I'm coming from Delphi and the paradigm may be different but I want to do define a custom event handler in Dart, something like the pseudo code below.
// -- I don't know how to define this type...
typedeft TCustomEvent = function (int pParam) of object;
class MyFirstClass {
TCustomEvent onDoSomething;
void doIt(int pValue){
onDoSomething(pValue);
}
}
class MySecondClass {
void makeMyFirstClassDoIt(){
MyFirstClass vObj = MyFirstClass();
vObj.onDoSomething = doSomething;
vObj.doIt(5);
}
doSomething(int pValue) {
print(pValue);
}
}
Any help would be greatly appreciated.
Related
is it possible to call generic types function.
if not is there a different approach to something like this.
someFunction<T>(){
T.anotherFunction();
}
EDIT
MyModel model = NetworkClient.sendRequest<MyModel>(url);
static Future<T> sendRequest<T>(String URL){
//send request
var res = data.toString();
return T.fromJson(json.decode(res))
}
void main() {
someFunction(Foo());
someFunction(Bar());
}
someFunction<T>(T t) {
if (t is Foo)
t.fooFunc();
else if (t is Bar)
t.barFunc();
else
throw Exception("Unknown type: ${t.runtimeType}");
}
class Foo {
void fooFunc() {
print("foo");
}
}
class Bar {
void barFunc() {
print("bar");
}
}
Put your method (lets say MyMethod) in an interface or base class. Lets say MyInterface.
Then use a constraint on your generic type:
someFunction<T extends MyInterface>(T t){
t.MyMethod();
}
Since you cannot have constructors in an interface, your plan with the fromJson might not work out. I don't know your class structure. But you can write generics with specific constraints in mind.
I want to recall a value of an edit method declared inside a form from a class main. How can I do it?
[Form]
public class AdvancedCustomerSchedule extends FormRun
{
Sorting sorting;
edit Sorting edtSorting(boolean set, Sorting _sorting)
{
if (set)
{
sorting = _sorting;
}
return sorting;
}
}
and the class:
class AdvancedCustomerScheduleService
{
static void main(Args args)
{
//I want to call the method edtSorting here.
}
}
UPDATE
FormRun callerForm;
if (args.caller() is FormRun)
{
callerForm = args.caller() as FormRun;
if (formHasMethod(callerForm, identifierStr(edtSorting)))
{
str test = callerForm.edtSorting();
info(test);
}
}
For calling methods defined on forms in general you usually use a pattern like the following:
...
FormRun callerForm;
...
if (_args.caller() is FormRun)
{
callerForm = _args.caller();
if (formHasMethod(callerForm, identifierStr(someMethod)))
{
callerForm.someMethod();
}
...
Have a look at class DirPartyContactInfoFormHandler and its static main method for an example.
Is there way to overriding method in Dart like JAVA, for example:
public class A {
public void handleLoad() {
}
}
And when overriding:
A a = new A() {
#Override
public void handleLoad() {
// do some code
}
};
No, Dart does not have anonymous classes. You have to create a class that extends A and instantiate it.
No but it much less useful in Dart because you can just reassign function:
typedef void PrintMsg(msg);
class Printer {
PrintMsg foo = (m) => print(m);
}
main() {
Printer p = new Printer()
..foo('Hello') // Hello
..foo = ((String msg) => print(msg.toUpperCase()))
..foo('Hello'); //HELLO
}
However you will need some extra boilerplate to access instance.
Use type Function:
class A {
final Function h
A(this.h);
void handleLoad(String loadResult) { h(loadResult); }
}
Or
class A {
final Function handleLoad;
A(this.handleLoad);
}
A a = new A((String loadResult){
//do smth.
});
I was wondering if is possible to create an instance of a generic type in Dart. In other languages like Java you could work around this using reflection, but I'm not sure if this is possible in Dart.
I have this class:
class GenericController <T extends RequestHandler> {
void processRequest() {
T t = new T(); // ERROR
}
}
I tried mezonis approach with the Activator and it works. But it is an expensive approach as it uses mirrors, which requires you to use "mirrorsUsed" if you don't want to have a 2-4MB js file.
This morning I had the idea to use a generic typedef as generator and thus get rid of reflection:
You define a method type like this: (Add params if necessary)
typedef S ItemCreator<S>();
or even better:
typedef ItemCreator<S> = S Function();
Then in the class that needs to create the new instances:
class PagedListData<T>{
...
ItemCreator<T> creator;
PagedListData(ItemCreator<T> this.creator) {
}
void performMagic() {
T item = creator();
...
}
}
Then you can instantiate the PagedList like this:
PagedListData<UserListItem> users
= new PagedListData<UserListItem>(()=> new UserListItem());
You don't lose the advantage of using generic because at declaration time you need to provide the target class anyway, so defining the creator method doesn't hurt.
You can use similar code:
import "dart:mirrors";
void main() {
var controller = new GenericController<Foo>();
controller.processRequest();
}
class GenericController<T extends RequestHandler> {
void processRequest() {
//T t = new T();
T t = Activator.createInstance(T);
t.tellAboutHimself();
}
}
class Foo extends RequestHandler {
void tellAboutHimself() {
print("Hello, I am 'Foo'");
}
}
abstract class RequestHandler {
void tellAboutHimself();
}
class Activator {
static createInstance(Type type, [Symbol constructor, List
arguments, Map<Symbol, dynamic> namedArguments]) {
if (type == null) {
throw new ArgumentError("type: $type");
}
if (constructor == null) {
constructor = const Symbol("");
}
if (arguments == null) {
arguments = const [];
}
var typeMirror = reflectType(type);
if (typeMirror is ClassMirror) {
return typeMirror.newInstance(constructor, arguments,
namedArguments).reflectee;
} else {
throw new ArgumentError("Cannot create the instance of the type '$type'.");
}
}
}
I don't know if this is still useful to anyone. But I have found an easy workaround. In the function you want to initialize the type T, pass an extra argument of type T Function(). This function should return an instance of T. Now whenever you want to create object of T, call the function.
class foo<T> {
void foo(T Function() creator) {
final t = creator();
// use t
}
}
P.S. inspired by Patrick's answer
2022 answer
Just came across this problem and found out that although instantiating using T() is still not possible, you can get the constructor of an object easier with SomeClass.new in dart>=2.15.
