I have a custom Widget called HorizontalList that receives a list and displays it horizontally.
I have made a vertical list whose rows are made of these horizontal lists.
My problem is, even if the list passed to each horizontal list is different from the others, all lines rendered are the same as if I built the widgets using the same list.
I check in the constructor of the widget itself, and the data are correct, but something goes wrong while rendering.
Widget Constructor:
HorizontalList(List<RestInfos> infos){
_rests = new List<RestInfos>();
_rests = infos;
print("horizontal list "+ _rests.length.toString());
if(_rests.length != 0)
print("horizontal list 1: " + _rests.elementAt(0).name);
}
Widget Build:
child: new ListView(
scrollDirection: Axis.horizontal,
children: new List <Widget>.generate(_rests.length, (int index) {
if(_rests.length == 0) {
//return null;
return new Card();
}
print("hotizontal " + _rests.length.toString());
print("indice "+ index.toString());
print("nome " + _rests.elementAt(index).name);
return new GestureDetector(
child:
new Card(...)
)
In the main page, I try to build the widget like this:
child: new ListView(
children: <Widget>[
new HorizontalList(list1),
new HorizontalList(list2),
new HorizontalList(list3),
new HorizontalList(list4),
]
)
I found out that the constructor i was using was "breaking" the rendering of the website.
I tried following strictly the documentation, as it is shown here and changing the code to the one below:
class HorizontalList extends StatelessWidget {
const HorizontalList({Key key, #required this.infoList}) : super(key: key);
final List<Infos> infoList;
#override
Widget build(BuildContext context) {
...
}
It's actually very strange, as I have used my old approach elsewhere in the app and didn't result in any problem.
Anyway, this solved my problem.
Related
class mButton extends StatefulWidget{
final VoidCallback onPressed;
final Widget child;
const mButton({Key key, this.onPressed, this.child}):super(key: key);
#override
_mButtonState createState()=>_mState();
}
class _mButtonState extends State<mButton>{
#override
Widget build(BuildContext context){
return Container(
child: RaisedButton(
color: _getColors(), #notice this line
child: widget.child,
onPressed: widget.onPressed,
)
);
}
Color _getColors(){
return _buttonColors.putIfAbsent(this, ()=> colors[next(0,5)]); #notice `this` in here
}
Map<_mButtonState, Color> _buttonColors = {};
final _random = Random();
int next(int min, int max) => min+_random.nextInt(max-min);
List<Color> colors = [
Colors.blue,
Colors.green,
Colors.orange,
Colors.purple,
Colors.amber,
Colors.lightBlue
];
}
In the above code, notice the two lines marked with #notice. The this keyword is supposed to refer to the current class instance, but from the above code, it almost makes the impression that the _getColors() method is trying to make this refer to every new instance of RaisedButton instantiated.
I'm a bit confused, does this refer to the instance of _mButtonState or
every new instance of RaisedButton instantiated(this would be done by instantiating mButton stateful widget) ?
it almost makes the impression that the _getColors() method is trying to make this refer to every new instance of RaisedButton instantiated
This wouldn't actually be possible here, because the instance of RaisedButton cannot be created until after _getColors() has been called (since the return value of _getColors() is passed into RaiseButton's constructor).
this will be _mButtonState. Although this can be a bit confusing in JavaScript, in Dart you can always tell what this will be from looking at the definition of the method, without concern for how/where it's called.
I'm building three pages of alarms (stored in different lists) that I have displayed via a Scaffold and TabViewer. Each alarm is stored as a row with a toggle switch to enable it. The rows for each page are stored as a List. Despite making sure to use set state when changing values and even trying to assign unique keys nothing I do seems to preserve the state of the switches when I changed tabs.
This is my first time coding in Flutter/Dart or designing an app for mobile in general. As such I'm still learning about some basic features of this language.
I've tried adding keys to everything using Uniquekey() to generate up keys didn't work so I've removed them.
I've made sure all variable changes are inside set state functions.
I've tried to store the variable inside the immutable super class of AlarmToggle which is both ill-advised and doesn't work anyways.
