I've created a small interface:
import ...
abstract class IController {
void navigateTo(BuildContext context, String routeName);
Future<LocationData> get location;
// registration process
void registerGender(Gender gender);
void registerBirthday(DateTime birthday);
Future<bool> register(String email, String password);
}
And then I tried to implement this:
import ...
class Controller implements IController {
static final Controller _instance = Controller._internal();
final ServiceAuthenticate _serviceAuth = ServiceAuthenticate();
final ServiceDatabase _serviceDb = ServiceDatabase();
final ServiceGPS _serviceGPS = ServiceGPS();
User _user;
String _routeName;
UserData _userData;
Controller._internal() {
this._routeName = ROUTE_WELCOME;
}
factory Controller() => _instance;
void navigateTo(BuildContext context, String routeName) {
this._routeName = routeName;
Navigator.pushReplacementNamed(context, routeName);
}
Future<LocationData> get location async{
this._userData.location = await this._serviceGPS.location;
print(this._userData.location);
return this._userData.location;
}
void registerGender(Gender gender){
this._userData = UserData();
this._userData.gender = gender;
}
void registerBirthday(DateTime birthday) {
this._userData.birthday = birthday;
}
Future<bool> register(String email, String password) async {
User user = await this._serviceAuth.registerWithEmailAndPassword(email, password);
if(user == null){
return false;
}
this._user = user;
return true;
}
}
But that code produces the following error:
error: 'Controller.navigateTo' ('void Function(BuildContext, String)') isn't a valid override of 'IController.navigateTo' ('void Function(dynamic, String)'). (invalid_override at [prototype] lib\controller\controller.dart:30)
It looks like Dart thinks, that the BuildContext in the IController is dynamic, but this is obviously not the case.
How can I fix this? I'm new to Dart and don't know what to do.
Thanks for help :)
I'm stupid.
My import statement was wrong.
The line
import 'package:prototype/Controller/IController.dart';
produced this error, because the folder controller starts with a lowercase Letter.
The correct import statement is
import 'package:prototype/controller/IController.dart';
But regardless of my stupid mistake is the error message quite interesting.
A had a similar error and in my case the problem was that the return type of the buggy function was such that there were two different classes in the codebase with the same name. And the interface was using one and the implementation the other.
The one line answer is :
Your import statement is wrong.
But now , you need to take care in which file the import statement is going wrong.
There can be many scenarios, but I would like to give an example where I was stuck.
I had two different files in different package, but both files were importing some method where the method names were same.
So while importing the file which contain this method, I had imported the same name method from one file, and at other place, the same name method from second file.
So that's where everything went wrong!
So if import file is correct in the file which is giving some error, check the other dependent file, where same method import statement is written, that may be wrong.
Related
I want to invoke functions of a class by their names inside a string. I know my best option are Mirrors.
var ref = reflect(new TestClass());
ref.invoke(Symbol("test"), []);
It works fine, I can call the function test by a string. But I also want to put "TestClass" inside a string. Is it possible somehow ?
var ref = reflect("TestClass");
ref.invoke(Symbol("test"), []);
Jonas
You can do something like this:
import 'dart:mirrors';
class MyClass {
static void myMethod() {
print('Hello World');
}
}
void main() {
callStaticMethodOnClass('MyClass', 'myMethod'); // Hello World
}
void callStaticMethodOnClass(String className, String methodName) {
final classSymbol = Symbol(className);
final methodSymbol = Symbol(methodName);
(currentMirrorSystem().isolate.rootLibrary.declarations[classSymbol]
as ClassMirror)
.invoke(methodSymbol, <dynamic>[]);
}
Note, that this implementation does require that myMethod is static since we are never creating any object but only operate directly on the class itself. You can create new objects from the class by calling newInstance on the ClassMirror but you will then need to call the constructor.
But I hope this is enough. If not, please ask and I can try add some more examples.
How can I abstract that a methods has optional parameters?
abstract class CopyWith<T>{
T copyWith({}); // Error : Expected an identifier.
}
If I add an identifier like {test} it works and subclasses can have additional arguments
What I want to achieve?
