For some reason the compiler is complaining about this:
SynchronousFuture<void> setNewRoutePath(AppLink newLink) {
_currentLink = newLink;
// return null; // compiler really wants to see this null return...why?
}
But is happy with this:
Future<void> setNewRoutePath(AppLink newLink) async {
_currentLink = newLink;
}
Seems like async keyword is handling the implicit return here. Is there some equivalent for SynchronousFuture?
SynchronousFuture<void> setNewRoutePath(AppLink newLink) {
_currentLink = newLink;
// return null; // compiler really wants to see this null return...why?
}
You need a return value because your function is declared to return a non-void value, so it must return something. You could argue that all functions could implicitly return null if there's no explicit return statement, but that would be error-prone:
int f(String someValue) {
if (someCondition) {
return 42;
}
} // f didn't return anything if someCondition is false. Accidental or intentional?
Futures themselves aren't really special. If you have a non-async function that returns a Future, you must still have an explicit return statement:
Future<void> f() {
print('Hello world!');
} // Error: f doesn't end with a return statement.
However, the async keyword does a few things:
Primarily it enables the use of the await keyword.
It automatically wraps returned values in a Future. This includes implicit return values from void functions.
SynchronousFuture is just an ordinary class provided by Flutter that implements the Future interface. It is not special. It's not part of the Dart language, so there is not going to be any keyword that does automatic return value wrapping like what the async keyword does.
Also note that you should not just sprinkle return null statements in your function that returns SynchronousFuture. With an async function, a return; statement (or exiting the function without an explicit return statement) ultimately returns a Future<void>() to the caller. Callers expect to be able to call methods (e.g. .then()) on the returned Future to add completion callbacks. If you return null for a SynchronousFuture, calling .then() on it will result in a null pointer exception at runtime. You instead would need to use return SynchronousFuture<void>(null);.
I also should point out that the SynchronousFuture documentation states:
In general use of this class should be avoided as it is very difficult to debug such bimodal behavior.
so unless you have some clear need to use a SynchronousFuture, you're better off avoiding it.
Unless you have created a class called SynchronousFuture the compiler will complain as there is no class called SynchronousFuture in the dart standard library, so either you or a library you import must define this class.
In dart all async functions must have a return value of type Future<T>.
// return null; // compiler really wants to see this null
return...why?
Because Future is not the same as void, when you return a value in an async function, that value is subsequently used as value for the future, as future is a generic class.
Related
I have some code like this:
File("foo.txt").readAsString().catchError((e)=>print(e));
The compiler is complaining
info: The return type 'void' isn't assignable to 'FutureOr<T>', as required by 'Future.catchError'.
I can't seem to give it what it wants and can't find a single clear usage example in any of the docs (just a long issue in git about how many ways there are to mis-use this). If I take the docs at face value, I should be able to return a bool, or a future, neither make the analyzer happy.
How do I provide this FutureOr?
The documentation for Future.catchError could be a lot clearer, but the relevant part is:
onError is called with the error and possibly stack trace, and the returned future is completed with the result of this call in exactly the same way as for then's onError.
Cross-referencing to the documentation for Future.then, the relevant portion is:
The onError callback must return a value or future that can be used to complete the returned future, so it must be something assignable to FutureOr<R>.
Since File.readAsString returns a Future<String>, your catchError callback also must return a Future<String>. Examples of doing that:
File("foo.txt").readAsString().catchError((e) {
print(e);
return Future.value('');
});
File("foo.txt").readAsString().catchError((e) async {
print(e);
return '';
});
Logically, this makes sense; because given:
String value = await File("foo.txt").readAsString().catchError(...);
then if readAsString succeeds, value should be assigned a String. If it fails, since you catch the exception without rethrowing it, value still needs to be assigned a String.
Put another way, your code is equivalent to:
Future<String> readFoo() async {
try {
return await File("foo.txt").readAsString();
} catch (e) {
print(e);
}
// Oops, missing return value.
}
In general, I strongly recommend using async/await with try-catch instead of using .catchError, which would avoid this confusion.
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 have some code like this:
File("foo.txt").readAsString().catchError((e)=>print(e));
The compiler is complaining
info: The return type 'void' isn't assignable to 'FutureOr<T>', as required by 'Future.catchError'.
I can't seem to give it what it wants and can't find a single clear usage example in any of the docs (just a long issue in git about how many ways there are to mis-use this). If I take the docs at face value, I should be able to return a bool, or a future, neither make the analyzer happy.
How do I provide this FutureOr?
The documentation for Future.catchError could be a lot clearer, but the relevant part is:
onError is called with the error and possibly stack trace, and the returned future is completed with the result of this call in exactly the same way as for then's onError.
Cross-referencing to the documentation for Future.then, the relevant portion is:
The onError callback must return a value or future that can be used to complete the returned future, so it must be something assignable to FutureOr<R>.
Since File.readAsString returns a Future<String>, your catchError callback also must return a Future<String>. Examples of doing that:
File("foo.txt").readAsString().catchError((e) {
print(e);
return Future.value('');
});
File("foo.txt").readAsString().catchError((e) async {
print(e);
return '';
});
Logically, this makes sense; because given:
String value = await File("foo.txt").readAsString().catchError(...);
then if readAsString succeeds, value should be assigned a String. If it fails, since you catch the exception without rethrowing it, value still needs to be assigned a String.
Put another way, your code is equivalent to:
Future<String> readFoo() async {
try {
return await File("foo.txt").readAsString();
} catch (e) {
print(e);
}
// Oops, missing return value.
}
In general, I strongly recommend using async/await with try-catch instead of using .catchError, which would avoid this confusion.
Future<void> saveEverything() {
_formKeyForDeposit.currentState?.save();
Navigator.of(this.context).pop(true);
return;
}
This^ is throwing the error:
A value must be explicitly returned from a non-void function.
I've tried returning void, I've tried return true, I've tried returning Future<void>, I've tried returning the Navigator.pop line.
There is an answer on Stackoverflow, but that doesn't work with enforced null safety, this function wants something returned despite being void. I don't understand it.
It won't compile, I'd love some clarity on what drives the issue, and a solution.
As far as I can see none of the called functions are async, so there is nothing you could await. This means that your function isn't asynchronous either and there is no need to use Future as a return type. void should work fine:
void saveEverything() {
_formKeyForDeposit.currentState?.save();
Navigator.of(this.context).pop(true);
}
Edit: To specifically answer:
this function wants something returned despite being void
The return type isn't void, it is a Future with a generic type of void . Future is a normal class and thus your method expects an object of type Future to be returned. The void here is defining what type the value of a successfully resolved Future should have.
like this
Future<void> saveEverything() async {
_formKeyForDeposit.currentState?.save();
Navigator.of(this.context).pop(true);
}