I can assign an arrow function, which returns a value to a function variable that requires a void signature. But I can not assign a block function that returns a value. Why? Should the not be arrow function restricted as well, as it returns a value?
arrow function
block function
That is because by doing
void Function(int) function=(a)=>b=a;
Dart assumes that you don't want to return the value of the assignation, that normally you could use, for example:
void main() {
int a=10,b;
print(b=a);
}
So, dart just thinks that you want to assign the variable, nothing more. Another more detailed example here:
int number=10;
late int target;
void main() {
print(test());
number+=10;
test2();
print(target);
}
int test()=>target=number;
void test2()=>target=number;
//Note: there aren't many best practices here (For example, global variables)
//Is just an example to make you understand, nothing more
but if you specify the return, dart will think you reeeeally want to return that value, and that's not possible, as it is a void function.
Related
I am writing a native plugin that, in some cases, has to call functions in the Flutter portion of the app, written in Dart.
How it's achieved, is explained here:
https://flutter.io/platform-channels/
Furthermore, an example of invoking a method from the native/platform part towards the Dart/non-native is here:
https://github.com/flutter/plugins/tree/master/packages/quick_actions
Now, this example is really nice in case the platform only needs to invoke a method, i.e. that call returns nothing/void, but in case it needs to invoke a function, i.e. needs a return value from the non-native/Dart part, I could not have found an example or documentation on the internet. I believe it can be implemented though, because in the native Java part, there is a method:
public void invokeMethod(String method, Object arguments, MethodChannel.Result callback)
So, there is a callback object that could have a return value from the non-native part - or, I am mistaken here, and there is currently no way of returning a value from the non-native Dart portion of the app?
The signature is void setMethodCallHandler(Future<dynamic> handler(MethodCall call)), so we need to provide a function at the Dart end that returns Future<dynamic>, for example _channel.setMethodCallHandler(myUtilsHandler);
Then implement the handler. This one handles two methods foo and bar returning respectively String and double.
Future<dynamic> myUtilsHandler(MethodCall methodCall) async {
switch (methodCall.method) {
case 'foo':
return 'some string';
case 'bar':
return 123.0;
default:
throw MissingPluginException('notImplemented');
}
}
At the Java end the return value is passed to the success method of the Result callback.
channel.invokeMethod("foo", arguments, new Result() {
#Override
public void success(Object o) {
// this will be called with o = "some string"
}
#Override
public void error(String s, String s1, Object o) {}
#Override
public void notImplemented() {}
});
In Swift, the return value is an Any? passed to the result closure. (Not implemented is signaled by the any parameter being the const NSObject value FlutterMethodNotImplemented.)
channel.invokeMethod("foo", arguments: args, result: {(r:Any?) -> () in
// this will be called with r = "some string" (or FlutterMethodNotImplemented)
})
I am writing a native plugin that, in some cases, has to call functions in the Flutter portion of the app, written in Dart.
How it's achieved, is explained here:
https://flutter.io/platform-channels/
Furthermore, an example of invoking a method from the native/platform part towards the Dart/non-native is here:
https://github.com/flutter/plugins/tree/master/packages/quick_actions
Now, this example is really nice in case the platform only needs to invoke a method, i.e. that call returns nothing/void, but in case it needs to invoke a function, i.e. needs a return value from the non-native/Dart part, I could not have found an example or documentation on the internet. I believe it can be implemented though, because in the native Java part, there is a method:
public void invokeMethod(String method, Object arguments, MethodChannel.Result callback)
So, there is a callback object that could have a return value from the non-native part - or, I am mistaken here, and there is currently no way of returning a value from the non-native Dart portion of the app?
The signature is void setMethodCallHandler(Future<dynamic> handler(MethodCall call)), so we need to provide a function at the Dart end that returns Future<dynamic>, for example _channel.setMethodCallHandler(myUtilsHandler);
Then implement the handler. This one handles two methods foo and bar returning respectively String and double.