I have a complex state manager, I make some abstraction , the following code is a minimal code, show my problem
import 'dart:collection';
abstract class CopyWith<T> {
T copyWith(OPTIONAL_NAMED_ARGUMENTS);
}
abstract class Manager<K, V extends CopyWith> {
final _map = HashMap<K, V>();
add(K key,V value){
_map[key] = value;
}
void copyWith(K key,OPTIONAL_NAMED_ARGUMENTS) {
assert(key != null);
if (_map.containsKey(key)) {
_map[key].copyWith(OPTIONAL_NAMED_ARGUMENTS);
}
}
}
class User implements CopyWith {
final int id;
final String name;
User({this.id, this.name});
User copyWith({int id, String name}) {
return User(
id: id ?? this.id,
name: name ?? this.name,
);
}
}
class UserManager extends Manager<int, User> {}
void main() {
final userManager = UserManager();
userManager.add(1,User(1,'test'));
userManager.copyWith(1,{test:'test2'})
}
As some one who has faced this issue in my library, I would say the only way is to not put a copyWith in your base class.
Why? Because you should only make a function polymorphic when there IS actually a shared calling convention and behavior. In your example, The way that these two classes perform copyWith is just different. It is, and should be, an error to send a name to Manager.copyWith, because Manager does not have a name to begin with. If you encounter a name inside a Manager.copyWith, that means there is some serious error in your code.
Also, if you actually try to invoke copyWith, as a responsible programmer, you will probably check if you are allowed to pass a name, which is,
if (someObj is User) {
someObj.copyWith(key, name: name);
} else if (someObj is Manager) {
throw IllegalStateError('You should not pass a name to a Manager! What am I supposed to do with the name now?');
}
There, you have already done type checking, so no need to make copyWith polymorphic.
However, some common behaviors can be made polymorphic, like updateKey. You can make Keyable as an interface, and Keyable updateKey(Key key) as an abstract method, and delegate to a non-polymorphic copyWith inside each subclasses.
There is nameof operator in C#, it allows to get property name at compile time:
var name = nameof(User.email);
Console.WriteLine(name);
//Prints: email
It is not possible to use reflection in flutter and I do not want to hardcode names of properties i.e. to be used for querying SQLite tables. Is there any workaround?
***Currently I'm using built_value library.
For the archives, I guess, this isn't possible as Dart doesn't store the names of variables after compiling, and as you mentioned, Flutter doesn't support reflection.
But you can still hardcode responsibly by grouping your properties as part of the object that they belong to, like with JSON:
class User {
final String email;
final String name;
const User({required this.email, required this.name});
Map toJson() => {
"email": email,
"name": name,
};
}
Instead of remembering to type out "email" and "name" whenever you use User, just call User.toJson(). Then, when you want to rename your variables, you can use your IDE's "rename all", or just skim over your User class to quickly change all of the names without missing any.
I'm currently monitoring the progress on the dart:mirrors package, which offers some neat reflective properties and methods, though, I hadn't found a simple way to just get the name of a symbol like nameof() does.
Example:
import 'dart:mirrors';
class User {
final String email;
final String name;
const User({required this.email, required this.name});
}
void main() {
reflectClass(User).declarations.forEach((key, value) {
print(value.simpleName);
});
}
Output:
Symbol("email")
Symbol("name")
Symbol("User")
These are of type Symbol.
More here: https://api.dart.dev/stable/2.4.0/dart-mirrors/dart-mirrors-library.html
So, until they develop a nameof, I've created an extension on symbol:
extension SymbolExtensions on Symbol {
String get nameof =>
RegExp(r'"(.*?)"').firstMatch(toString())!.group(1).toString();
}
So, you could do:
print(reflectClass(User)
.declarations[#email)]!
.simpleName
.nameof);
Output:
email
It's a start. Dart is still growing.
You can use code generation.
The basic idea is to create a nameof annotation class and mark parts of your code with it. You also need to create a code generator that handles your annotated code. Look at the json_serializable package for an example and create your own code generator.
If you do not want to create your own generator, use a ready-made package nameof: https://pub.dev/packages/nameof
Short how-to with this package.
Mark your class with nameof annotation.
#nameof
class Car {
final double price;
final double weigth;
final int year;
final String model;
Car(this.price, this.weigth, this.year, this.model);
Car.sedan(double price, double weigth, int year)
: this(price, weigth, year, 'Sedan');
}
Run the code generator.
flutter pub run build_runner build
Then use the generated class, which will look something like this.
