I have this class:
class Entry {
final String id;
final List<ListEntry> listEntries;
Entry({this.listEntries}):
id = Uuid().v4();
Entry.withId({this.id, this.listEntries});
}
// create new class instance
var e = Entry();
Now when I call any method on e.listEntries I will get a NPE because it is not initialized. Is there a way to have it default to an empty list in case the constructor argument is not provided?
You can use a Factory constructor:
class Entry {
final String id;
final List<String> listEntries;
factory Entry({List<String> listEntries}) {
return Entry._(listEntries ?? []);
}
Entry._(this.listEntries):
id = Uuid().v4();
Entry.withId({this.id, this.listEntries});
}
You can initialize the field in the initializer list instead of using this.ListEntries (an initializing formal).
class Entry {
final String id;
final List<ListEntry> listEntries;
Entry({List<ListEntry> listEntries})
: this.withId(id: Uuid().v4(), listEntries: listEntries);
Entry.withId({this.id, this.listEntries})
: listEntries = listEntries ?? [];
}
// create new class instance
var e = Entry();
Here I reuse the Entry.withId constructor so I only have to write things once.
Related
I remember that Dart objects have a method which does an object return a value by default without pointing to an property. Example:
class A {
final String name;
A(this.name);
...
}
main() {
var obj = A('chesu');
print(obj + ' locuaz');
}
Output: chesu locuaz
But I don't remember that method or decorator and it is not toString().
Use interpolation to compose strings and values. It will automatically call toString method of value.
class A {
A(this.name);
final String name;
#override
toString() => name;
}
main() {
var obj = A('chesu');
print('$obj locuaz');
}
I finally found what I was looking for, it's called callable classes.
In dart we can execute some code when value of field is changed using something like
class Name{
String fname;
String lname;
}
class Person extends ChangeNotifier{
Name _name = Name();
set name(Name n){
notifyListeners();
_name = n;
}
get name=>_name;
}
//inside main()
Person p = Person();
p.name = Name();
I want to be able to perform similar action while setting inner fields. Such as while doing
p.name.fname ="FooBar";
But I want to be able to do it from Person class.
Because I am extending ChangeNotifier in Person class. And I want to call
notifyListeners()
that is not accessible in Name class. This is best I've come up with
Name newName = Name(p.name); //copy constructor
newName.fname = "Foo Bar";
p.name = newName;
Is there a better way?
What you can do depends on how you can constrain the API.
If Name objects are routinely being created by third-party code and passed around, and are expected to retain their identity when stored in a Person object, then here isn't much you can do. So I wouldn't design the Person object that way.
Instead I'd say that the Name object of a Person object is linked to that, and setting the name of a Person is the same as setting both name parts.
Example:
class Person {
_PersonName _name;
Person(...) : ... {
_name = _PersonName(this);
}
...
void set name(Name name) {
_name.fname = name.fname;
_name.lname = name.lname;
notifyListeners();
}
Name get name => _name;
}
class _PersonName extends Name {
final Person _owner;
_PersonName(this._owner);
void set fname(String fname) {
super.fname = fname;
_owner.notifyListeners();
}
void set lname(String lname) {
super.lname = lname;
_owner.notifyListeners();
}
}
That has the disadvantage that the extracted _PersonName is forever linked to the Person object, even if you try to write a different Name object.
Another option is to create a new _PersonName on every store a new name object, and detach the old object from the Person at that point:
class Person {
_PersonName _name = _PersonName;
Person(...) : ... {
_name = _PersonName(this, null, null);
}
void set name(Name name) {
_name.owner = null;
_name = _PersonName(this, name.fname, name.lname);
notifyListeners();
}
Name get name => _name;
}
class _PersonName extends Name {
Person _owner;
_PersonName(this._owner, String fname, String lname) {
super.fname = fname;
super.lname = lname;
}
void set fname(String fname) {
super.fname = fname;
owner?.notifyListeners();
}
void set lname(String lname) {
super.lname = lname;
owner?.notifyListeners();
}
}
This approach behaves mostly like the plain storing of name objects, except that if you do:
var p = Person();
var n = Name();
p.name = n;
print(identical(n, p.name)); // false?
you don't preserve the identity of the Name object stored into the Person object.
