Should I get the following error:
class.dart:11:11: Error: The getter '_privateID' isn't defined for the class 'Y'.
- 'Y' is from 'class.dart'.
Try correcting the name to the name of an existing getter, or defining a getter or field named '_privateID'.
From the following code?
mixin.dart:
class Mixin {
static int _nextID = 0;
int publicID = _nextID++; // I only need one of these lines
int _privateID = _nextID++; // but this variable is inaccessible
}
class.dart:
import 'mixin.dart';
class X with Mixin {
void run() {
print(publicID); // no error here
}
}
class Y with Mixin {
void run() {
print(_privateID); // Error: _privateID not defined
}
}
void main() {
Y().run();
}
Or is this a bug? If it's not a bug, I'd like to understand why this behavior is reasonable.
When I instead define the mixin in the same file as the above classes, I get no error.
(Dart SDK 2.4.1.)
It is not a bug.
The private field is inherited, but you cannot access it because its name is private to a different library.
Dart's notion of "privacy" is library private names.
The name _privateID in the mixin.dart library introduces a library private name. This name is special in that it can only be written inside the same library.
If someone writes _privateID in a different library, it is a different name, one unique to that library instead.
It is as if private names includes the library URI of the library it is written in, so what you really declare is a name _privateID#mixin.dart.
When you try to read that field in class.dart, you write ._privateID, but because it is in a different library, what you really write is ._privateID#class.dart, a completely different name, and the classs does not have any declarations with that name.
So, if one class needs to access a private member of another class (or mixin, or anything), then the two needs to be declared in the same library, because otherwise they cannot even write the name of that variable.
That is why the code works if you write the mixin in the same library.
If you want to move the mixin to a separate file, but not necessarily a separate library, you can use a part file.
Related
I'm using the Dart annotation #protected because I want to limit a member to only be called from a single other class, like so
class Foo {
#projected
void doSomethingSpecial() {}
}
class Bar {
final foo = Foo();
consumeDoSomethingSpecial() {
foo.doSomethingSpecial();
}
}
Understandably, foo.doSomethingSpecial() triggers the warning
The member 'doSomethingSpecial' can only be used within instance members of subclasses...
In the C++ world I would annotate Bar as a friend of Foo to permit this call but I'm not seeing the equivalent annotation in Dart?
I did see that I can suppress a warning by adding
// ignore: the_appropriate_lint_rule
above the line with the warning but I'm not seeing a lint rule that applies to using the #protected annotation?
"friend" in Dart is essentially implemented by ensuring that the identifiers are "library-local" (begin with underscore), and all friends are in the same library. A library in Dart is typically just a single file; however, using part/part-of, it can span multiple files.
Any reference to an underscore-prefixed identifier is in-scope for the same library, but out of scope anywhere else, even if you import that file.
I have to access to a private method in a Class. I created a library with inside the Class, imported it but it can't still recognize. I have the same problem also with private variables.
Example:
file buffer.dart:
library buflib;
class Buffer{
void _record(){
[...]
}
}
in the same folder: engine.dart
import 'buffer.dart';
class Engine {
Buffer _buff = Buffer()
[...]
void myMethod(){
[...]
this._buff._record();
}
}
I have this error:
The method '_record' isn't defined for the type 'Buffer'.
Try correcting the name to the name of an existing method, or defining >a method named '_record'.dartundefined_method
Any suggestions?
By default, each separate .dart file is a separate library. Since private identifiers are private to the library, they won't be visible to other .dart files.
You can use the library and part of directives to group multiple .dart files into the same library, but those directives aren't documented.
u cant call private methods or variables from out there classes .... u should change it to public by removing "under score"
I am newbie for Guice and seeking help for the following use case :
I have developed one package say (PCKG) where the entry class of that package depends on other class like:
A : Entry point class --> #Inject A(B b) {}
B in turn is dependent on C and D like --> #Inject B(C c, D d) {}
In my binding module I am doing :
bind(BInterface).to(Bimpl);
bind(CInterface).to(CImpl);
...
