I'm trying to learn firebase with flutter and i ran into this problem
here is my code :
FirebaseFirestore.instance
.collection('attendees')
.doc(user.uid)
.snapshots()
.listen((snapshot) {
if (snapshot.data() != null) {
if (snapshot.data()!['attending']) {
_attending = Attending.yes;
} else {
_attending = Attending.no;
}
} else {
_attending = Attending.unknown;
}
notifyListeners();
});
what is the solution ?
the exact problem is within this line :
if (snapshot.data()!['attending']) {
how can I rewrite this so i wont ruin the current functionality ?
I appreciate your help inadvance
The reason you are getting error -
Conditions must have a static type of 'bool'
because on line snapshot.data()!['attending'] an = sign is missing.
To make your code work just do
if (snapshot.data() != snapshot.data()!['attending']) {
_attending = Attending.yes;
} else {
_attending = Attending.no;
}
Understanding The Error
I would also like to point out that Dart a stricter language (more like Java in terms of 'truthy' values).
In JavaScript you can use any ‘truthy’ value in a conditional statement. In Dart you cannot use ‘truthy’ values. For example:
var name = 'Joe';
if (name) {
// do something...
OR
var a = 1
if(a){
//this would work in JavaScript
}
You cannot do such things in Java or Dart. The reason is that Dart requires that a condition is a bool true not just a 'truthy' value. You could correct the code by changing it to:
if (name.isNotEmpty)
OR
if(a==1)
{
//these == signs are really important
}
Just store the snapshot.data() to the local map variable and do the operations by that.
_attendingSubscription = FirebaseFirestore.instance
.collection('attendees')
.doc(user.uid)
.snapshots()
.listen((snapshot) {
final Map<String, dynamic>? data = snapshot.data();
if (data != null) {
_attending = data['attending'] ? Attending.yes : Attending.no;
} else {
_attending = Attending.unknown;
}
notifyListeners();
});
Related
I'm trying to handle some data for a flutter application, however I am getting the following error on my code:
The method 'add' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').
Map<String, List<SourcefulListing>> sortedSkills = {};
QuerySnapshot listingSnapshot = await listingsRef.get();
List<SourcefulListing> listings = [];
for (int i = 0; i < listingSnapshot.docs.length; i++) {
listings.add(SourcefulListing.fromJson(
listingSnapshot.docs[i].data() as Map<String, dynamic>));
}
for (String skill in skills) {
for (SourcefulListing listing in listings) {
if (listing.selectedSkill == skill) {
if (sortedSkills[skill] == null || sortedSkills[skill] != []) {
sortedSkills[skill] = [listing];
} else {
sortedSkills[skill] = sortedSkills[skill].add(listing);
}
}
}
}
Basically I have a Map with Strings as key and List for the values. The for each loop should add the SourcefulListing object to the map, however there is an error on the .add method.
Any help would be much appreciated.
Try this,
Map<String, List<SourcefulListing>> sortedSkills = {};
QuerySnapshot listingSnapshot = await listingsRef.get();
List<SourcefulListing> listings = [];
for (int i = 0; i < listingSnapshot.docs.length; i++) {
listings.add(SourcefulListing.fromJson(
listingSnapshot.docs[i].data() as Map<String, dynamic>));
}
for (String skill in skills) {
for (SourcefulListing listing in listings) {
if (listing.selectedSkill == skill) {
if (sortedSkills[skill] == null || sortedSkills[skill] != []) {
sortedSkills[skill] = [listing];
} else {
sortedSkills[skill]?.add(listing); // changes made here
setState(() {}); // update ui
}
}
}
}
Null Safety : https://dart.dev/null-safety
it's just a small question about dart/flutter code.