I haven't tired using PageStorageKey as I'm not sure how they'd be implemented in my code but I feel this is likely the only solution.
class Alarms {
List<Widget> allAlarms = []; // Store all alarms for the object
buildAlarm(
GlobalKey<ScaffoldState>
pageKey,
[int hour,
int minute,
List<bool> alarmDaysOfWeek]) {
TimeOfDay alarmTime = TimeOfDay(hour: hour, minute: minute);
AlarmRow _newAlarm = new AlarmRow(UniqueKey(), alarmTime, alarmDaysOfWeek);
allAlarms.add(_newAlarm);
}
void removeAlarm(GlobalKey<ScaffoldState> pageKey) {allAlarms.removeLast();}}
class AlarmRow extends StatefulWidget {
final TimeOfDay _alarmTime;
final List<bool> _alarmDaysofWeek;
final UniqueKey key;
AlarmRow(this.key, this._alarmTime, this._alarmDaysofWeek);
AlarmRowState createState() => new AlarmRowState();
}
class AlarmRowState extends State<AlarmRow> {
bool _alarmIsActive;
AlarmRowState(){_alarmIsActive = _alarmIsActive ?? false;}
void toggleChanged(bool state) {this.setState(() {_alarmIsActive = state;});}
#override
Widget build(BuildContext context) {
return new Container(
child: Row(
children: <Widget>[
new AlarmIcon(_alarmIsActive),
new Column(
children: <Widget>[
new AlarmTime(widget._alarmTime),
new AlarmWeekly(widget._alarmDaysofWeek),
],
),
new AlarmToggle(
_alarmIsActive,
() => toggleChanged(!_alarmIsActive),
),
],
),
);
} // Build
} // Class
No matter what I seem to try the _alarmIsActive variable in AlarmRow() gets reset to null each time the tab is changed. I'm trying to preserve its state when changing pages.
The solution is as jdv stated to use AutomaticKeepAliveClientMixin, it's not hard to use but I figured I'd include the instruction that I found after searching here in case anyone searches and finds this and doesn't know automatically how to implement it like myself.
class AlarmRowState extends State<AlarmRow> with AutomaticKeepAliveClientMixin {
#override bool get wantKeepAlive => true;
It's implemented in the state class with any modifiable variables you want to preserve. The 'with' after the 'extends' adds a Mixin which is a sort of class inheritance. Finally, it requires you to set the 'wantKeepAlive' to true and it compiles and state is no longer lost while the widget is not being rendered.
Why a stateful widget loses state while it's not rendered is something I'm still searching for. But at least I have a solution.
I hope this will help you #Ender ! check for this code, you can create a Global shared preference and use it, as shown below:-
class AlarmRow extends StatefulWidget {
#override
State<StatefulWidget> createState() => new _AlarmRowState();
}
class _AlarmRowState extends State<AlarmRow>{
bool alarmIsActive;
#override
void initState() {
alarmIsActive = Global.shared.alarmIsActive;
super.initState();
}
#override
Widget build(BuildContext context) {
....
.....
....
....
body: new Container(
child: Switch(
value: alarmIsActive,
onChanged: (bool isEnabled) {
setState(() {
alarmIsActive = isEnabled;
Global.shared.alarmIsActive = isEnabled;
isEnabled =!isEnabled;
});
},
.....
......
),
),
);
}
}
class Global{
static final shared =Global();
bool alarmIsActive = false;
}
Switch enabled and will maintain its state
Edit: I've edited the code below to feature the method that fetches the data along with the widgets that build the train estimates (replacing any API information along the way with "API_URL" and "API_STOP_ID"). I hope this even better helps us figure out the problem! I really appreciate any information anyone can give -- I've been working very hard on this project! Thank you all again!
Original post:
I have a ListView of ListTiles that each have a trailing widget which builds train arrival estimates in a new Text widget. These trailing widgets are updated every five seconds (proven by print statements). As a filler for when the app is fetching data from the train's API, it displays a "no data" Text widget which is built by _buildEstimatesNull().
However, the problem is that "no data" is still being shown even when the app has finished fetching data and _isLoading = false (proven by print statements). Still, even if that was solved, the train estimates would become quickly outdated, as the trailing widgets are updating every five seconds on their own but this would not be reflected in the actual app as the widgets were built on page load. Thus, I need a way to rebuild those trailing widgets whenever they fetch new information.
Is there a way to have Flutter automatically rebuild the ListTile's trailing widget every five seconds as well (or whenever _buildEstimatesS1 is updated / the internals of the trailing widget is updated)?