Note I am not providing binding information for A as i want to provide its binding by its consumer class. (this is how the design is so my request is to keep the discussion on main problem rather than design).
Now my consumer class is doing like:
AModule extends PrivateModule {
protected void configure() {
bind(AInterface.class).annotatedWith(AImpl.class);
}
}
Also in my consumer package:
.(new PCKGModule(), new AModule())
Q1. Am i doing the bindings correctly in consumer class. I am confused because when i am doing some internal testing as below in my consumer package:
class testModule {
bind(BInterface).to(Bimpl);
bind(CInterface).to(CImpl)...
}
class TestApp {
public static void main(..) {
Guice.createInstance(new testModule());
Injector inj = Guice.createInstance(new AModule());
A obj = inj.getInstance(A.class);
}
}
It is throwing Guice creation exception.Please help me get rid of this situation.
Also one of my friend who is also naive to Guice was suggesting that I need to create B's instance in AModule using Provides annotation. But i really didn't get his point.
Your main method should look like this:
class TestApp {
public static void main(..) {
Injector injector = Guice.createInjector(new TestModule(), new AModule());
A obj = injector.getInstance(A.class);
}
Note that the Java convention is for class names to have the first letter capitalised.
I'm pretty sure your implementation of AModule isn't doing what you think it's doing either, but it's hard to be certain based on the information you've provided. Most likely, you mean to do this:
bind(AInterface.class).to(AImpl.class)`
There's no need to do anything "special" with A's binding. Guice resolves all the recursion for you. That's part of its "magic".
annotatedWith() is used together with to() or toInstance(), like this:
bind(AInterface.class).to(AImpl.class).annotatedWIth(Foo.class);
bind(AInterface.class).to(ZImpl.class).annotatedWIth(Bar.class);
Then you can inject different implementations by annotating your injection points, e.g.:
#Inject
MyInjectionPoint(#Foo AInterface getsAImpl, #Bar AInterface getsZImpl) {
....
}
It's worth also pointing out that you can potentially save yourself some boilerplate by not bothering with the binding modules (depending how your code is arranged) and using JIT bindings:
#ImplementedBy(AImpl.class)
public interface AInterface {
....
}
These effectively act as "defaults" which are overridden by explicit bindings, if they exist.
So in Apple documentation:
Any type members added in an extension have the same default access level as type members declared in the original type being extended. If you extend a public or internal type, any new type members you add will have a default access level of internal.
Giving a sub class of UIView extension:
extension UIViewSubClass
{
var helloWorld : String {
get {
return "helloWorld"
}
}
}
This will mark helloWorld as internal, I have no problem, and I cannot see it in my Objective-C based project.
However, if I mark the extension to be public:
public extension UIViewSubClass
{
var helloWorld : String {
get {
return "helloWorld"
}
}
}
Now helloWorld apprears in my Objective-C based code, which means it is marked as public.
However, I don't see Apple mentions this, did it?
I just saw the documentation said a public class will still has implicit internal level.
public class SomePublicClass { // explicitly public class
public var somePublicProperty = 0 // explicitly public class member
var someInternalProperty = 0 // implicitly internal class member
private func somePrivateMethod() {} // explicitly private class member
}
Marking public for extension seems have different effect than marking a Class definition. This makes me confused.
Could someone help me, is this supposed to be so, or this is a sort of swift bug? I am with swift 2.1 and Xcode 7.2
Answer: Yes, placing access level modifiers (in your case, public, specifically) in front of extensions modify the default access level of all new types in the scope of that extension.
Note that the modifier does not affect the access level of the class/struct/etc being extended (only it's members. However, there are things that ought to be considered, as I will discuss below.
In this discussion, I'll post a few important facts regarding regarding access levels in Swift. All these are from the Swift Language Guide - Access Control - Access Levels.
Let's first assure what you've already stated:
All entities in your code (with a few specific exceptions, as
described later in this chapter) have a default access level of
internal if you do not specify an explicit access level yourself.