So what you could do is:
class MyClass<T> {
final T Function() creator;
MyClass(this.creator);
T getGenericInstance() {
return creator();
}
}
and when using it:
final myClass = MyClass<SomeOtherClass>(SomeOtherClass.new)
Nothing different but looks cleaner imo.
Here's my work around for this sad limitation
class RequestHandler {
static final _constructors = {
RequestHandler: () => RequestHandler(),
RequestHandler2: () => RequestHandler2(),
};
static RequestHandler create(Type type) {
return _constructors[type]();
}
}
class RequestHandler2 extends RequestHandler {}
class GenericController<T extends RequestHandler> {
void processRequest() {
//T t = new T(); // ERROR
T t = RequestHandler.create(T);
}
}
test() {
final controller = GenericController<RequestHandler2>();
controller.processRequest();
}
Sorry but as far as I know, a type parameter cannot be used to name a constructor in an instance creation expression in Dart.
Working with FLutter
typedef S ItemCreator<S>();
mixin SharedExtension<T> {
T getSPData(ItemCreator<T> creator) async {
return creator();
}
}
Abc a = sharedObj.getSPData(()=> Abc());
P.S. inspired by Patrick
simple like that.
import 'dart:mirrors';
void main(List<String> args) {
final a = A<B>();
final b1 = a.getInstance();
final b2 = a.getInstance();
print('${b1.value}|${b1.text}|${b1.hashCode}');
print('${b2.value}|${b2.text}|${b2.hashCode}');
}
class A<T extends B> {
static int count = 0;
T getInstance() {
return reflectClass(T).newInstance(
Symbol(''),
['Text ${++count}'],
{Symbol('value'): count},
).reflectee;
}
}
class B {
final int value;
final String text;
B(this.text, {required this.value});
}
Inspired by Patrick's answer, this is the factory I ended up with.
class ServiceFactory<T> {
static final Map<Type, dynamic> _cache = <String, dynamic>{};
static T getInstance<T>(T Function() creator) {
String typeName = T.toString();
return _cache.putIfAbsent(typeName, () => creator());
}
}
Then I would use it like this.
final authClient = ServiceFactory.getInstance<AuthenticationClient>(() => AuthenticationClient());
Warning: Erik made a very good point in the comment below that the same type name can exist in multiple packages and that will cause issues. As much as I dislike to force the user to pass in a string key (that way it's the consumer's responsibility to ensuring the uniqueness of the type name), that might be the only way.
I'd like to port this javascript code to dart.
function Beagle() {
this.argv_ = null;
this.io = null;
};
Beagle.prototype.run = function() {
this.io = this.argv_.io.push();
};
runCommandClass(Beagle);
the probleme is
How to create object Beagle
How to create prototype object Beagle.prototype.run
This kind of Js code (function definition and prototype changes) can be ported to a Dart class. You can follow these main rules :
function Xxxx(){/* js code to init */} (pseudo Js class) translates to :
class Xxxx {
/// constructor
Xxxx() {
/* Dart code to init */
}
}
when you have contructor parameters like in function Xxxx(param1, param2){/* js code to init */} you have to create an other constructor with those parameters :
class Xxxx {
/// constructor with parameters
Xxxx(param1, param2) {
/* Dart code to init with param1, param2 */
}
}
Xxxx.prototype.method1 = function(p1, p2, p3){/* js code for method */} are like methods that have to be translated to :
class Xxxx {
// .... other code
/// method
method1(p1, p2, p3) {
/* Dart code */
}
}
To make your code more clear you can also add type annotations on methods and constructors. This is recommanded by the Dart Style Guide.
Type annotations are important documentation for how a library should be used. Annotating the parameter and return types of public methods and functions helps users understand what the API expects and what it provides.
For instance :
class Xxxx {
/// constructor
Xxxx(String param1, int param2) {
/* Dart code to init with param1, param2 */
}
/// method
void method1(num p1, String p2, DateTime p3) {
/* Dart code */
}
}
class Beagle { //
Map argv_;
int io;
Map portInfo;
// could make sense to make this a constructor, that depends how the Terminal class uses it (didn't look)
void run(this.argv_) {
this.portInfo_ = JSON.parse(this.argv_['argString']); // not tested
io = argv_['io'].length;
}
void sendString_(String s) { // no idea what the underlines at the end of argv_, sendString_, ... are for
// ...
}
void onRead_(String s) {}
void onTerminalResize_(int width, int height) {}
void exit(code) {
// ...
}
void close() {
// ...
}
}
var b = new Beagle(); // not translated from the JS source - just added to show how to create a new object from the Beagle class
b.run(argvFromSomewhere);
This includes a some guessing about what the intention of the JavaScript code might be.
I prefer using specific types when porting from JavaScript. It helped me a lot finding bugs and understanding the intention. When I guessed the wrong type I get an exception at runtime, then I can reason about why I got an unexpected type and which of my assumptions were wrong.