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
Related
I have a class FoodDetails which accepts a list of names
class FoodDetails extends StatefulWidget {
FoodDetails({this.namesList});
final List namesList;
#override
State<FoodDetails> createState() => FoodDetailsState();
}
I want to create objects of a class NameClass based on the names of namesList.
For example:
If namesList=["John", "Doe"];
I want to create lists like
List<NameClass> John;
List<NameClass> Doe;
How can I achieve this? I tried the following but it doesn't work
class FoodDetailsState extends State<FoodDetails> {
void initState() {
super.initState();
for (int i = 0; i < widget.namesList.length; i++) {
List<NameClass> widget.namesList[i];
}
}
}
You cannot create objects dynamically like javascript in dart.
Easy workaround would be using HashMap in which the key will be the desired dynamically allocated name and the value will be the instance of the object.
Code snippet which would help you start with this:
import 'dart:collection';
class Class {
String? var1;
String? var2;
Class({var1, var2}) {
this.var1 = var1;
this.var2 = var2;
}
}
void main() {
var list = ["John", "Doe"];
HashMap<String, Class> map = new HashMap();
list.forEach((element) {
map.addAll({element: new Class()}); // Just adding the key to the Map
});
print(map.containsKey("John"));
print(map.containsKey("Doe"));
// Can easily manipulate the value known the key
map["John"]!.var1 = "variable1";
map["Doe"]!.var2 = "variable2";
map.forEach((key, value) {
print("Key: " +
key +
" has values var1: " +
(value.var1 ?? "") +
" and var2: " +
(value.var2 ?? ""));
});
}
The output would be:
true
true
Key: John has values var1: variable1 and var2:
Key: Doe has values var1: and var2: variable2
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'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;
Is there any way using which I can return a Class as a function return type?
My sample code:
class Data_1{
static int price = 1;
}
class Data_2{
static int price = 2;
}
getData() {
if (widget.data_name == "Data_1"){
return Data_1;
} else if (widget.data_name == "Data_2"){
return Data_2;
}
}
void print_price() {print(getData.price);}
Using the above code getting the below error:
The getter 'price' isn't defined for the type 'dynamic Function()'.
Try importing the library that defines 'price', correcting the name to the name of an existing getter, or defining a getter or field
named 'names'.
There are a few issues with the code above, to fix them:
We should define a property getter in the classes:
class Data_1 {
static int _price = 1;
get price => _price;
}
class Data_2 {
static int _price = 2;
get price => _price;
}
getData() should return an instance of the class, not the Type:
getData() {
if (widget.data_name == "Data_1")
{return Data_1();} // not Data_1
else if (widget.data_name == "Data_2")
{return Data_2();} } // not Data_2
}
We can now call the getData() function and access the price getter:
void main() {
print(getData().price);
}
Follows the full code example:
class Data_1 {
static int _price = 1;
get price => _price;
}
class Data_2 {
static int _price = 2;
get price => _price;
}
getData() {
return Data_1();
}
void main() {
print(getData().price);
}
I made this simple model to work with an API
class BooksModel {
List<_Book> _books = [];
BooksModel.fromJson(Map<dynamic, dynamic> parsedJson) {
List<_Book> temp = [];
for (int i = 0; i < parsedJson['books'].length; i++) {
_Book book = _Book(parsedJson['books'][i]);
temp.add(book);
}
_books = temp;
}
List<_Book> get books => _books;
}
class _Book {
int _id;
String _name;
_Book(book) {
_id = book['id'];
_name = book['name'];
}
int get id => _id;
int get name => _name;
}
The problem is i have to turn all '_Book' class properties to 'String', if i made only one 'int' as given in the above example, it throws this error.
type 'String' is not a subtype of type 'int'
I don't even use that 'id' which is 'int' in my app, so it's not about the usage of it, the problem is in this model
Can you just show the example of your json, so that i can tell you that what your model should be or where it is going wrong.
So maybe your issue is not that big , id required is integer and you are passing the String.
Is book['id'] a string?
Try it:
// _id = book['id'];
_id = int.parse(book['id']);