In Dart (Flutter) I would like to have some static code run without being explicitly invoked.
I tried this:
// File 1
class MyClass {
static int member = 42;
}
int dummy = 42;
and file 2:
// File 2
void main() {
int tmp = MyClass.member;
}
I put a breakpoint on the dummy = 2; line but it seemed to never be invoked.
I also tried:
// File 1
class MyClass {
static int member1 = 42;
static int member2 = SomeOtherClass.someFunc();
}
and file 2:
// File 2
void main() {
int tmp1 = MyClass.member1;
int tmp2 = MyClass.member2;
}
With this, SomeOtherClass.someFunc() was invoked when the int tmp2 = ... line was invoked.
I would like SomeOtherClass.someFunc() to be invoked without explicitly accessing MyClass.member2. I would like it invoked on any of the following triggers:
When the program starts (before main() is called).
OR, when code in a file in which MyClass is imported is invoked for the first time.
Is either of these possible in Dart?
This behavior is intentional and cannot be changed. As jamesdlin also explain, all static variables (class and global) in Dart are lazy evaluated and will first get a value with first attempt to access the value.
This is design is described in the Dart specification followed up with a reason for that design choice:
Static variable declarations with an initializing expression are initializedlazily.
The lazy semantics are given because we do not want a language where one tends to define expensive initialization computations, causing long application startup times. This is especially crucial for Dart, which must support the coding of client applications.
https://dart.dev/guides/language/specifications/DartLangSpec-v2.2.pdf
Related
Why is it an error in Dart to try to set a class's static variable in the global space?
Example:
class Name {
static String? firstName;
}
Name.firstName = 'Mike'; // Error
void main() {
Name.firstName = 'Mike'; // Ok
}
It's not a big deal. I just came across this and then couldn't find an explanation for why it is. Where in the documentation does it describe the nuance here?
[UPDATE]
The actual error thrown is, among others: "Variables must be declared using the keywords 'const', 'final', 'var', or a type name."
You actually can execute statements outside of a function, but they have to be statements that declare scoped variables. Maybe these aren't technically statements, but just variable instantiations.
class Name {
static String? staticName;
String? lastName;
}
final me = Name(); // Ok
me.lastName = 'Jones'; // Error
void main() {
Name.staticName = 'Mike'; // Ok
final you = Name(); // Ok
you.lastName = 'Smith'; // Ok
}
Without the variable scoping, the compiler thinks I must be defining a function and it gets confused when there is no parameter list or function body.
It makes sense that statements are restricted to variable instantiations of function definitions only, so that there won't be side effects related to execution order to other importers of the file, as per #jamesdlin answer.
Name.firstName = 'Mike'; is a statement. You can't execute arbitrary statements in the global namespace. In what order would they execute? Suppose you had:
name.dart:
class Name {
static String? firstName;
}
and mike.dart:
import 'name.dart';
Name.firstName = 'Mike';
and spike.dart:
import 'name.dart';
Name.firstName = 'Spike';
and finally:
import 'name.dart';
import 'mike.dart';
import 'spike.dart';
void main() {
print(Name.firstName);
}
What should happen? Should it be illegal for multiple libraries to assign to Name.firstName? Should the last one imported win? If so, then suddenly importing a library would have side-effects, and order would matter. What would happen if an imported library imports other libraries with side-effects?
It's a huge headache that is completely unnecessary since you could have just done:
class Name {
static String? firstName = 'Mike';
}
in the first place.
I am new to Dart and Flutter. While I am going through tutorials, I got that we can make singleton using factory keyword. But after that, I got this code.
class AccountService {
static final _instance = AccountService._internal();
AccountService._internal();
static AccountService getInstance() {
return _instance;
}
}
My questions.
How does the code work?
when getInstance() get called?
is AccountService._internal() a constructor?
static final _instance = AccountService._internal(); - When this get called?
Please help me
Static fields in Dart are all lazy evaluated so they will first get its value the first time you access the field.
So:
When you call getInstance(), it will return the value of the field _instance. If this is the first time the field will be evaluated so AccountService._internal() is called. If it is second time, the value from previous access is reused.
First time you call the method somewhere in your code? If you are never calling the method, the object referenced by _instance will never be created.
Yes, it is a named constructor and because the name starts with "_" it is only available from the library this class is part of. By doing so, it is possible to restrict new objects from this class so only the class itself are allowed to create an instance.
