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.
Related
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.
If you can figure out how to rename this question, I'm open for suggestions.
In the Dart language, one can write a class with final fields. These are fields that can only be set before the constructor body runs. That can be on declaration (usually for static constants inside a class), in an initialiser list syntax when declaring the constructor or using the this.field shorthand:
class NumBox{
final num value;
NumBox(this.value);
}
Let's say I actually needed to do some processing on instance creation and can't just initialise the field before the constructor. I can switch to using a private non-final field with a getter:
class NumBox{
num _value;
NumBox(num v) {
_value = someComplexOperation(v);
}
num get value => _value;
}
Or I can get a similar behavior using a factory constructor:
class NumBox{
final num value;
factory NumBox(num v) {
return new NumBox._internal(someComplexOperation(v));
};
NumBox._internal(this.value);
}
I hit a similar bump when I tried learning Dart a few years back and now that I have more baggage, I still don't know. What's the smarter way to do this?
A factory constructor is a good way, it allows to pre-calculate without limitations any values that you then pass to a normal constructor to forward to final fields.
An alternative way is initializer list which is executed before the constructor body and therefore allows to initializer final fields:
class NumBox{
final num value;
NumBox(num v) : value = someComplexOperation(v)
}
In the initializer list you are not allowed to read this because the instance isn't fully initialized yet.
DartPad example
You should design your API with your user in mind, then implement it in whatever way is simpler and more maintainable to you. This question is about the latter part.
Making fields final is great when it's possible, and when it isn't, making them private with a public getter is a good alternative. It's your choice what to do, because it's you who is going to maintain your class, nobody else should need to look behind the public API.
If you need a complex computation, Günther Zöchbauer's suggestion is the first to turn to: Use a helper function. In some cases, you can even do it inline
class MyThing {
final x;
MyThing(args) : x = (() { complex code on args; return result;} ());
}
It gets ugly quickly, though, so having it as a static helper function is usually better.
If your complex computation doesn't match this, which ususally means that there is more than one field being initialized with related values, then you need a place to store an intermediate value and use it more than once when initializing the object.
A factory constructor is the easy approach, you can compute everything you need and then call the private generative constructore at the end. The only "problem" is that by not exposing a generative constructor, you prevent other people from extending your class. I quoted "problem" because that's not necessarily a bad thing - allowing people to extend the class is a contract which puts restrictions on what you can do with the class.
I tend to favor public factory constructors with private generative constructors even when it's not needed for any practical reason, just to disable class extension.
class MyClass {
const factory MyClass(args) = MyClass._; // Can be const, if you want it.
const MyClass._(args) : ... init .. args ...;
}
If you need a generative constructor, you can use a forwarding generative constructor to introduce an intermediate variable with the computed value:
class MyClass {
final This x;
final That y;
MyClass(args) : this._(args, _complexComputation(args));
MyClass._(args, extra) : x = extra.theThis, y = extra.theThat, ...;
}
All in all, there is no strict rule. If you prefer final fields, you can do extra work to make them final, or you can just hide the mutability behind a getter - it's an implementation and maintainability choice, and you're the one maintaining the code.
As long as you keep the abstraction clean, and keeps track of what you have promised users (generative constructor? const constructor?) so you won't break that, you can change the implementation at any time.
I have an F# class that uses the following to declare, but not initialize class members:
[<DefaultValue>] val mutable myVariable : myType
How can I check in the code whether this value has been initialized? I tried:
x.myVariable = null
but that doesn't seem to work.
From your description, it is a bit hard to say what you are actually trying to achieve - using both uninitialized values and inheritance is generally not the preferred way of doing things in F# (but they are both sometimes necessary for interoperability with .NET), so if you follow this direction, you might not be getting that many advantages from using F#.
Wouldn't the following work for you instead? The idea is that we define a base class that takes the value of the private thing through a constructor:
type Base(myThing : Random) =
member x.MyThing = myThing
And an inherited class can then provide a value, but also access it using a member:
type MySubclass() =
inherit Base(new Random(0))
member x.Next() =
x.MyThing.Next()
I am new to Dart language. So I would like to know more about some conventions that programmers follow while developing in this language.
Should I always encapsulate my class members as I do, for example in Java? Whenever I create property of class, should I make it private and provide getters/setters? Or there are situations when I should leave them public? If so, what are examples of these situations?
In my opinion, type annotations such as String, int, etc. increase readability of code. They serve as a documentation for other developers who are reading/using my code. Programmer should not think value of what type is storing in this variable right now. So again, what are the situations, that require using var keyword when declaring a variable?
Dmitry.
Thank you.
Thanks for checking out Dart!
No need to encapsulate class fields. Dart creates implicit getters and setters for you. If you need to actually compute something for that field, you can then implement the getter or setter manually. Bonus: this doesn't break consumers of your API.
Example:
class Person {
int age;
}
Later, you want to calculate age:
class Person {
DateTime birthdate;
int get age => new DateTime.now().difference(birthdate).inDays ~/ 365;
}
In both cases, you can do this:
print(person.age);
Pretty cool! No change in API, and no defensive getters and setters (just add them when you need them).
You should use type annotations for the "surface area" of your code. For example, use type annotations for method and function signatures. For cases where the variable's type is very obvious, you should consider using var, because it's more terse and readable.
For example:
String doCoolStuff(int bar) {
var clearlyABool = true;
return 'Hello world';
}
Note that the return type and bar parameter are type annotated, but the clearlyABool uses var because we initialize with a bool.
Feel free to use type annotations everywhere, it's programmer choice. Anecdote: the dart2js source code uses type annotations pretty much everywhere.
I would like to cast instances of my custom class A to int. What is the syntax of the implicit cast operator? (I thought I remembered that there is such a feature but I can't find it on the web)
int a = (new A());
You can also use as to help tell the tools "no, really, treat this object as this type".
A good example of this is when you have to deal with dart:html's querySelector() function.
FormElement form = querySelector('#sign-up') as FormElement;
In the above, the object returned by querySelector('#sign-up') is checked that it is really an instance of FormElement.
Learn more at https://www.dartlang.org/docs/dart-up-and-running/ch02.html#operators
Type annotations are not allowed to affect behavior in Dart. If you're not running in checked mode, then this:
int a = new A();
will work the same as this:
var a = new A();
at run-time. In other words, when not in checked mode, you're welcome to store your A in a variable annotated as an int, but no actual conversion takes place.
If you are running in checked mode, the first form will give you a runtime exception.
I'm not sure, but I think what you're asking for is a way to define a conversion between your class A and int that will happen automatically when "cast" to an int. No such thing exists, to my knowledge. You should simply define a method to do so. For example:
int a = new A().to_i();