There is no way to do so, and also change the behavior of setting strings directly on the name using person.name.fname = ..., so something has to be sacrificed.
Is it really not possible to create multiple constructors for a class in dart?
in my Player Class, If I have this constructor
Player(String name, int color) {
this._color = color;
this._name = name;
}
Then I try to add this constructor:
Player(Player another) {
this._color = another.getColor();
this._name = another.getName();
}
I get the following error:
The default constructor is already defined.
I'm not looking for a workaround by creating one Constructor with a bunch of non required arguments.
Is there a nice way to solve this?
You can only have one unnamed constructor, but you can have any number of additional named constructors
class Player {
Player(String name, int color) {
this._color = color;
this._name = name;
}
Player.fromPlayer(Player another) {
this._color = another.getColor();
this._name = another.getName();
}
}
new Player.fromPlayer(playerOne);
This constructor
Player(String name, int color) {
this._color = color;
this._name = name;
}
can be simplified to
Player(this._name, this._color);
Named constructors can also be private by starting the name with _
class Player {
Player._(this._name, this._color);
Player._foo();
}
Constructors with final fields initializer list are necessary:
class Player {
final String name;
final String color;
Player(this.name, this.color);
Player.fromPlayer(Player another) :
color = another.color,
name = another.name;
}
If your class uses final parameters the accepted answer will not work. This does:
class Player {
final String name;
final String color;
Player(this.name, this.color);
Player.fromPlayer(Player another) :
color = another.color,
name = another.name;
}
If you already used a constructor with params in the project and now you figured out that you need some no params default constructor you can add an empty constructor.
class User{
String name;
User({this.name}); //This you already had before
User.empty(); //Add this later
}
Try the below code on DartPad
class MyClass {
//These two are private attributes
int _age;
String _name;
//This is a public attribute
String defaultName = "My Default Name!";
//Default constructor
MyClass() {
_age = 0;
_name = "Anonymous";
}
MyClass.copyContructor(MyClass fromMyClass) {
this._age = fromMyClass._age;
this._name = fromMyClass._name;
}
MyClass.overloadedContructor(String name, int age) {
this._age = age;
this._name = name;
}
MyClass.overloadedContructorNamedArguemnts({String name, int age}) {
this._age = age;
this._name = name;
}
//Overriding the toString() method
String toString() {
String retVal = "Name:: " + _name + " | " + "Age:: " + _age.toString();
return retVal;
}
}
//The execution starts from here..
void main() {
MyClass myClass1 = new MyClass();
//Cannot access oprivate attributes
//print(myClass1.name);
//print(myClass1.age);
//Can access the public attribute
print("Default Name:: " + myClass1.defaultName);
print(myClass1.toString());
MyClass myClass2 = new MyClass.copyContructor(myClass1);
print(myClass2.toString());
MyClass myClass3 = new MyClass.overloadedContructor("Amit", 42);
print(myClass3.toString());
MyClass myClass4 =
new MyClass.overloadedContructorNamedArguemnts(age: 42, name: "Amit");
print(myClass4.toString());
}
Dart doesn't support parameter overloading (having multiple functions of the same name but with different parameters). This applies to constructors as well - that's the reason why in SDK there're so many classes with named constructors.
