I have a situation where I defined a couple of module-level instance variables using the 'private' scope identifier. I require to do this because these variables will be used in several functions within the module. Also, some of these variables are 'lists' or 'sets'. I realized that values of these variables persist between repeated calls to a certain function within the module. This is as expected.
I am also creating a test where I call one of the functions repeatedly. I would prefer to have a fresh copy of the instance variables (just like with instance members in Java). I can't seem to do so. If I try to nullify the content of the list/set, I get into trouble as follows:
module foo::bar
private set[DataType_1] data1;
public void nullifyInstanceVars( )
{
//tried
data1={}
}
//method that gets called repeatedly:
public void repeatCallMe(..)
{
nullifyInstanceVars( );
...
..
//Throws an error like: trying to add an element of type 1 to set[void]
data1 += anElementOfType1
}
So, I modified the nullifyInstanceVars( ) method to have set[DataType1] data1={ }. It doesn't work because I believe that simply creates a new variable scoped only within the function and really doesn't clear the element!
Any help is appreciated...
This really looks like a bug in the Rascal interpreter. I will file a bug report for it.
The work around is to initialize data1 in the declaration as well:
private set[int] data1 = {};
Can you confirm that this solves your problem?
Related
Why does this code:
class _SequentialTextPageState {
String jsonTextPref = 'seqtext';
int jsonTextSuff = 10;
String jsonText = jsonTextPref + jsonTextSuff.toString();
}
generate these errors?
Error: The instance member 'jsonTextPref' can't be accessed in an initializer.
Error: The instance member 'jsonTextSuff' can't be accessed in an initializer.
It seems to me that concatenation between String and int is correct?
Dart initializes objects in multiple phases. Initializing members directly ("field initializers") occurs early in object initialization, before this becomes valid, so that phase cannot initialize members that depend on other parts of the object.
Dart provides multiple ways to initialize members, so if one member needs to depend on another, you can initialize it in a later phase by using a different mechanism. For example, you could do one of:
Add the late keyword to make the dependent member lazily initialized.
Move initialization of the dependent member into the constructor body.
In the case of a Flutter State subtype, you could initialize the dependent member in its initState method, which in some cases is more appropriate.
Note that in some cases you additionally can consider replacing the member variable with a read-only getter instead. For example, in your case, perhaps you could use:
String get jsonText => jsonTextPref + jsonTextSuff.toString();
That would be appropriate if jsonText should always depend on jsonTextPref and jsonTextSuff, would never need to have an independent value, and if it's acceptable for jsonText to return a new object every time it's accessed.
Dart does not allow field initializers to refer to the object itself. Fields must always be fully initialized before any access is given to the object begin created.
The initializers can only access static and top-level variables, not any instance variables on the object itself.
With null safety, you will be allowed to write late String jsonText = this.something + this.other;. That field will then not be initialized until it's first read or written, which is necessarily after the object itself has been created.
You can only use constant expressions as initializers. x=this.y is not constant.
The error was displayed when I did following:
class MyClass {
String id;
String[] imagePaths;
}
It will mark the String in the line String id; as error, but the error is in the next line, it should be List<String> imagePaths; instead of String[] imagePaths; then the error in the line above also disappears. This can be very confusing if you have a big class and the actual error is many lines underneath the first marked line (talking from experience...)
I am trying out the new Dart FFI in making wrappers for libsodium. Libsodium needs to be initialized with calling init(). But I don't think the user should be burdened to remember this and I also wouldn't like to check some global state variable.
I know that Go has package init() functions which run when a package gets included. Is there something similar in Dart?
Of course I could just wrap everything up into a class and run init() in the constructor but there isn't much sense in instatiating a class which basically only exposes static methods. Besides, I would like to preserve the procedural style of libsodium.
Of course I could just wrap everything up into a class and run init() in the constructor but there isn't much sense in instatiating a class which basically only exposes static methods. Besides, I would like to preserve the procedural style of libsodium.
You could have a singleton instance and expose library functions as methods on the instance, and you could provide a public getter function that automatically does initialization.
For example, something like:
Libsodium? _instance;
Libsodium get libsodium => _instance ??= Libsodium._();
class Libsodium {
Libsodium._() {
// Do initialization.
}
void foo() {
// ...
}
void bar() {
// ...
}
}
and then callers would need to use it via:
import 'libsodium.dart';
libsodium.foo();
This would hide the class instantiation and wouldn't look any different to callers than if you were to use only top-level functions with a namespace (import 'libsodium.dart' as libsodium).
Dart does not have any way to implicitly run code. No code runs before main, and all code running after main does so because it was invoked directly or indirectly from the main method. So, no.
