Get pointer to a struct from a Dart_NativeArguments struct in C - dart

I'm trying to wrap a C library using Dart. I call into a C function from dart and pass in the arguments through a Dart_NativeArguments struct in C:
void _sayHello(Dart_NativeArguments arguments) {
string from;
Dart_Handle seed_object = HandleError(Dart_GetNativeArgument(arguments, 0));
if (Dart_IsString(seed_object)) {
const char* seed;
HandleError(Dart_StringToCString(seed_object, &seed));
from = seed;
}
num = (int)Dart_GetNativeArgument(arguments, 1);
Dart_SetReturnValue(arguments, HandleError(Dart_NewStringFromCString(sayHello(from, num).c_str())));
}
In Dart, I call the function and pass in the necessary arguments
String sayHello(String from) native "sayHello";
main() {
print(sayHello("Dart"));
}
I was wondering how I could pass in pointers (to a struct I made) instead of just strings and ints as arguments. There are functions in Dart to convert Dart_Handles into Strings and ints but not pointers. What is the internal structure of the Dart_Handle and how would I go about converting it back to a pointer? For example:
Dart code:
String sayHello(info from) native "sayHello";
class info
{
String message;
int num;
}
main() {
info tester = new info();
tester.message = "Dart";
tester.num = 2;
print(sayHello(tester));
}
C Code:
void sayHello(Dart_NativeArguments arguments) {
/*What do I do here to get back a pointe to the struct/class I passed
in as an argument in Dart?*/
}

Your Dart_NativeArguments will consist of just one item, which will be an instance - the instance of the class info that you created with new info(). You can test whether it's an instance with bool Dart_IsInstance(Dart_Handle object). So what you have is an handle to an instance of info. This allows you to access its instance fields (message and num) to get and set them, using Dart_GetField and Dart_SetField.
Dart_Handle instance = Dart_GetNativeArgument(arguments, 0);
Dart_Handle message_handle = Dart_GetField(retobj, NewString("message"));
char* message;
Dart_StringToCString(message_handle, &message);
Dart_Handle number_handle = Dart_GetField(retobj, NewString("num"));
int64_t number;
Dart_IntegerToInt64(number_handle, &number);
// message contains the string, number contains the number
// use them, copy them etc
I know this is just an example, but it might be easier to redefine sayHello to take 2 arguments (a string and an int) rather than passing an object instance. There isn't a way to access the fields of a class in one step, you need to access them individually. Consider these two versions of the Dart code, one passing an object instance and one just the values. The second version is simpler at the Dart and C side (no GetField steps). The first version is more powerful, though, because you could update the fields using SetField, which you couldn't in the second.
class Info {
String message;
int num;
Info(this.message, this.num);
}
version1() {
sayHelloV1(new Info('Dart', 2));
}
version2() {
sayHelloV2('Dart', 2);
}
If your C API requires you to pass in a struct you will have to create that in your C code by copying the values you extract using Dart_IntegerToInt64etc into it, then pass the pointer to your C struct to the API.
If your API is very precise about the packing/padding of the data into the struct, you could use Dart typed_data to pack the Dart types into a ByteData and pass the underlying byte array.

Related

Dart: how to run static function/code "automatically"?

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

How can I use `class` as Map's key?

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.

Is there a way to pass a primitive parameter by reference in Dart?

