Based on the SharedPreferences class, I try to retrieve a preference value like so:
String loadIPAddress() {
SharedPreferences.getInstance().then((SharedPreferences prefs) {
try {
var loadedValue = prefs.getString('serverIPAddress');
print('loadIPAddress <= ' + loadedValue);
return loadedValue; // [1]
} catch (e) {
print('loadIPAddress <= NOPE');
return '---'; [2]
}
});
}
Unfortunately, this doesn't return a value each time.
Q: Does the 1 and [2] return statements return the value of loadIPAddress()?
No, as you've guessed, those returns return from the then callback.
To return from loadIPAddress, refactor it like this:
Future<String> loadIPAddress() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
try {
var loadedValue = prefs.getString('serverIPAddress');
print('loadIPAddress <= ' + loadedValue);
return loadedValue;
} catch (e) {
print('loadIPAddress <= NOPE');
return '---';
}
}
Note that having made loadIPAddress async, it now returns a Future, so you should call it like:
String ip = await loadIPAddress();
// or
loadIPAddress().then((String ip) {
// do something with ip - probably setState
});
Related
I'm trying to transform a Stream of a list of one type into a Stream of a list of another type, and having an issue with this.
I have this list of Habits that I'm streaming from Firebase, and I want to accept that stream in a function, and return a new stream that is a list of ViewModels of another type from it. But my function is returning a stream of the wrong type.
Here is my code:
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.map((habitsList) {
habitsList.map(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) => completion.date
.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
I am getting a compile error because the result variable is showing as type Stream<Null> when I hover over it, where I would expect it to be Stream<List<HabitCompletionViewModel>>. Any idea what I'm doing wrong?
Your outer .map call does not have a return statement which is why you are getting a Stream<Null>.
So add a return statement like so:
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.map((habitsList) {
// added return statement here
return habitsList.map(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) =>
completion.date.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
However the above code still has an error because it is now returning a Stream<List<Future<HabitCompletionViewModel>>> instead of the desired Stream<List<HabitCompletionViewModel>>. To solve this you can use .asyncMap instead of .map.
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.asyncMap((habitsList) {
return Stream.fromIterable(habitsList).asyncMap(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) =>
completion.date.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
How can I await a stream (or any other event queue) multiple times?
I tried Stream.first & Stream.single, both doesn't work.
What I want to do:
//next is a fake member
Future<void> xxx() async {
final eventStream = foo();
await eventStream.next; // wait for first event
//do some work
await eventStream.next; // wait for second event
//do some other differnet work
await eventStream.next; // wait for 3rd event
// another differnet work
return;
}
equvalent to:
Future<void> xxx() async {
final eventStream = foo();
int i=0;
await for (final _ in eventStream){
if(i==0);//do some work
else if(i==1);//do some other differnet work
else if(i==2){;break;}//another differnet work
++i;
}
return;
}
Try StreamQueue from package:async.
var q = StreamQueue(eventStream);
var v1 = await q.next;
var v2 = await q.next;
var v3 = await q.next;
// other work.
await q.cancel(); // If not listened to completely yet.
I end up with a custom class
class CompleterQueue<T> {
final _buf = <Completer<T>>[];
var _iWrite = 0;
var _iRead = 0;
int get length => _buf.length;
Future<T> next() {
if (_iRead == _buf.length) {
_buf.add(Completer<T>());
}
final fut = _buf[_iRead].future;
_iRead += 1;
_cleanup();
return fut;
}
void add(T val) {
if (_iWrite == _buf.length) {
final prm = Completer<T>();
_buf.add(prm);
}
_buf[_iWrite].complete(val);
_iWrite += 1;
_cleanup();
}
void _cleanup() {
final minI = _iWrite < _iRead ? _iWrite : _iRead;
if (minI > 0) {
_buf.removeRange(0, minI);
_iWrite -= minI;
_iRead -= minI;
}
}
}
I have DocumentReference locationDocumentRef; in my state.
I'm changing locationDocumentRef based on the references, whether I gather by querying or by adding new document.
So I have this function to check the documents, if there is one set its reference to the locationDocumentRef, or add a new one and set its ref to the locationDocumentRef. I'm resetting its value everytime by setting it to null, since I didn't want to get previous result. But it prints null.