Future<dynamic> myUtilsHandler(MethodCall methodCall) async {
switch (methodCall.method) {
case 'foo':
return 'some string';
case 'bar':
return 123.0;
default:
throw MissingPluginException('notImplemented');
}
}
At the Java end the return value is passed to the success method of the Result callback.
channel.invokeMethod("foo", arguments, new Result() {
#Override
public void success(Object o) {
// this will be called with o = "some string"
}
#Override
public void error(String s, String s1, Object o) {}
#Override
public void notImplemented() {}
});
In Swift, the return value is an Any? passed to the result closure. (Not implemented is signaled by the any parameter being the const NSObject value FlutterMethodNotImplemented.)
channel.invokeMethod("foo", arguments: args, result: {(r:Any?) -> () in
// this will be called with r = "some string" (or FlutterMethodNotImplemented)
})
I'm using a lambda expression in my C# script in my Unity project to call a function with a parameter when a DOTween callback is called.
It simply looks like this: animation.OnComplete(() => DestroyOnCompleted(children));
It works just fine, but I am note sure why a lambda expression is used in this case. I know it has something to do with delegates, but other than that, I'm not too sure; and I would like to explain it to my exam, if I get asked about it.
Could anyone enlighten me? :-)
Why not? ;)
I don't know that API too much but it seems like it is simply expecting something like
OnComplete(TweenCallback callback)
where TweenCallback from your usage basically seems to equal the c# built-in Action delegate and basically simply a parameter less void
public delegate void TweenCallback();
so whether you pass in this callback as a lambda like
animation.OnComplete(() => DestroyOnCompleted(children));
or anonymous method using the delegate operator like
animation.OnComplete(delegate { DestroyOnCompleted(children); });
or using a method
animation.OnComplete(OnCompletedAnimation);
...
private void OnCompletedAnimation()
{
DestroyOnCompleted(children);
}
is basically equivalent.
The main difference between the first two and the last one is: Where does children come from?
The lambda and delegate way allows you to pass in children from the current scope variables without having to store it in any field!
If you look at the documentation of DotTween, you see that row:
Now looking at the Source Code of DotTween, you can see the definition of TweenCallback:
public delegate void TweenCallback();
So the question now is, what is a delegate void in c#?
A delegate in c# is basically an object that "represent" a function.
But functions are not all the same, they can have parameters in input and return something (or return void).
To understand what kind of function does a delegate represent, try to just remove the keyword delegate.
For example, the TweenCallback without the keyboard delegate is:
public void TweenCaalback()
So the delegate represent a void function that has no parameters in input! (And it is Public).
What does it means represent a function?
It means that this is valid code:
void DoNothing()
{
}
TweenCallback x = DoNothing;
x();
So you can "assign functions" to a delegate that has the same function signature.
In this case, TweenCallback is a delegate void (), so you can assign to it a void() function.
What is a lambda?
A lambda is an expression of that style:
(string name, int age) => { return 3 };
you can read that as "string name and int age go in return 3"
That's a more concise way to describe that function:
int AnonymousFunction (string name, int age) {}
The main difference is that lambdas do not have any name. If you have not any parameter in input the lambda become like this:
() => {return 3;}
If you have only one statement inside the {} you are allowed to write it more shortly as
() => 3;
Final step
Is this valid code?
void DoNothing()
{
}
TweenCallback x = () => DoNothing();
Yes it is! Tween callback is expects a void () function.
() => DoNothing(); Is a lambda (un-named function) that takes nothing in input and calls some other function. It's the shorter version of () => {DoNothing();} that you have to think as void () {DoNothing();}
So when writing
animation.OnComplete(() => DestroyOnCompleted(children));
You are just passing a void () function to OnComplete Method, that makes sense because TweenCallback is a void () delegate.
Notes
As you can see, functions and lambdas expression can be converted implicitly to delegates. But you have to understand that they are all different things, and in more advanced coding scenarios that distinction is not just pure theory.