It is called first time _instance is accessed. Since this name also starts with "_" it is only available from the library this class is part of.
The lazy initialization of static fields is described in the Dart specification with the following reasoning:
Static variable declarations with an initializing expression are initializedlazily (8.1).
The lazy semantics are given because we do not want a language where one tends to define expensive initialization computations, causing long application startup times. This is especially crucial for Dart, which must support the coding of client applications.
https://dart.dev/guides/language/specifications/DartLangSpec-v2.2.pdf
Added code example
class AccountService {
static final _instance = AccountService._internal();
AccountService._internal() {
print(':: Calling AccountService._internal constructor');
}
static AccountService getInstance() {
print(':: Calling getInstance()');
return _instance;
}
}
void main() {
print(':: Step 1');
AccountService.getInstance();
print(':: Step 2');
AccountService.getInstance();
print(':: End');
}
Output:
:: Start
:: Step 1
:: Calling getInstance()
:: Calling AccountService._internal constructor
:: Step 2
:: Calling getInstance()
:: End
I use Piece class as Map's key.
But when this code ran, error occured Uncaught exception:
C.JSNull_methods.$indexSet is not a function.
class Piece {
int type;
Piece(this.type);
}
void main() {
Map<Piece, int> hand;
hand[Piece(5)] = 5;
if (hand.containsKey(Piece(5))) {
print("contains");
}
print('${hand[Piece(5)]}');
}
In dart-lang, how can I use class as Map's key?
First, the error you got has nothing to do with using types as keys but are before you never initialize the hand variable. So you need to do this:
Map<Piece, int> hand = {};
Now, you will not get the exception but your code will properly not work as expected since hand.containsKey(Piece(5)) will return false and print('${hand[Piece(5)]}') will return null.
This is because the map Map<Piece, int> are not using the Type as key but instead objects of the type Piece. So if we take your code here:
Map<Piece, int> hand = {};
hand[Piece(5)] = 5;
if (hand.containsKey(Piece(5))) {
print("contains");
}
print('${hand[Piece(5)]}');
You are here creating a new object instance of the Piece type each type you are writing "Piece(5)". Since each of this objects will be a separate instance of a Piece then you will not receive the value 5 you have saved because the value 5 has been saved for a different object than you are requesting.
There are multiple solutions for that and I don't know which one are the best for you. But the simple solution in this case is to either only creating one instance of Piece and reuse that:
void main() {
Map<Piece, int> hand = {};
final piece = Piece(5);
hand[piece] = 5;
if (hand.containsKey(piece)) {
print("contains");
}
print('${hand[piece]}');
}
Or make a const constructor for your Piece class so instances with the same arguments are made into the same object. This solution requires that the int type are final since you cannot edit a const constructed object (since it is constant):
class Piece {
final int type;
const Piece(this.type);
}
void main() {
Map<Piece, int> hand = {};
hand[const Piece(5)] = 5;
if (hand.containsKey(const Piece(5))) {
print("contains");
}
print('${hand[const Piece(5)]}');
}
Note that you need to prefix you object instantiation with const like "const Piece(5)" each time you want a instance where you are sure it will returns the same object for the same arguments.
I want to create a private variable but I cannot.
Here is my code:
void main() {
var b = new B();
b.testB();
}
class A {
int _private = 0;
testA() {
print('int value: $_private');
_private = 5;
}
}
class B extends A {
String _private;
testB() {
_private = 'Hello';
print('String value: $_private');
testA();
print('String value: $_private');
}
}
When I run this code, I get the following result:
String value: Hello
int value: Hello
Breaking on exception: type 'int' is not a subtype of type 'String' of 'value'.
Also I not get any error or warnings when editing this source code.
How can I create a private variable in Dart?
From Dart documentation:
Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore _, it’s private to its library.
Libraries not only provide APIs, but are a unit of privacy: identifiers that start with an underscore _ are visible only inside the library.
A few words about libraries:
Every Dart app is a library, even if it doesn’t use a library directive. The import and library directives can help you create a modular and shareable code base.
You may have heard of the part directive, which allows you to split a library into multiple Dart files.
Dart documentation "libraries-and-visibility"
Privacy in Dart exists at the library, rather than the class level.