I would like to pass a primitive (int, bool, ...) by reference. I found a discussion about it (paragraph "Passing value types by reference") here: value types in Dart, but I still wonder if there is a way to do it in Dart (except using an object wrapper) ? Any development ?
The Dart language does not support this and I doubt it ever will, but the future will tell.
Primitives will be passed by value, and as already mentioned here, the only way to 'pass primitives by reference' is by wrapping them like:
class PrimitiveWrapper {
var value;
PrimitiveWrapper(this.value);
}
void alter(PrimitiveWrapper data) {
data.value++;
}
main() {
var data = new PrimitiveWrapper(5);
print(data.value); // 5
alter(data);
print(data.value); // 6
}
If you don't want to do that, then you need to find another way around your problem.
One case where I see people needing to pass by reference is that they have some sort of value they want to pass to functions in a class:
class Foo {
void doFoo() {
var i = 0;
...
doBar(i); // We want to alter i in doBar().
...
i++;
}
void doBar(i) {
i++;
}
}
In this case you could just make i a class member instead.
No, wrappers are the only way.
They are passed by reference. It just doesn't matter because the "primitive" types don't have methods to change their internal value.
Correct me if I'm wrong, but maybe you are misunderstanding what "passing by reference" means? I'm assuming you want to do something like param1 = 10 and want this value to still be 10 when you return from your method. But references aren't pointers. When you assign the parameter a new value (with = operator), this change won't be reflected in the calling method. This is still true with non-primitive types (classes).
Example:
class Test {
int val;
Test(this.val);
}
void main() {
Test t = new Test(1);
fn1(t);
print(t.val); // 2
fn2(t);
print(t.val); // still 2, because "t" has been assigned a new instance in fn2()
}
void fn1(Test t) {
print(t.val); // 1
t.val = 2;
}
void fn2(Test t) {
t = new Test(10);
print(t.val); // 10
}
EDIT
I tried to make my answer more clear, based on the comments, but somehow I can't seem to phrase it right without causing more confusion. Basically, when someone coming from Java says "parameters are passed by reference", they mean what a C/C++ developer would mean by saying "parameters are passed as pointers".
As dart is compiled into JavaScript, I tried something that works for JS, and guess what!? It worked for dart!
Basically, what you can do is put your value inside an object, and then any changes made on that field value inside that function will change the value outside that function as well.
Code (You can run this on dartpad.dev)
main() {
var a = {"b": false};
print("Before passing: " + a["b"].toString());
trial(a);
print("After passing: " + a["b"].toString());
}
trial(param) {
param["b"] = true;
}
Output
Before passing: false
After passing: true
One of the way to pass the variables by reference by using the values in List. As arrays or lists are Pass by reference by default.
void main() {
List<String> name=['ali' ,'fana'];
updatename(name);
print(name);
}
updatename(List<String> name){
name[0]='gufran';
}
Try this one, This one of the simplest way to pass by reference.
You can use ValueNotifier
And, you can pass it as ValueListenable to classes or methods that needs to know up-to-date value, but should not edit it:
class Owner {
final theValue = ValueNotifier(true);
final user = User(theValue);
...
}
class User {
final ValueListeneble<bool> theValue;
User(this.theValue);
...
}
It provides more functionality than actually needed, but solves the problem.
If ValueNotifier + ValueListenable do not work for you (you want to make sure the client does not listen to every change of the value, or your package is pure Dart package and thus cannot reference Flutter libraries), use a function:
class Owner {
int _value = 0;
int getValue() => _value;
void increase() => _value++;
}
void main() {
final owner = Owner();
int Function() obtainer = owner.getValue;
print(obtainer());
owner.increase();
print(obtainer());
}
Output will be:
0
1
This approach has memory usage related downside: the obtainer will hold the reference to the owner, and this, even if owner is already not referenced, but obtainer is still reachable, owner will be also reachable
and thus will not be garbage collected.
If you do not want the downside, pass the smaller container than the entire owner:
import 'package:flutter/foundation.dart';
class ListenableAsObtainer<T> implements ValueObtainer<T> {
ListenableAsObtainer(this._listenable);
final ValueListenable<T> _listenable;
#override
T get value => _listenable.value;
}
class FunctionAsObtainer<T> implements ValueObtainer<T> {
FunctionAsObtainer(this._function);
final T Function() _function;
#override
T get value => _function();
}
class ValueAsObtainer<T> implements ValueObtainer<T> {
ValueAsObtainer(this.value);
#override
T value;
}
/// Use this interface when the client needs
/// access to the current value, but does not need the value to be listenable,
/// i.e. [ValueListenable] would be too strong requirement.
abstract class ValueObtainer<T> {
T get value;
}
The usage of FunctionAsObtainer will still result in holding the owner from garbage collection, but two other options will not.
Just to make it clear:
void main() {
var list1 = [0,1,2];
var modifiedList1 = addMutable(list1, 3);
var list2 = [0,1,2];
var modifiedList2 = addImmutable(list2, 3);
print(list1);
print(modifiedList1);
print(list2);
print(modifiedList2);
}
List<int> addMutable(List<int> list, int element){
return list..add(element);
}
List<int> addImmutable(List<int> list, int element){
return [...list, element];
}
Output:
[0, 1, 2, 3]
[0, 1, 2, 3]
[0, 1, 2]
[0, 1, 2, 3]
All variables are passed by value. If a variable contains a primitive (int, bool, etc.), that's it. You got its value. You can do with it whatever you want, it won't affect the source value. If a variable contains an object, what it really contains is a reference to that object.
The reference itself is also passed by value, but the object it references is not passed at all. It just stayed where it was. This means that you can actually make changes to this very object.
Therefore, if you pass a List and if you .add() something to it, you have internally changed it, like it is passed by reference. But if you use the spread operator [...list], you are creating a fresh new copy of it. In most cases that is what you really want to do.
Sounds complicated. Isn't really. Dart is cool.