So my question is, how can I resolve them and get the value? I think I'm resolving too early in my code, so I can't await a non-future value. How can I fix it?
void firestoreCheckAndPush() async {
setState(() {
locationDocumentRef = null;
});
bool nameExists = await doesNameAlreadyExist(placeDetail.name);
if (nameExists) {
print('name exist');
} else {
print('name will be pushed on firestore');
pushNameToFirestore(placeDetail);
}
var resolvedRef = await locationDocumentRef;
print(resolvedRef.documentID); // I get null here
}
These are the functions that I have used
Future<bool> doesNameAlreadyExist(String name) async {
QuerySnapshot queryDb = await Firestore.instance
.collection('locations')
.where("city", isEqualTo: '${name}')
.limit(1)
.getDocuments();
if (queryDb.documents.length == 1) {
setState(() {
locationDocumentRef = queryDb.documents[0].reference;
});
return true;
} else {
return false;
}
}
And the other
void pushNameToFirestore(PlaceDetails pd) async {
DocumentReference justAddedRef =
await Firestore.instance.collection('locations').add(<String, String>{
'city': '${pd.name}',
'image': '${buildPhotoURL(pd.photos[0].photoReference)}',
});
setState(() {
locationDocumentRef = justAddedRef;
});
}
there is two mistakes i saw first here
var resolvedRef = await locationDocumentRef;
why you await for locationDocumentRef,
second you dont wait for pushNameToFirestore(PlaceDetails pd) firestoreCheckAndPush() function which is weird since pushNameToFirestore(String) is sync and this means you wouldnt wait for it to finish so if you are adding a new name it would print null.
correct me if i am wrong.
you can find more about sync and future here https://www.dartlang.org/tutorials/language/futures
look at the graph at the middle of the page
Try this
Future<List<DocumentSnapshot>> doesNameAlreadyExist(String name) async {
QuerySnapshot data = await Firestore.instance
.collection('locations')
.where("city", isEqualTo: name)
.limit(1)
.getDocuments();
return data.documents;
}
void firestoreCheckAndPush() async {
var data = await doesNameAlreadyExist('yourname');
if (data.length > 0) {
print('name exist');;
print('Document id '+ data[0].documentID);
} else {
print('name will be pushed on firestore');
}
}
Take a look into following code.
void firestoreCheckAndPush() async {
DocumentReference documentReference;
var data = await doesNameAlreadyExist('yourname');
var dataRef = await doesNameAlreadyExist('yourname');
if (data.length > 0) {
print('name exist');
documentReference = dataRef[0].reference;
print('Document id ' + data[0].documentID);
documentReference = dataRef[0].reference;
print('Document reference ');
print(documentReference);
} else {
print('name will be pushed on firestore');
}
}
I have a function which does some asynchronous operations and I want to return the status of the operation back to the caller. How can I achieve this?
Future<bool> setData() async {
Firestore.instance.collection("test").document('$id').setData({
'id': 'test'
}).then((onValue) {
print('Data set success');
return true;
}).catchError((onError) {
print('Data set Error!!!');
return false;
});
}
//Calling
final status = await setData();
if(status){
//do success
}
But this function complains that it doesn't end with a return statement. What is the logical mistake I'm making here?
You miss a return in your setData function
return Firestore.instance....
am trying to call a method which calls another method .. and depending on that method result i will continue with my method .. something like this:
void submit() async{
if (login) {
....
bool result = await Login("966" + phone, _data.code);
if (result) {
successpage();
} else {
.....
}
and login:
bool Login(String phone, String SMScode) {
http.post(baseUrl + loginURL + "?phone=" + phone + "&smsVerificationCode="+ SMScode,
headers: {
'content-type': 'application/json'
}).then((response) {
final jsonResponse = json.decode(Utf8Codec().decode(response.bodyBytes));
print("LOGIN: " + jsonResponse.toString());
Map decoded = json.decode(response.body);
print(decoded['success']);
if (decoded['success']) {
globals.token = decoded['token'];
globals.login = true;
}else{
globals.login = false;
}
});
return globals.login;
}
but this doesn't work and doesn't give me the result of the last bool i need .. how to solve this?
The asynchronous handling is incorrect in your program. Basically your Login function returns without waiting the http post.
The following update should work.
Future<bool> Login(String phone, String SMScode) async {
final response = await http.post('$baseUrl$loginURL?phone=$phone&smsVerificationCode=$SMScode',
headers: {'content-type': 'application/json'});
final jsonResponse = json.decode(Utf8Codec().decode(response.bodyBytes));
print("LOGIN: " + jsonResponse.toString());
Map decoded = json.decode(response.body);
print(decoded['success']);
if (decoded['success']) {
globals.token = decoded['token'];
globals.login = true;
} else {
globals.login = false;
}
return globals.login;
}