If you were to put class A into a separate library file (eg, other.dart), such as:
library other;
class A {
int _private = 0;
testA() {
print('int value: $_private'); // 0
_private = 5;
print('int value: $_private'); // 5
}
}
and then import it into your main app, such as:
import 'other.dart';
void main() {
var b = new B();
b.testB();
}
class B extends A {
String _private;
testB() {
_private = 'Hello';
print('String value: $_private'); // Hello
testA();
print('String value: $_private'); // Hello
}
}
You get the expected output:
String value: Hello
int value: 0
int value: 5
String value: Hello
In dart '_' is used before the variable name to declare it as private. Unlike other programming languages, here private doesn't mean it is available only to the class it is in, private means it is accessible in the library it is in and not accessible to other libraries. A library can consists of multiple dart files as well using part and part of. For more information on Dart libraries, check this.
The top answer as of now is definitely correct.
I'll try to go into more detail in this answer.
I'll answer the question, but lead with this: That's just not how Dart is intended to be written, partly because library-private members make it easier to define operators like ==. (Private variables of a second object couldn't be seen for the comparison.)
Now that we've got that out of the way, I'll start out by showing you how it's meant to be done (library-private instead of class-private), and then show you how to make a variable class-private if you still really want that. Here we go.
If one class has no business seeing variables on another class, you might ask yourself whether they really belong in the same library:
//This should be in a separate library from main() for the reason stated in the main method below.
class MyClass {
//Library private variable
int _val = 0;
int get val => _val;
set val(int v) => _val = (v < 0) ? _val : v;
MyClass.fromVal(int val) : _val = val;
}
void main() {
MyClass mc = MyClass.fromVal(1);
mc.val = -1;
print(mc.val); //1
//main() MUST BE IN A SEPARATE LIBRARY TO
//PREVENT MODIFYING THE BACKING FIELDS LIKE:
mc._val = 6;
print(mc.val); //6
}
That should be good. However if you really want private class data:
Though you technically aren't allowed to create private variables, you could emulate it using the following closure technique. (HOWEVER, you should CAREFULLY consider whether you really need it and whether there is a better, more Dart-like way to do what you're trying to accomplish!)
//A "workaround" that you should THINK TWICE before using because:
//1. The syntax is verbose.
//2. Both closure variables and any methods needing to access
// the closure variables must be defined inside a base constructor.
//3. Those methods require typedefs to ensure correct signatures.
typedef int IntGetter();
typedef void IntSetter(int value);
class MyClass {
IntGetter getVal;
IntSetter setVal;
MyClass.base() {
//Closure variable
int _val = 0;
//Methods defined within constructor closure
getVal = ()=>_val;
setVal = (int v) => _val = (v < 0) ? _val : v;
}
factory MyClass.fromVal(int val) {
MyClass result = MyClass.base();
result.setVal(val);
return result;
}
}
void main() {
MyClass mc = MyClass.fromVal(1);
mc.setVal(-1); //Fails
print(mc.getVal());
//On the upside, you can't access _val
//mc._val = 6; //Doesn't compile.
}
So yeah. Just be careful and try to follow the language's best-practices and you should be fine.
EDIT
Apparently there's a new typedef syntax that's preferred for Dart 2. If you're using Dart 2 you should use that. Or, even better, use inline function types.
If you use the second, it will be less verbose, but the other problems remain.
Given the following class, Dart Editor (build 5549) gives me some conflicting feedback (per the comments in the constructor body):
class Example {
final int foo;
Example() :
foo = 0
{
foo = 1; // 'cannot assign value to final variable "foo"'
this.foo = 2; // ok
}
}
Even more confusingly, it will happily generate equivalent (working) javascript for both lines. The situation seems to be the same with methods as it is with the constructor; this especially leads me to believe that it was intended to be disallowed in both cases.
The Dart Style Guide suggests using public final fields instead of private fields with public getters. I like this in theory, but non-trivial member construction can't really go into the initializer list.
Am I missing a valid reason for the former to be reported as an error while the latter is not? Or should I be filing a bug right now?
This is surely a bug in the JavaScript generator if you run the following in the Dart VM:
main() {
new Example();
}
class Example {
final int foo;
Example() : foo = 0 {
foo = 1; // this fails in the dart vm
this.foo = 2; // this also fails in the dart vm
}
}
then it refuses to execute both the line foo = 1 and this.foo = 2. This is consistent with the spec which requires (if I read it correctly) that final fields to be final in the constructor body.