In Dart you can use Default Constructor, Named Constructor, Factory Method and Static Method to instantiate classes:
class A {
// Default constructor
A() : msg = '1';
// Named constructor with positional param
A.message(this.msg);
// Factory method with named param
factory A.underscore({String msg = ''}) {
return A.message('_'+msg);
}
// Factory method with arrow func body
static A bang(msg) => A.message('!'+msg);
final String msg;
}
void main() {
print(A().msg);
print(A.message('2').msg);
print(A.underscore(msg: '3').msg);
print(A.bang('4').msg);
}
Output:
1
2
_3
!4
You can use factory constructors
factory Player.fromPlayer(Player another) => Player(another.name, another.color);
i had found solution to solve this problem depend on checked the type of data you are passed it to function
Try this Solution
As Günter Zöchbauer already specified in his answer:
You can only have one unnamed constructor, but you can have any number of additional named constructors in Flutter.
By using named constructor you can create multiple constructors in the same class.
Each constructor will have a unique name. So that you can identify each of them.
Syntax for named constructor :
class_name.constructor_name (arguments) {
// If there is a block of code, use this syntax
// Statements
}
or
class_name.constructor_name (arguments);
// If there is no block of code, use this syntax
For more insights Click Here
To know about various types of constructors in Flutter Click Here
Class User{
User();
User.fromName(this.name);
String? name;
}
If you want to do some more elaborated property calculation (I'm a Swift guy), you can do like this:
class FooProvider {
int selectedFoo;
FooProvider(List<String> usageObjects)
: selectedFoo = firstOne(usageObjects);
static int firstOne(List<String> usageObjects) {
return 2;
}
}
Basically, that's what I'm trying to do:
ClassName
{
final OtherClass field;
ClassName()
{
field = new OtherClass(this);
}
}
It's not possible to assign a final field in a constructor body. The final field needs to be assigned before the constructor body, in the initializer list or on declaration:
class ClassName
{
final OtherClass field = new OtherClass(); // Here
ClassName()
: field = new OtherClass() // or here
{
}
}
As you can't use this in the initializer list or on the declaration, you can't do what you plan to do.
With null safety, you can initialize a final field in different ways:
At declaration:
class Foo{
final int bar = 1;
}
In constructor parameter (initializing formal).
class Foo {
final int bar;
// Initializing in constructor parameter.
Foo(this.bar);
}
In the initializer list.
class Foo {
final int bar;
// Initializer list
Foo() : bar = 1;
}
Combination of above two.
class Foo {
final int bar;
Foo(int value) : bar = value;
}
Use late keyword for lazy initialization.
class Foo {
late final int bar; // Initialize it later, maybe in a method
}
Since Dart 2.12 it is possible by using late keyword.
The code below prints 5:
class ClassName
{
final int var1 = 5;
late final OtherClass field;
ClassName()
{
field = new OtherClass(this);
}
}
class OtherClass {
OtherClass(ClassName object) {
print(object.var1);
}
}
void main() {
final object = ClassName();
}
Please see this and the following sections
Like the question at Dynamic class method invocation in PHP I want to do this in Dart.
var = "name";
page.${var} = value;
page.save();
Is that possible?
There are several things you can achieve with Mirrors.
Here's an example how to set values of classes and how to call methods dynamically:
import 'dart:mirrors';
class Page {
var name;
method() {
print('called!');
}
}
void main() {
var page = new Page();
var im = reflect(page);
// Set values.
im.setField("name", "some value").then((temp) => print(page.name));
// Call methods.
im.invoke("method", []);
}
In case you wonder, im is an InstanceMirror, which basically reflects the page instance.
There is also another question: Is there a way to dynamically call a method or set an instance variable in a class in Dart?
You can use Dart Mirror API to do such thing. Mirror API is not fully implemented now but here's how it could work :
import 'dart:mirrors';
class Page {
String name;
}
main() {
final page = new Page();
var value = "value";
InstanceMirror im = reflect(page);
im.setField("name", value).then((_){
print(page.name); // display "value"
});
}
You can use Serializable
For example:
import 'package:serializable/serializable.dart';
#serializable
class Page extends _$PageSerializable {
String name;
}
main() {
final page = new Page();
var attribute = "name";
var value = "value";
page["name"] = value;
page[attribute] = value;
print("page.name: ${page['name']}");
}