/// Container for names of elements belonging to the [Car] class
abstract class NameofCar {
static const String className = 'Car';
static const String constructor = '';
static const String constructorSedan = 'sedan';
static const String fieldPrice = 'price';
static const String fieldWeigth = 'weigth';
static const String fieldYear = 'year';
static const String fieldModel = 'model';
}
You can implement your own nameOf function:
String? nameOf(dynamic o) {
if (o == null) return "null";
try {
if (o is List) {
var first = o.length > 0 ? o[0] : null;
if (first != null) {
var elementType = nameOf(first)!;
Log.debug("nameOf: List<$elementType>");
if (!isMinified(elementType))
return "List<$elementType>";
}
} else {
Function? getTypeName = o.getTypeName;
if (getTypeName != null) return getTypeName();
}
} catch (e) {
Log.debug("ignored nameOf error: $e, falling back to o.runtimeType: ${o.runtimeType}");
}
return o.runtimeType.toString();
}
bool isMinified(String type) => type.startsWith("minified:");
I'm new to dart. Currently, working on a mobile app through flutter. I have a Helper class which has some common methods which I've planned throughout the app. I've included that Helper class in another class. But, can't able to fig. out how to access its methods.
My commom Helper class code:
import 'dart:async';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';
class Helper {
Map userDetails = {};
Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
// --- Method for getting user details from shared preference ---
Future<Map>getUserDetailsFromSharedPreference () async {
try {
final SharedPreferences prefs = await _prefs;
if(prefs.getString('user') != null) {
this.userDetails = json.decode(prefs.getString('user'));
} else {
print('Shared preference has no data');
}
} catch (e){
print('Exception caught at getUserDetails method');
print(e.toString());
}
return this.userDetails;
}
}
Here is my main program code where I've included the Helper class & trying to access it's getUserDetailsFromSharedPreference (). In this case, I'm getting an error like Only static memebers can be accessed in initializers. I also tried to extends Helper class in UserProfile class. But, there also I'm getting a different kind of errors. Can't able to identify how to do this thing.
import 'package:flutter/material.dart';
import 'helper.dart';
class UserProfile extends StatefulWidget {
#override
UserProfileState createState() => new UserProfileState();
}
class UserProfileState extends State<UserProfile> {
Helper helper = new Helper();
var userData = helper.getUserDetailsFromSharedPreference();
}
#Günter Zöchbauer I've made my Helper.dart file like this as you've suggested -
import 'dart:async';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';
class Helper {
Map userDetails = {};
Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
static Helper _instance;
factory Helper() => _instance ??= new Helper._();
Helper._();
// --- Method for getting user details from shared preference ---
Future<Map>getUserDetailsFromSharedPreference () async {
try {
final SharedPreferences prefs = await _prefs;
if(prefs.getString('user') != null) {
this.userDetails = json.decode(prefs.getString('user'));
} else {
print('Shared preference has no data');
}
} catch (e){
print('Exception caught at getUserDetails method');
print(e.toString());
}
return this.userDetails;
}
}
Now, in my tryint to access that getUserDetailsFromSharedPreference() method I'm getting the same error Only static memebers can be accessed in initializers .
You could ensure a singleton instance of the class using a public factory constructor with a private regular constructor:
class Helper {
static Helper _instance;
factory Helper() => _instance ??= new Helper._();
Helper._();
...
}
If you call new Helper(), you'll always get the same instance.
You need to import the file that contains class Helper {} everywhere where you want to use it.
