The issue seems to be around calling a method with a _ before it, such as Future _showFirstPolylineMapMarkers
In my main.dart at the end of a method I call 2 entities, one is within main.dart and works fine, the other is in another Class, _mapItemsExample2.
...
_sendIntermodalDataToSecondScreen(context, mappedValues, dest); //works fine
_mapItemsExample2?.getroute(deplat, deplong, deplocname);
}
});
}
It will pass data to
Future getroute(deplat, deplong, deplocname) async {
print('getroutetest');
_showFirstPolylineMapMarkers(deplat, deplong, deplocname);
}
void _showFirstPolylineMapMarkers(deplat, deplong, deplocname) async {
...
But it goes dead and doesn't print or call _showFirstPolylineMapMarkers
If i replace .getroute... with ._showFirstPolylineMapMarkers(deplat, deplong, deplocname) to call the method directly it shows an error
The method '_showFirstPolylineMapMarkers' isn't defined for the type 'MapItemsExample2'.
If i remove the underscore at the start _ errors go away but it doesnt call it when ran
Any guidance appreciated
Thank you
The _ indicates a private field or method, meaning that it can only be used within the file it's defined in.
It is a good practice to use the _ to make methods private to ensure they're only used where they are defined.
Related
I cannot use the ".obs" property variables that I have created as parameters in my api service methods. and it gives the error The prefix 'coinTwo' can't be used here because it is shadowed by a local declaration.
Try renaming either the prefix or the local declaration.
I wrote the problem in getx, but they did not understand the problem.
I would appreciate it if you could take a look at my issue on github.
İssue link: text
I would appreciate it if you could take a look at my issue on github.
İssue link: text
Your problem is in the declaration of getOrderBookData
Instead of
getOrderBookData(coinOne.value, coinTwo.value) async {
...
you should either have
getOrderBookData() async {
...
//Do stuff with coinOne.value, coinTwo.value
...
OR
getOrderBookData(String firstCoin, String secondCoin) async {
coinOne.value = firstCoin;
coinTwo.value = secondCoin;
...
Edit
Seeing your post on GetX's Github, it looks like you're missing some basic understanding on how functions and methods work.
The code you wrote is the equivalent of the following method signature :
getOrderBookData("aValue", "anotherValue") async {
and it doesn't makes any sense.
Your method's signature should only declare the parameters it's expecting and their type.
You can also define a default value for those parameters if needed.
getOrderBookData({String firstCoin = coinOne.value, String secondCoin = coinTwo.value}) async {
in function declarations you just tell what type it is, and not actual values
So either do
getOrderbookData(String one, String two) async {
var res = await api.fetchOrderBookData(one, two);
purchase.value = res.result!.buy!;
sales.value = res.result!.sell!;
}
or hardcode it to always use coinOne and coinTwo and give no parameters
getOrderbookData() async {
var res = await api.fetchOrderBookData(coinOne.value, coinTwo.value);
purchase.value = res.result!.buy!;
sales.value = res.result!.sell!;
}
Make these changes and the error will be gone.
First, make a minor change to your getOrderBookData method:
void getOrderBookData() async {...} // Don't pass any parameter here.
And then make a change to your onInit method too.
#override
void onInit() {
getOrderBookData();
super.onInit();
}
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);
}
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.
I have an async method. Can I call it from an non-async method? Like the following
My method
void method() async{
await 'something'
}
Case 1
onPressed:() {
method();
}
Case 2
onPressed:() async{
await method();
}
Which of the above is correct? It seems to me two of them is OK. However, the second one I think works much more slower, am I wrong?
In general, the caller of an async function must also be asynchronous if it wants to wait for the call to complete. This makes asynchronous-ness contagious.
In your case, your async function is a "fire-and-forget" function; callers cannot wait for it to complete, so it doesn't matter. Your second case (with await method()) is wrong because you should use await only on Future/FutureOr, but method returns void, so there nothing to wait for. (The Dart analyzer would warn you about this if you have the await_only_futures lint enabled.)
You also could simplify your code further by using a tear-off instead of creating an unnecessary closure:
onPressed: method
coming from the JS world I'm having a bit of problem wrapping my head around promise kit flavor of promises, I need a bit of help with the following.
Assume I have a function that returns a promise, say an api call, on some super class I await for that promise, then do some other action (potentially another network call), on that parent call I also have a catch block in order to set some error flags for example, so in the end I have something close to this:
func apiCall() -> Promise<Void> {
return Promise { seal in
// some network code at some point:
seal.fulfill(())
}
}
// in another class/object
func doApiCall() -> ? { // catch forces to return PMKFinalizer
return apiCall()
.done {
// do something funky here
}
.catch {
print("Could not do first request"
}
}
now I'm trying to write some unit tests for this functionality, so the response is mocked and I know it will not fail, I just need to await so I can verify the internal state of my class:
// on my test file
doApiCall().done {
// test my code, but I get an error because I cannot pipe a promise that already has a `.catch`
}
How would one go about solving this problem? I could use finally to chain the PMKFinalizer but that feels wrong
Another tangential question would be, is it possible to re catch the error on a higher level, let's say a UI component so it can hold some temporary error state? as far as I see I did not see a way to achieve this.
Many thanks 🙏