How to create private variables in Dart?

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.

How to pass objective-c function as a callback to C function?

I want to call a c function from objective-c and pass objective-c function as a callback
the problem is this function has a callback as parameter, so I have to pass objective-c function as a call back to c function
here is the header of the c function
struct mg_context *mg_start(const struct mg_callbacks *callbacks,
void *user_data,
const char **configuration_options);
here is where I try to call it
- (void)serverstarted
{
NSLog(#"server started");
}
- (IBAction)startserver:(id)sender {
NSLog(#"server should start");
const char *options[] =
{
"document_root", "www",
"listening_ports", "8080",
NULL
};
mg_start(serverstarted(), NULL, options);
}
I have tried several ways to do it and searched the web to just get a clue how to do it but with not luck
here is the library I am incuding in my code
https://github.com/valenok/mongoose
Your chief problem is the first parameter to mg_start(), which is described in the declaration as const struct mg_callbacks *callbacks. You are trying pass a pointer to a function. (Actually you are trying to pass the result of a call to that function, which is even further from the mark.) That isn't what it says: it says a pointer to a struct (in particular, an mg_callbacks struct).
The example code at https://github.com/valenok/mongoose/blob/master/examples/hello.c shows you how to configure this struct. You have to create the struct and put the pointer to the callback function inside it. Then you pass the address of that struct.
Other problems with your code: your callback function itself is all wrong:
- (void)serverstarted
{
NSLog(#"server started");
}
What's wanted here is a C function declared like this: int begin_request_handler(struct mg_connection *conn), that is, it takes as parameter a pointer to an mg_connection struct. Your serverstarted not only doesn't take that parameter, it isn't even a C function! It's an Objective-C method, a totally different animal. Your use of the term "Objective-C function" in your title and your question is misleading; C has functions, Objective-C has methods. No Objective-C is going to be used in the code you'll be writing here.
What I suggest you do here is to copy the hello.c example slavishly at first. Then modify the content / names of things slowly and bit by bit to evolve it to your own code. Of course learning C would also help, but you can probably get by just by copying carefully.
As matt already said, you cannot pass an Objective-C method as callback where a C function
is expected. Objective-C methods are special functions, in particular the receiver ("self")
is implicitly passed as first argument to the function.
Therefore, to use an Objective-C method as request handler, you need an (intermediate) C function as handler and you have to pass self to that function, using the user_data argument. The C function can then call the Objective-C method:
// This is the Objective-C request handler method:
- (int)beginRequest:(struct mg_connection *)conn
{
// Your request handler ...
return 1;
}
// This is the intermediate C function:
static int begin_request_handler(struct mg_connection *conn) {
const struct mg_request_info *request_info = mg_get_request_info(conn);
// Cast the "user_data" back to an instance pointer of your class:
YourClass *mySelf = (__bridge YourClass *)request_info->user_data;
// Call instance method:
return [mySelf beginRequest:conn];
}
- (IBAction)startserver:(id)sender
{
struct mg_callbacks callbacks;
memset(&callbacks, 0, sizeof(callbacks));
callbacks.begin_request = begin_request_handler;
const char *options[] =
{
"document_root", "www",
"listening_ports", "8080",
NULL
};
// Pass "self" as "user_data" argument:
mg_start(&callbacks, (__bridge void *)self, options);
}
Remarks:
If you don't use ARC (automatic reference counting) then you can omit the (__bridge ...)
casts.
You must ensure that the instance of your class ("self")
is not deallocated while the server is running. Otherwise the YourClass *mySelf
would be invalid when the request handler is called.

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