class ShuttleApp extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new ShuttleState();
}
}
class ShuttleState extends State<ShuttleApp> {
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new HomeScreen(),
);
}
}
class HomeScreen extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new HomeState();
}
}
class HomeState extends State<HomeScreen> {
var _isLoading = true;
void initState() {
super.initState();
_fetchData();
const fiveSec = const Duration(seconds: 5);
new Timer.periodic(fiveSec, (Timer t) {
_fetchData();
});
}
var arrivalsList = new List<ArrivalEstimates>();
_fetchData() async {
arrivalsList.clear();
stopsList.clear();
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
globals.serviceActive = false;
} else {
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
arrivalsList.add(arrivalEstimate);
});
}
});
}
setState(() {
_isLoading = false;
});
}
Widget _buildEstimateNull() {
return new Container(
child: new Center(
child: new Text("..."),
),
);
}
Widget _buildEstimateS1() {
if (globals.serviceActive == false) {
print('serviceNotActive');
_buildEstimateNull();
} else {
final String translocStopId = "API_STOP_ID";
final estimateMatches = new List<String>();
arrivalsList.forEach((arrival) {
if (arrival.stopId == translocStopId) {
estimateMatches.add(arrival.arrivalAt);
}
});
estimateMatches.sort();
if (estimateMatches.length == 0) {
print("zero");
return _buildEstimateNull();
} else {
return new Container(
child: new Center(
child: new Text(estimateMatches[0]),
),
);
}
}
}
#override
Widget build(BuildContext context) {
return new Scaffold(
backgroundColor: const Color(0xFF171717),
appBar: new AppBar(),
body: new DefaultTextStyle(
style: new TextStyle(color: const Color(0xFFaaaaaa),),
child: new ListView(
children: <Widget>[
new ListTile(
title: new Text('S1: Forest Hills',
style: new TextStyle(fontWeight: FontWeight.w500, fontSize: 20.0)),
subtitle: new Text('Orange Line'),
contentPadding: new EdgeInsets.symmetric(vertical: 16.0, horizontal: 16.0),
trailing: _isLoading ? _buildEstimateNull() : _buildEstimateS1(),
),
],
),
)
);
}
class ArrivalEstimates {
final String routeId;
final String arrivalAt;
final String stopId;
ArrivalEstimates(this.routeId, this.arrivalAt, this.stopId);
}
Thank you so much in advance for any help you can give! I really super appreciate it! :)
There are a few ways you could tackle this. It is slightly difficult however to tell what's going on without seeing a bit more of your code - specifically how you're getting the data and what you're doing with it. But I think I can give you a sufficient answer anyways.
The simple way of doing this is to either:
Have a StatefulWidget which keeps track of the build estimates for all of the items in the list. It should request data from your API, get the results, and then call setState(() => this.listData = data);. The call to setState is what tells the widget that it needs to rebuild.
Have a StatefulWidget for each item in the list. They would all each perform an API request every 5 seconds, get the results, and then each would call setState(() => this.itemData = data);. This means multiple calls to the API etc.
The advantage of #1 is that you can batch API calls, whereas the advantage to #2 is that your build would change less overall (although the way flutter works, this would be pretty minimal)... so I would probably go with #1 if possible.
However, there is a better way of doing this!
The better way of doing this is to have some sort of API Manager (or whatever you want to call it) which handles the communication with your API. It probably would live higher up in your widget tree and would be started/stopped with whatever logic you want. Depending on how far up the widget tree is, you could either pass it into each child or more likely hold it in an InheritedWidget which could then be used to retrieve it from each list element or from the overall list.
The API manager would provide various streams - either with a bunch of named fields/methods or with a getStream(id) sort of structure depending on your API.
Then, within your various list elements, you would use StreamBuilder widgets to build each of the elements based on the data - by using a StreamBuilder you get a ConnectionState object that lets you know whether the stream has received any data yet so you can choose to show an isLoading type widget instead of the one that shows data.
By using this more advanced method, you get:
Maintainability
If your API changes, you only have to change the API manager
You can write better testing as the API interactions and the UI interactions are separated
Extensibility
If you, later on, use push notifications for updates rather than pinging a server every 5 seconds, that can be incorporated into the API manager so that it can simply update the stream without touching the UI
EDIT: as per OP's comments, they have already implemented more or less the first suggestion. However, there are a few problems with the code. I'll list them below and I've posted the code with a couple of changes.
The arrivalsList should be replaced each time a new build is done rather than simply being changed. This is because dart compares the lists and if it finds the same list, it doesn't necessarily compare all of the elements. Also, while changing it in the middle of a function isn't necessarily going to cause problems, it's generally better to use a local variable and then change the value at the end. Note that the member is actually set within setState.
If serviceActive == false, the return was missed from return _buildEstimateNull();.
