],
where: 'id = ?',
whereArgs: [id]);
Using the dart:mirrors package you can dynamically access your class properties and invoke methods using their string names.
https://api.dart.dev/stable/2.4.0/dart-mirrors/dart-mirrors-library.html
import 'dart:mirrors';
class Animal {
int _id;
String _title;
Animal(this._id, this._title);
int get id => _id;
String get title => _title;
}
main() {
var r = reflect(Animal(1, 'Dog'));
print(r.getField(Symbol('id')).reflectee);
print(r.getField(Symbol('title')).reflectee);
}
import 'dart:mirrors';
class MyClass {
int i, j;
void my_method() { }
int sum() => i + j;
MyClass(this.i, this.j);
static noise() => 42;
static var s;
}
main() {
MyClass myClass = new MyClass(3, 4);
InstanceMirror myClassInstanceMirror = reflect(myClass);
ClassMirror MyClassMirror = myClassInstanceMirror.type;
InstanceMirror res = myClassInstanceMirror.invoke(#sum, []);
print('sum = ${res.reflectee}');
var f = MyClassMirror.invoke(#noise, []);
print('noise = $f');
print('\nMethods:');
Iterable<DeclarationMirror> decls =
MyClassMirror.declarations.values.where(
(dm) => dm is MethodMirror && dm.isRegularMethod);
decls.forEach((MethodMirror mm) {
print(MirrorSystem.getName(mm.simpleName));
});
print('\nAll declarations:');
for (var k in MyClassMirror.declarations.keys) {
print(MirrorSystem.getName(k));
}
MyClassMirror.setField(#s, 91);
print(MyClass.s);
}
the output:
sum = 7
noise = InstanceMirror on 42
Methods:
my_method
sum
noise
All declarations:
i
j
s
my_method
sum
noise
MyClass
91