Let's say I've a stream() which returns Stream<int>. stream().listen returns StreamSubscription.
var subs = stream().listen((e) {
if (someCondition) subs.cancel(); // Error
});
I don't understand why is there an error, because by the time I start listening for events in the listen method, I would have definitely a valid object subs.
Note: I know this can be done by creating a StreamSubscription instance/top-level variable but why they have prevented the use of local variable like this?
We know that Stream.listen does not call its callback until after a value is returned, but the Dart compiler does not.
Consider the following function, which simply calls a callback and returns the result:
T execute<T>(T Function() callback) => callback();
Now, consider using it to assign a variable:
int myVariable = execute(() => myVariable + 1);
The problem here is that the given callback is called synchronously, before myVariable is assigned, but it tries to use myVariable to calculate a value!
To resolve this issue with your stream question, you can use the new late keyword. Using late tells the compiler that you know the variable will be assigned by the time it's accessed.
late final StreamSubscription<MyType> subscription;
subscription = stream().listen(/* ... */);
Likely because it's possible that subs will be used before it's assigned. We know that the callback passed to listen will be called on stream events, but it's also possible that the callback is called immediately and it's return value or a calculation done by it may be required for the return value of the function it was passed to.
Take this fakeFunc for instance, which I made an extension on the int class for convenience:
extension FakeListen on int {
int fakeFunc(int Function(int x) callback, int val) {
return callback(val);
}
}
The return value depends on the result of callback!
int subs = x.fakeFunc((e) {
print(e);
subs.toString();//error
return e + 1;
}, 5);
I can't use subs because subs will be guaranteed to not be exist at this point. It's not declared. This can be easily solved by moving the declaration to a separate line, but also forces you to make it nullable. Using late here won't even help, because subs won't exist by the time you try to use it in the callback.
Your scenario is different, but this is an example of where allowing that would fail. Your scenario involves a callback that is called asynchronously, so there shouldn't be any issues with using subs in the callback, but the analyzer doesn't know that. Even async-marked methods could have this issue as async methods run synchronously up until its first await. It's up to the programmer to make the right decision, and my guess is that this error is to prevent programmers from making mistakes.
What is the difference between main() and void main() in Flutter
The only answers that I could find on the internet are about C++ or sth like that.
Well, let's see:
void main() {
print(main.runtimeType);
}
will print () => void. That means a function with no parameters returning void.
While
main() {
print(main.runtimeType);
}
will print () => dynamic. That means a function with no parameters returning dynamic.
The valid return type for the main function is void:
Every app must have a top-level main() function, which serves as the entrypoint to the app. The main() function returns void and has an optional List<String> parameter for arguments.
Source
So strictly speaking, having no return type (that means defaulting to the type dynamic) is wrong. But that is the simplified beginners instructions, so there are exceptions. But when in doubt, do it by the book.
the different is when return a value from the function in void read() you can't return any type of value ..
While in read()you can return any type of value weather is int , double , String , list and also a set Because it's dynamic also you can type dynamic read() it's same as read()
void main () {
// print(iifo.runtimeType);
var k = iifo() ;
print(k) ;
}
iifo ( ) {
return ('Stack') ;
}
output
Stack
in return line you can return any type of data (int , double , String , bool , list, etc....).
This answer does not address your question directly, since I see that it has been already answered properly. However, you might find good to know that the main function in a Dart application can also return Future<void> instead of void, since you can tag the main function as an async function.
In the particular case of a Flutter application, this means that you can do whatever you need to do before running runApp(...) in the main() method body, even asynchronously. This way, the splash screen will be shown until your asynchronous result is retrieved, before calling runApp(...). For example, setting a landing widget depending on the result of an asynchronous task:
Future<void> main() async {
// Do whatever you need to do here
final home = await createHomeWidgetDependingOnAsyncResult();
return runApp(MyApp(home: home));
}
I found it the problem, I wrote:
import 'LanguagesScreen.dart';
Instead of:
import 'package:flutter_projects/LanguagesScreen.dart';