Here's the code:
class HomeState extends State<HomeScreen> {
var _isLoading = true;
void initState() {
super.initState();
_fetchData();
const fiveSec = const Duration(seconds: 5);
new Timer.periodic(fiveSec, (Timer t) {
_fetchData();
});
}
var arrivalsList = new List<ArrivalEstimates>();
_fetchData() async {
var arrivalsList = new List<ArrivalEstimates>(); // *********** #1
stopsList.clear();
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
print("no service id");
globals.serviceActive = false;
} else {
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
arrivalsList.add(arrivalEstimate);
});
}
});
}
setState(() {
_isLoading = false;
this.arrivalsList = arrivalsList; // *********** #1
});
}
Widget _buildEstimateNull() {
return new Container(
child: new Center(
child: new Text("..."),
),
);
}
Widget _buildEstimateS1() {
if (globals.serviceActive == false) {
print('serviceNotActive');
return _buildEstimateNull(); // ************ #2
} else {
final String translocStopId = "API_STOP_ID";
final estimateMatches = new List<String>();
print("arrivalsList length: ${arrivalsList.length}");
arrivalsList.forEach((arrival) {
if (arrival.stopId == translocStopId) {
print("Estimate match found: ${arrival.stopId}");
estimateMatches.add(arrival.arrivalAt);
}
});
estimateMatches.sort();
if (estimateMatches.length == 0) {
print("zero");
return _buildEstimateNull();
} else {
return new Container(
child: new Center(
child: new Text(estimateMatches[0]),
),
);
}
}
}
#override
Widget build(BuildContext context) {
return new Scaffold(
backgroundColor: const Color(0xFF171717),
appBar: new AppBar(),
body: new DefaultTextStyle(
style: new TextStyle(color: const Color(0xFFaaaaaa),),
child: new ListView(
children: <Widget>[
new ListTile(
title: new Text('S1: Forest Hills',
style: new TextStyle(fontWeight: FontWeight.w500, fontSize: 20.0)),
subtitle: new Text('Orange Line'),
contentPadding: new EdgeInsets.symmetric(vertical: 16.0, horizontal: 16.0),
trailing: _isLoading ? _buildEstimateNull() : _buildEstimateS1(),
),
],
),
)
);
}
Instead of clearing and re-using the arrivalsList, create a new list every time the data is fetched. Otherwise Flutter is unable to detect if the list has changed.
Also, the code would clearer if you called setState whenever you change the list.
_fetchData() async {
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
globals.serviceActive = false;
setState(() {
_isLoading = false;
});
} else {
final newArrivalsList = new List<ArrivalEstimates>();
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
newArrivalsList.add(arrivalEstimate);
});
}
});
setState(() {
arrivalsList = newArrivalsList;
_isLoading = false;
});
}
}
A few side notes:
I'm not sure if you actually want to clear the list before you fetch the data. If the state was updated properly, that would cause a flicker every 5 seconds.
I'm not sure if you simplified the code, but calling the _fetchData method every five seconds may become a problem if the network is slow.
If you are certain that you want a child widget to rebuild every time you call setState() and it is stubbornly refusing, you can give it a UniqueKey(). This will ensure that when setState() triggers a rebuild the child widget keys will not match, the old widget will be popped and disposed of, and, the new widget will replace it in the widget tree.
Note that this is using keys in sort of the opposite way for which they were intended (to reduce rebuilding) but if something beyond your control is hindering necessary rebuilds then this is a simple, built-in way to achieve the desired goal.
Here is a very helpful Medium article on keys from one the Flutter team members, Emily Fortuna:
https://medium.com/flutter/keys-what-are-they-good-for-13cb51742e7d
I am not sure if this is what your looking for but and im probably late on this but i believe you can use a change notifier efficiently to achieve this. Basically a change notifier is hooked to your backed logic() for instance an api data fetch. A widget is then registered with a change notifier of the same type as the change notifier provider. In event of data change, the widgets registered with the change notifier will be rebuild.
For instance
// extend the change notifier class
class DataClass extends ChangeNotifier {
....
getData(){
Response res = get('https://data/endpoint')
notifyListeners()
}
void onChange() {
notifyListeners();
}
....
}
Every time there is change in data you call the notifyListeners() that will trigger rebuild of consuming widgets.
Register you widget with a changenotifier
class View extends StatefulWidget {
Widget create(BuildContext context) {
return ChangeNotifierProvider<ModelClass>(
builder: (context) => DataClass(auth: auth),
child: Consumer<ModelClass>(
builder: (context, model, _) => View(model: model),
),
);
}
}
You can also user a Consumer for the same. Get more on this from the Documentation
I had an issue about updating image using Image.network in stateful widget, it doesn't update when change the url inside set State but when i do hot reload, the image updated.
anyone have idea why it's happen ?
