If you open int.dart class source code, you'll find isEven is written as:
/** Returns true if and only if this integer is even. */
bool get isEven;
I want to know in which class this method is implemented?
It's implemented in the implementation class which implements the int interface.
Which class that is (or if there really is a class) depends on the backend.
The VM has a number of classes implementing int, but all of them get their isEven implementation from the same superclass.
When compiling to JavaScript, integers are represented directly by JavaScript numbers.
The compiled code treats those as Dart objects implementing the internal class, and that too implements isEven.
Related
I am writing code for a game in Godot and I'm using F# as my programming language. I'm kind of pushing the boundary a bit since they are still in late alpha support for C#. It isn't always easy to find solutions to problems from converting patterns from C# to F#. I'm running into an issue declaring my delegates in a way that the Godot Editor will recognize my delegate Signals. In my eyes, this borders on the x-y problem but I'm working in the constraints of the Godot system.
I'm looking to be able to declare the F# equivalent of this class. The key here is that the [Signal] function annotation is what tells the editor that this delegate can be used as a signal in their observer pattern. Having the delegate declared within the class also locks the scope of that delegate to that class. Only that class will raise that signal.
using Godot;
public class GodotNode : Node
{
[Signal]
public delegate void MyDelegate();
public void SendSignal
{
EmitSignal(nameof(MyDelegate));
}
}
I have done by best to try to recreate this within F# but I'm really falling short on how to nest the delegate within another type. From everything I have seen this isn't exactly possible but I was wondering if there was something else I can do.
open Godot
(* This is the closest I have been able to get to. *)
[<Signal>]
type MyDelegateSignal = delegate of Unit -> Unit
type GodotNodeFs()
inherit Node
(* But I need the equivalent of this
[<Signal>]
type MyDelegateSignal = delegate of Unit -> Unit *)
member this.SendSignal =
this.EmitSignal(nameof MyDelegate)
Worst case scenario, I know I can at least get this working by pushing this back down to the C# code. At the moment in Godot, every class needs to be in C#. So all F# classes are inherited by a C# wrapper class.
public class GodotNode : GodotNodeFs
{ // ... put signals and emitters in base class
}
I'm familiar with Java's Class methods
int x = Integer.parseInt("9");
in dart (which I'm new to) it's kinda weird that the method is called with the primitive type
var x = int.parse('9');
any explanation, Thank you.
In Dart everything (including primitives) is an object, so has methods. There's no need for the artificial boxing class Integer since int is already a class and can host the static string parsing method.
In ObjC, it is using Messaging, static binding, dynamic typing, dynamic binding, dynamic method resolution, dynamic loading, introspector and so on.
Importantly, the core method objc_msgSend is responsible for taking the selector you're sending and the object you're sending it to, and looking that up in the class method tables to figure out exactly which piece of code is supposed to handle it.
My concerns here are:
Is Swift doing something similar like ObjC on runtime?
How does Swift runtime find the implementation code for some object/class method?
In ObjC, classes and objects are compiled to some runtime types such as C struct on runtime. Then what are such classes and objects compiled to on runtime in Swift?
Does Swift runtime have something like class / meta class / isa pointer / super pointer?
In short there are Dynamic and Static types of method call dispatching.
Static - the function address to be called is determined in the compilation time, so that expense of such call is similar to C-function calling. This mechanism is used for private methods or final classes methods call dispatching.
Dynamic dispatching is mechinism which allows to implement polymorphism concept of OOP - the function address to be called is determined in running time. Swift has two subtypes of it:
2.1. Obj-C - you already described in the question. This mechanism is used when object inherits from NSObject or calling method has #objc prefix.
2.2. Virtual table based (like in C++) - there is similar witness tables. What it does during method call dispatching is just single arithmetic operation - calculation of actual function address based on function offset in the base class witness table and the object class witness table location. So that's a relatively cheap operation comparing to Obj-C. It explains why "pure" Swift approximates to C++ performance.