OK, this goes in line with what you've quoted in you question: any new type members, whether in a class or structure definition, will have a default access level of internal.
Now, lets look at the access level modifier that you can add in front of extensions:
You can extend a class, structure, or enumeration in any access
context in which the class, structure, or enumeration is available.
Any type members added in an extension have the same default access
level as type members declared in the original type being extended. If
you extend a public or internal type, any new type members you add
will have a default access level of internal. If you extend a private
type, any new type members you add will have a default access level of
private.
Alternatively, you can mark an extension with an explicit access level
modifier (for example, private extension) to set a new default access
level for all members defined within the extension. This new default
can still be overridden within the extension for individual type
members.
This sorts things out. We look at your example, and assume that your class UIViewSubClass has access level public (or compile time error, se below):
/* no access level modifier: default access level will be 'internal' */
extension UIViewSubClass
{
// default access level used: internal
var helloWorld : String {
get {
return "helloWorld"
}
}
}
// modify default access level to public
public extension UIViewSubClass
{
// default access level used: public
var helloWorld : String {
get {
return "helloWorld"
}
}
}
With the discussion above in mind, it's expected that your helloWorld in you public extension ... is marked as internal, as this is, in this context, the default access level. In the context of extensions, access level modifiers work differently than when applied to types.
Finally, we should point out that using a public access modifier when extending a non-public class will yield a compile time error in Swift. So in your case above:
If UISubViewClass is an internal or a private class, then the public extension ... on the class will yield an compile time error.
If UISubViewClass is a public class, then adding the public extension will be redundant as the default access modifier of a public class is already, by definition, public.
I'd say that the error described above is not really an error to avoid runtime errors, but rather to avoid redundant (and confusing) code: public member types or private or internal classes will never make use of itspublic` access level.
class MyImplicitlyInternalClass {
private var myExplicitlyPrivateVar = 0
var myImplicitlyInternalVar = 0
public var myExplicitlyPublicVar = 0 // warning, see (Note 1) below
// redundant 'public': can never be accessed publicly
// myExplicitlyPublicVar will behave as 'internal'
}
public extension MyImplicitlyInternalClass { // error, see (Note 2)
var myNewVarIsInternal : Int { get { return 0 } }
}
/* (Note 1) Compile type warning:
"Declaring a public var for an internal class."
(Note 2) Compile time error:
"Extension of internal class cannot be declared public."
Summary: in theory, these two above are the same type of
'faults', but only the latter is flagged as and error. */
Hence, it only ever makes sense to use the access level modifiers on extensions to make the default access level more restrictive, i.e., using internal extension ... of a public class, or private extension or an internal class.
I wish to have the sub-classes of a super-class "registered" by an arbitrary name - whenever I declare a sub-class I wish to also have it entered into the super-class.sub Map.
Is there any way to accomplish this outside of main()?
// base class
class Mineral{
final String formula;
static Map<String,Mineral> sub = {}
Mineral( this.formula );
}
// sub class - declare and register
class Mica extends Mineral{
Mica( String formula ) : super( formula );
}
Mineral.sub['mica'] = Mica; // oops!
when I run this, I get
Error: line 10 pos 1: unexpected token 'Mineral' Mineral.sub['mica'] = Mica;
assuming that executable code is not allowed outside main().
cannot put within the super-class since other sub-classes may declared later, outside the library.
Dart has no way to run code as part of a library being loaded.
Executable code can only be put inside methods, or in field initializers, and static field initializers are lazy so they won't execute any code until you try to read them.
This is done to ensure quick startup - a Dart program doesn't have to execute any code before starting the main library's "main" method.
So, no, there is no way to initialize something that isn't constant before main is called.
Either
Mineral.sub['mica'] = new Mica();
or
static Map<String,Type> sub = {};
When you assign Mica you assign the Type Mica. new Mica() is an instance of Mica that is of the kind Mineral and can be assigned to the map you declared.
edit
Maybe you want to initialize the sub map:
static Map<String,Mineral> sub = {'mica': new Mica()};
hint: the semicolon is missing in this line in your question.