I saw this code today :
Future<List<String>?> getAtSignListFromKeychain() async {
var atsignMap = await _getAtSignMap();
if (atsignMap.isEmpty) {
// no atsigns found in biometric storage
// read entries from flutter keychain
// for mobile platforms only
if (Platform.isAndroid || Platform.isIOS) {
atsignMap = await checkForValuesInFlutterKeychain();
if (atsignMap.isEmpty) {
return null;
}
} else {
return null;
}
}
var atsigns = atsignMap.keys.toList();
_logger.info('Retrieved atsigns $atsigns from Keychain');
return atsigns;
}
I don't understand interest of returning null with List? . Isn't better to write this code ?:
Future<List<String>> getAtSignListFromKeychain() async {
var atsignMap = await _getAtSignMap();
if (atsignMap.isEmpty) {
// no atsigns found in biometric storage
// read entries from flutter keychain
// for mobile platforms only
if (Platform.isAndroid || Platform.isIOS) {
atsignMap = await checkForValuesInFlutterKeychain();
if (atsignMap.isEmpty) {
return atsignMap;
}
} else {
return List.empty();
}
}
var atsigns = atsignMap.keys.toList();
_logger.info('Retrieved atsigns $atsigns from Keychain');
return atsigns;
}
Or I'm missing something ? Thank you !
First of all, there are always different solutions for different problems. I believe it is better to return a null value for some cases instead of creating an empty list in the memory and returning that. Either way, you have to check the returning value, handle errors, etc. So instead of checking if the list is empty or not, you can use just the ?? operator.
And in some cases, the list you expect with the items in it may be empty. If you return an empty list by default in such cases, you would not know if the list that you expected is really empty or is there a problem.
I am still coming up to speed with dart and wanted to know if there was an easier way to not execute a statement if the value is null. See example below:
I can always do the if statements below for setting field3 and field4, but felt like something like field5 should work. But when I try to do that, it complains that a null check operator is used on a null value.
Also I don't want to change the Map to have a dynamic value.
Is there a single one liner to do what I am trying to do, or do I just need to check for null before setting the value.
Map<String, Object> myMap = {};
print('running now');
try {
myMap['field1'] = DummyClass.getString('hello');
myMap['field2'] = DummyClass.getString('good');
//Is there a more concise way to do this than the 2 options below?
if (DummyClass.getOptionalString('goodbye') != null) {
myMap['field3'] = DummyClass.getOptionalString('goodbye')!;
}
String? temp = DummyClass.getOptionalString('go');
if (temp != null) {
myMap['field4'] = temp;
}
// This gives an error 'null check operator used on a null value'
// myMap['field5'] ??= DummyClass.getOptionalString('to')!;
} catch (e) {
print('error condition, $e');
}
print(myMap);
}
class DummyClass {
static String getString(String? strParam) {
String? retString = getOptionalString(strParam);
if (retString == null) {
throw ('nulls are not allowed');
}
return retString;
}
static String? getOptionalString(String? strParam) {
if (strParam == null || strParam.length < 3) {
return null;
}
return strParam;
}
}
There's no built-in way to do what you want, but you could write a function (or extension method) to do it. For example:
extension MapTrySet<K, V> on Map<K, V> {
void trySet(K key, V? value) {
if (value != null) {
this[key] = value;
}
}
}
and then you could do:
myMap.trySet('field3', DummyClass.getOptionalString('goodbye'));
myMap.trySet('field4', DummyClass.getOptionalString('go'));
Alternatively, if you really want to use normal Map syntax, you could create your own Map class that has a void operator []=(K key, V? value) override and does nothing when the value is null, but that probably would not be worth the effort.
The issue is that the ??= operator assigns to the left if it is null. Expanded, it would look something like this:
a ??= b;
// Equivalent to:
if (a == null) {
a = b;
}
Which is not something that you're trying to achieve. AFAIK, there is no such operator yet in Dart. However, you can try this:
final possiblyNullValue = '';
final myMap = <String, String>{};
myMap['key'] = possiblyNullValue ?? myMap['key'];
// Equivalent to:
if (possiblyNullValue != null) {
myMap['key'] = possiblyNullValue;
}
// or:
myMap['key'] = possiblyNullValue != null? possiblyNullValue : myMap['key'];
Which would work in your case as a one-liner.