??= means new Helper._() is only executed when _instance is null and if it is executed the result will be assigned to _instance before it is returned to the caller.
update
getUserDetailsFromSharedPreference is async and can therefore not be used in the way you use it, at least it will not lead to the expected result. getUserDetailsFromSharedPreference returns a Future that provides the result when the Future completes.
class UserProfileState extends State<UserProfile> {
Helper helper = new Helper();
Future<Map> _userData; // this with ??= of the next line is to prevent `getUserDetailsFromSharedPreference` to be called more than once
Future<Map> get userData => _userData ??= helper.getUserDetailsFromSharedPreference();
}
If you need to access userData you need to mark the method where you do with async and use await to get the result.
foo() async {
var ud = await userData;
print(ud);
}
To access other class method you can simply put static on the method.
class Helper {
static printing(String someText){
print(someText);
}
}
void main() {
Helper.printing('Hello World!');
}
I think this question is more related to accessing one class data in another class. So I explained on the basis of my understanding of the question but if I'm not correct about it.
but if you want to access data of class A, not directly but through class B.
so first you have to make an object of A in class B but remember one thing you would have to make the object static in order to get access to the data of class A within Class B
If you still are confused about all this, I made a solution video.
Check it out: https://youtu.be/shK7ZraruCI
I need to "fetch" the methods in a Dart Class.
How can I do this?
And I want to be able to call the methods.
May I see an example?
Here's an easy copy-pasteable code sample:
import 'dart:mirrors';
import 'dart:io';
main() {
var im = reflect(new File('test')); // Retrieve the InstanceMirror of some class instance.
im.type.methods.values.forEach((MethodMirror method) => print(method.simpleName));
}
Output is:
existsSync
_delete
exists
directory
_getDecodedLines
readAsTextSync
readAsBytesSync
readAsLinesSync
_directory
throwIfError
lastModifiedSync
readAsLines
open
_ensureFileService
deleteSync
delete
_exists
length
openInputStream
create
_create
readAsText
_openStdioSync
openOutputStream
_fullPath
_lastModified
fullPathSync
readAsBytes
lastModified
_openStdio
_open
openSync
lengthSync
directorySync
fullPath
createSync
_lengthFromName
Here is a basic example:
(Note: You will want to have a (very) up to date version of the SDK for this, this was done in Dart Editor version 0.2.1_r14167, Dart SDK version 0.2.1.2_r14167 Nov 2, 2012)
My most sincere Thanks to Gilad of the Google Dart Team for providing this example!
#import('dart:mirrors');
class MyClass {
String _test;
String get test => _test;
set test(String paramVal) => _test = paramVal;
void my_method() {
}
void print_test(){
print("test string is: ${_test}");
}
MyClass(String test) {
_test = test;
}
}
main() {
MyClass myClass = new MyClass("Make my day, PUNK.");
myClass.print_test();
//ClassMirror myClassMirror = reflect(myClass).type;
InstanceMirror myClassInstanceMirror = reflect(myClass);
ClassMirror MyClassMirror = myClassInstanceMirror.type;
Map<String, MethodMirror> map = MyClassMirror.methods;
print("map = ${map}");
map.values.forEach( (MethodMirror mm){
myClassInstanceMirror.invoke(mm.simpleName,[]);
});
}
Concerning Reflection I have just written a couple of "Helper Functions" for fetching a LIST of the method names (not a Map) and invoking the method... all I can say for sure is that it works right now. There are likely technical reasons for not doing it this way - but in my case this does not run in a complex environment. However, I do think they nicely mask over a lot of details that not everybody is going to want to deal with.
Here's a functioning demonstration with and without the Helper functions:
#import('dart:mirrors');
class MyClass {
String _test;
String get test => _test;
set test(String paramVal) => _test = paramVal;
void my_method1(){print("in method1");}
void my_method2(){print("in method2");}
void print_test(){
print("test string is: ${_test}");
}
MyClass(String test) {
_test = test;
}
}
//Helper Methods
InstanceMirror hMr;
List REFLECT_methods(Object myClass) {hMr=reflect(myClass);return(hMr.type.methods.values);}
REFLECT_invoke(MethodMirror mm){hMr.invoke(mm.simpleName, []);}
main() {
MyClass myClass = new MyClass("Make my day, PUNK.");
print("\n=======Using Raw Reflection================");
InstanceMirror myClassInstanceMirror = reflect(myClass);
ClassMirror MyClassMirror = myClassInstanceMirror.type;
Map<String, MethodMirror> map1 = MyClassMirror.methods;
map1.values.forEach( (MethodMirror mm){
myClassInstanceMirror.invoke(mm.simpleName,[]);
});
print("\n==========Using Helper functions=============");
List list2 = REFLECT_methods(myClass);
list2.forEach((method){
REFLECT_invoke(method);
});
}