If the URL is the same as before, try to add some random querystring to the url, like:
Image.network(url + "?v=1");
Give the Image.network a key with the url as a value.
key: ValueKey(url)
First you need to add a key to your widget
This wiil ensure that when the widget tree get rebuilded the image widget get
rebuilded too
Image(
image: NetworkImageSSL(_imageUrlPath),
fit: BoxFit.cover,
key: ValueKey(new Random().nextInt(100)),),
),
Then clear the image cache
In my case have to clear image cache when image is uploaded to server
Ex=>
saveDataToServer(){
if(onSucess){
imageCache.clear();
imageCache.clearLiveImages();
}
}
Clearing the two image caches and adding a key worked, but it seemed like overkill.
Ian's answer was using a parameter in the url, but I wanted a bit of caching and did not want to request the server with a parameter.
So I ended up with the following solution that uses DateFormat from the intl package.
// Make parameter using DateFormat. This image will be cached for a minute.
var nowParam = DateFormat('yyyyddMMHHmm').format(DateTime.now());
// Use # instead of ? or & in url as nothing after # is transmitted to the server.
var img = Image.network(url + '#' + nowParam);
I solved this issue by adding a key vaule to the Image.newtwork then u need to delete the cache evertime before it reolards.
imageCache.clear();
Image myimage = Image.network('url', key: ValueKey(new Random().nextInt(100)),);
If we attach some random number to the url or some random key to the widget whenever you call setState(() {}) method image will be requested from the server which is not good, so maintain version number for the url & update the version number whenever you want image to be updated.
class ImageRefreshDemo extends StatefulWidget {
ImageRefreshDemo({
Key? key,
}) : super(key: key);
#override
_ImageRefreshDemoState createState() => _ImageRefreshDemoState();
}
class _ImageRefreshDemoState extends State<ImageRefreshDemo> {
final String _imageUrl = 'image url';
int _imageVersion = 1;
Future<void> _refreshImage() async {
//call API & update the image
_imageVersion++;
setState(() {});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Image Refresh Demo'),
),
body: Image.network(
'$_imageUrl?v=$_imageVersion',
),
floatingActionButton: FloatingActionButton(
child: Icon(
Icons.edit,
color: Colors.white,
),
onPressed: _refreshImage,
),
);
}
}
Not sure since I have just started building things with Flutter and Dart. If anyone can take a look at the code and can share inputs on:
How to display listview having fixed number of items, lets say in my example we are fetching 100 items
How to implement paging, lets say initially I want to fetch 1st page and then while scrolling page 2nd and so on.
Issues:
In current implementation, I am finding 2 issues:
Able to scroll endlessly at bottom
Finding exception in logcat output:
03-15 06:14:36.464 3938-3968/com.technotalkative.flutterfriends I/flutter: Another exception was thrown: RangeError (index): Invalid value: Not in range 0..99, inclusive: 100
I have posted the same issue on my Github repository: https://github.com/PareshMayani/Flutter-Friends/issues/1
Would appreciate if you contribute to this repo!
That is beacuse you are using ListView.builder which actually renders an infinite list when itemCount is not specified. Try specifying itemCount to 100.
For pagination, the simplest solution with a ListView.builder would be to show a refresh widget when the list has reached its end and initiate a refresh api call, and then add new items to the list and increase item count.
Example:
class Example extends StatefulWidget {
#override
_ExampleState createState() => new _ExampleState();
}
class _ExampleState extends State<Example> {
// initial item count, in your case `_itemCount = _friendList.length` initially
int _itemCount = 10;
void _refreshFriendList() {
debugPrint("List Reached End - Refreshing");
// Make api call to fetch new data
new Future<dynamic>.delayed(new Duration(seconds: 5)).then((_){
// after new data received
// add the new data to the existing list
setState(() {
_itemCount = _itemCount + 10; // update the item count to notify newly added friend list
// in your case `_itemCount = _friendList.length` or `_itemCount = _itemCount + newData.length`
});
});
}
// Function that initiates a refresh and returns a CircularProgressIndicator - Call when list reaches its end
Widget _reachedEnd(){
_refreshFriendList();
return const Padding(
padding: const EdgeInsets.all(20.0),
child: const Center(
child: const CircularProgressIndicator(),
),
);
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(),
// ListView Builder
body: new ListView.builder(
itemCount: _itemCount + 1,
itemBuilder: (_, index) {
final Widget listTile = index == _itemCount // check if the list has reached its end, if reached end initiate refresh and return refresh indicator
? _reachedEnd() // Initiate refresh and get Refresh Widget
: new Container(
height: 50.0,
color: Colors.primaries[index%Colors.primaries.length],
);
return listTile;
},
),
);
}
}
Hope that helps!
Note: I'm not claiming this is the best way or is optimal but this is one of the ways of doing it. There is an example social networking app of git which does it in a different way, you can take a look at it here.