If you don't mark you method with private keyword or your class is not final and same time you class is "pure" Swift (it does not inherit NSObject) then this virtual table based mechanism is used. It means that all the methods by default are virtual.
P.S.
Helpful link for proving my vision regarding "Types":
https://developer.apple.com/swift/blog/?id=27
"Subtypes" explanation is based on my understanding.
According to Apple :
When you mark a member declaration with the dynamic modifier, access to that member is always dynamically dispatched. Because declarations marked with the dynamic modifier are dispatched using the Objective-C runtime, they’re implicitly marked with the #objc attribute.
According to Wikipedia:
dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time.
Dynamic dispatch is often used in object-oriented languages when different classes contain different implementations of the same method due to common inheritance. For example, suppose you have classes A, B, and C, where B and C both inherit the method foo() from A. Now suppose x is a variable of class A. At run time, x may actually have a value of type B or C and in general you can't know what it is at compile time.
Right now, I'm studying the dependency injection framework : Typhoon and when I open the sample project for Swift in all the classes that inherit from the Objective-C class TyphoonAssembly all the methods relatives to inject dependencies have the dynamic modifier included in the following way :
public dynamic func weatherReportDao() -> AnyObject {
return TyphoonDefinition.withClass(WeatherReportDaoFileSystemImpl.self)
}
I thought that I'm missing something, but I don't understand where is the polymorphic operation (method or function) to call at run time here.
What's is the purpose of the dynamic dispatch here?
The answer to your question is addressed in this post:
https://github.com/appsquickly/typhoon/wiki/TyphoonAssembly
Basically at runtime the Typhoon Framework is going to replace your method with its own routine that implements the features of the framework and calls your method to do whatever work you've defined for it.
In order for the framework to be able to replace the method, the method must be dynamically dispatched.
What does external mean in Dart? For example: external DateTime._now();
I'm new to Dart, I can't find documentation for external, so can you give an example to help explain？
9.4 External Functions
An external function is a function whose body is provided separately from its
declaration. An external function may be a top-level function (17), a method
The body of the function is defined somewhere else.
As far as I know this is used to fix different implementations for Dart VM in the browser and Dart VM on the Server.
When we make an external function inside a class like toString()
external String toString();
means this method is abstract and the child of the parent class will add the function body, that's because in Dart we only can make an abstract class.
Summary:
external function = abstract function in not abstract classes
I don't think external keyword is meant to be used to mark methods as abstract, even if that's possible
It's enough to leave a method with no implementation to set it abstract, inside an abstract class
It's the equivalent of declare in TypeScript, and extern in C#, those are used for interoperability with other runtimes, which means you're telling the compiler "Don't worry about this method's implementation, I promise it will exist at runtime", the runtime may be in C or Javascript or whatever
In case, if you are wondering why or where should I even use external keyword, here is a one more example for Flutter.
class MyStruct extends Struct {
#Int32()
external int a;
#Float()
external double b;
external Pointer<Void> c;
}
Sometimes, but not often when you play with native libraries, in this case with Struct to access native struct's field in memory. Under Struct We must declare all fields as external because it will be external fields from dart:ffi (C / C++).
So, external is more than just way to declare "abstract method".
9.4 External Functions
An external function is a function whose
body is provided separately from its
declaration.
What this does mean is that you define the function, but without implementation. It's exactly how you define the abstract method, but the only difference is that with external you don't implement the method in dart but in C or something else.
Something like String class it can be considered as external functions except that the String class it marked with #pragma('vm:entry-point') which make the entire class use native code.
See the following example to understand:
This dart's side.
https://github.com/dart-lang/sdk/blob/main/sdk/lib/core/string.dart#L711
This the implementation in C++.
https://github.com/dart-lang/sdk/blob/main/runtime/lib/string.cc#L467-#L472
In my opinion it is an equivalent of Java native keyword. For example, since current time milliseconds is implemented differently on Android, iOS, Linux etc, DateTime.now().millisecondsSinceEpoch will be linked to different implementations at runtime. So it is not initially known how this method will look like. For this reason it is marked as external meaning it is platform dependent.