You could create your map with all entries, even null, and then filter the null values out:
void main() {
try {
final myMap = <String, dynamic>{
'field1': DummyClass.getString('hello'),
'field2': DummyClass.getString('good'),
'field3': DummyClass.getOptionalString('goodbye'),
'field4': DummyClass.getOptionalString('go'),
}..removeWhere((k, v) => v == null);
print(myMap);
} catch (e) {
print('error condition, $e');
}
}
I want to use a variable to access a certain value in my hive database:
In the code below if I use myBox.getAt(i).attributeSelect I get an error because attributeSelect is not defined for the box.
If I use myBox.getAt(i).test it works. How can I make flutter recognise that attributeSelect is a variable and put the value there? I have a total of 181 different variables the user can choose from. Do I really need that many if clauses? The variables are booleans. So I want to check if that attribute is true for the document at index i.
Error: NoSuchMethodError: 'attributeSelect'
method not found
Receiver: Instance of 'HiveDocMod'
attributeSelect = 'test'; //value depends on user choice
Future<void> queryHiveDocs() async {
final myBox = await Hive.openBox('my');
for (var i = 0; i < myBox.length; i++) {
if (attributeSelect == 'All Documents') {
_hiveDocs.add(myBox.getAt(i)); // get all documents
//print(myBox.getAt(24).vesselId);
} else {
// Query for attribute
if (myBox.getAt(i).attributeSelect) {
_hiveDocs.add(myBox.getAt(i)); // get only docs where the attributeSelect is true
}
}
}
setState(() {
_hiveDocs = _hiveDocs;
_isLoading = false;
});
}
I solved it the annoyingly hard way:
if (attributeSelect == 'crsAirCompressor') {
if (myBox.getAt(i).crsAirCompressor) {
_hiveDocs.add(myBox.getAt(i));
}
} else if (attributeSelect == 'crsBatteries') {
if (myBox.getAt(i).crsBatteries) {
_hiveDocs.add(myBox.getAt(i));
}...
I want to
1.map item from observable to another one if it has already saved in database.
2.otherwise, use it as it is.
and keep their order in result.
Saved item has some property like tag, and item from observable is 'raw', it doesn't have any property.
I wrote code like this and run testMethod.
class Item {
final String key;
String tag;
Item(this.key);
#override
String toString() {
return ('key:$key,tag:$tag');
}
}
class Sample {
///this will generate observable with 'raw' items.
static Observable<Item> getItems() {
return Observable.range(1, 5).map((index) => Item(index.toString()));
}
///this will find saved item from repository if it exists.
static Future<Item> findItemByKey(String key) async {
//simulate database search
await Future.delayed(Duration(seconds: 1));
if (key == '1' || key == '4') {
final item = Item(key)..tag = 'saved';
return item;
} else
return null;
}
static void testMethod() {
getItems().map((item) async {
final savedItem = await findItemByKey(item.key);
if (savedItem == null) {
print('not saved:$item');
return item;
} else {
print('saved:$savedItem');
return savedItem;
}
}).listen((item) {});
}
The result is not expected one.
expected:
saved:key:1,tag:saved
not saved:key:2,tag:null
not saved:key:3,tag:null
saved:key:4,tag:saved
not saved:key:5,tag:null
actual:
not saved:key:2,tag:null
not saved:key:3,tag:null
not saved:key:5,tag:null
saved:key:1,tag:saved
saved:key:4,tag:saved
How to keep their order in result?
I answer myself to close this question.
According to pskink's comment, use asyncMap or concatMap solve my problem. Thanks!!
below is new implementation of testMethod.
asyncMap version:
getItems().asyncMap((item) {
final savedItem = findItemByKey(item.key);
if (savedItem != null)
return savedItem;
else
return Future.value(item);
}).listen(print);
concatMap version:
getItems().concatMap((item) {
final savedItem = findItemByKey(item.key);
if (savedItem != null)
return Observable.fromFuture(savedItem);
else
return Observable.just(item);
}).listen(print);