If you need something initialized, there is a number of options.
You can use a lazily initialized static variable:
var _initialState = _init();
int doSomething(arguments) {
_initialState;
do the something.
}
The reading of _initialState will ensure that init is invoked the first time
it's needed, and not after that. You can even store things in the state for
later use.
The singleton implementation object suggested by #jamesdlin. It basically does the
same thing, then puts the methods on the object instead of being static.
Another variant is to do exactly that, but with a private instance, and have the
public top-level functions forward to the singleton object. Then you get the
procedural API and still ensures that the state object is initialized.
(This may be better for testing, allowing you to have multiple state objects alive
at the same time).
Require people to call init.
I have two functions which return a value.
protected function createbox():player
{
var face:player = _world.CreateBody(bodydef);
return face;
}
They are identical except that they return a different variable. One returns face, the other returns eyes.
They are executed like this:
_elements.push(new Item(createbox(), new BoxSprite()));
The elements are created. But the problem comes when I try to referece face or eyes. I don't know how to reference them. They don't appear in the debug list of variables. They are null value (outside their functions).
Item is a class file. Which is also a function
public function Item(body:player,sprite:Graphic)
_elements is a vector array.
player is a class
I don't know the syntax to access face and eyes. They always return NULL or property not found no matter where you access them.
Try creating a Global variable. A global variable is available both inside and outside the function definition. So try assigning the variable out of the function and updating its value inside the function.
See example:
var face:String = "This is created out of the function";
function scopeTest(){
face = "This was updated inside the function";
}
scopeTest();
trace(face);
According to your post, faces and eyes are return objects from createbox(), which then are involved in the instantiation process of an Item object, which then is pushed into the _elements vector. Therefore, in order to access faces and eyes, you have to pass these objects to some public references during the constructor of the Item class. After that, you should be able to access them by enumerating wanted Item objects in the vector and using dot syntax on the Item objects.
I'm new to Dart, so maybe I'm missing something here:
This works:
In my main(), I have this:
var a = _someFunction;
var b = _someFunction;
print("${a == b}"); // true. correct!
Where _someFunction is another top-level function.
This does NOT work: (at least not how I'm expecting it to)
Given this class...
class Dummy {
void start() {
var a = _onEvent;
var b = _onEvent;
print(a == b); // false. ???????
}
void _onEvent() {
}
}
Instantiating it from main() and calling its start() method results in false. Apparently a new instance of some function or closure object is created and returned whenever my code obtains a reference to _onEvent.
Is this intentional behaviour?
I would expect that obtaining multiple references to the same method of the same instance returns the same object each time. Perhaps this is intended for some reason. If so; what reason? Or is this a bug/oversight/limitation of VM perhaps?
Thanks for any insights!
Currently, the behaviour seems to be intentional, but the following defect is open since May 2012: https://code.google.com/p/dart/issues/detail?id=144
If I were to guess, I'd say that setting "var a = _onEvent;" creates a bound method, which is some sort of object that contains both the function as well as this. You are asking for bound methods to be canonicalized. However, that would require the team to create a map of them, which could lead to worries about memory leaks.
I think they made "var a = _someFunction;" work early on because they needed static functions to be constants so that they could be assigned to consts. This was so that they could write things like:
const logger = someStaticLoggingFunction;
This was in the days before statics were lazily evaluated.
In any case, I would say that comparing closures for equality is a edge case for most languages. Take all of the above with a grain of salt. It's just my best guess based on my knowledge of the system. As far as I can tell, the language spec doesn't say anything about this.
Actually, now that I've read (https://code.google.com/p/dart/issues/detail?id=144), the discussion is actually pretty good. What I wrote above roughly matches it.
I was wondering if anyone had an idea for the best way to provide the
functionality of bindData() outside of my grails controllers. In my current
project I have created several groovy classes to model objects returned by
an api. In these classes I have a static method that parses xml and returns
a List of objects of the class. I would like to skip all the type casting
nonsense by using the bindData method in these classes. Any suggestions on
how to do this would be appreciated.
I was looking for a similar solution, to use bindData in a service class. I found a solution in JT's blog. The solution is basically to import:
import org.codehaus.groovy.grails.web.metaclass.BindDynamicMethod
then add this to your code:
def foo = new Foo()
BindDynamicMethod bind = new BindDynamicMethod()
def args = [ foo, params, [exclude:['name', 'mail']] ] // for example
bind.invoke( foo, 'bind', (Object[])args)
The (Object[]) cast is necessary du to Groovy/Java compatability. (Groovy is treating the ‘args’ object as an ArrayList, not an array of Objects.)