I'm looking for a generic way of obtaining a reference to the Scrollable object which triggered a ScrollUpdateNotification.
I've seen this answer but only the last option seems viable, and ultimately seems to fall short. I can ascertain the scroll offset, but not a reference to the scrollable object itself.
Consider the following code...
final NotificationListener<ScrollUpdateNotification> scrollNotifier = NotificationListener<ScrollUpdateNotification>(
child: widget,
onNotification: (ScrollUpdateNotification value) {
ScrollableState scrollable1 = Scrollable.of(value.context);
ScrollableState scrollable2 = Scrollable.of(context);
return false;
},
);
Both scrollables are always null. The first because value.context refers to the context of the gesture being listened to. The second because the context is that in which the NotificationListener exists. Is there any other way to achieve this generically?
That is because when you do Scrollable.of(context), internally it doesn't actually fetch the Scrollable.
It fetches a _ScrollableScope which holds the instance of ScrollableState.
The problem being, the widget tree is roughly similar to this:
Scrollable
RawGestureDetector
_ScrollableScope
Where it's the RawGestureDetector's context that is passed to notifications.
Taking this into consideration, you can still obtain the Scrollable instance by doing the following:
NotificationListener<ScrollNotification>(
onNotification: (notification) {
notification.context.ancestorStateOfType(TypeMatcher<ScrollableState>());
},
child: child,
),
Related
I have a widget with a list and a button with a tree dot icon in every row that shows and hides a panel in its own row. I only want one panel open in the list. When I click on a row button, I'd like to close the panels of the other rows list. All the buttons in the list are siblings. I'd like to send an event to the other rows' code to close the panels. Which is the correct manner of flutter?
I have tried NotificationListener but it does not work because the components to be notified are not their parents.
The question is if the correct thing to do is to use the event_listener library or to use streams. I'm new to flutter/dart and streams seem too complex to me. It's a very simple use case and in this entry
Flutter: Stream<Null> is allowed?
they say
*
Some peoples use streams as a flux of events instead of a value
changing over time, but the class isn't designed with this in mind.
They typically try to represent the following method as a stream:
So with simple events with 0 or 1 argument. event_listener or Streams?
This is the screen I'm working on. I want that when one yellow button panel opens the other one closes.
Your question is broad and it seems to be a design question, i.e. it doesn't have a right answer.
However, I don't think you should use Streams or EventListeners at all in this case, because you should not make components in the same layer communicate with each other. Components should only communicate with their parents and children, otherwise your code will increase in complexity really fast. That's even documented in flutter_bloc.
Other than that, if you don't lift state up, i.e. move the responsibility of triggering the removal of the other rows to a parent Widget, than you're fighting against Flutter instead of letting it help you.
It's easy to create a parent Widget, just wrap one Widget around it. What you want to do is hard, so why would try to communicate with sibling widgets instead of using what's Flutter designed to do?
This is a suggestion:
class _NewsSectionState extends State<NewsSection> {
Widget build(BuildContext context) {
return ListView.builder(
itemCount: newsInSection.length;
itemBuilder: (_, int index) => NewsTile(
title: Text('${newsInSection[index].title}')
onDismiss: () => onDismiss(index),
// I don't know how you set this up,
// but () => onDismiss(Index)
// should animate the dismiss of the Row with said index
),
);
}
}
class NewsRow extends StatefulWidget {
final void Function() onDismiss;
#override
State<NewsRow> _createState => _NewsRowState();
}
class _NewsRowState extends State<NewsRow> {
Widget build(BuildContext context) {
return Row(
children: [
// title
// home button
// fav button
// remove button
IconButton(
Icons.close,
onPressed: widget.onDismiss,
),
],
);
}
}
I have a scrollcontroller which I need to add a listener to do some pagination functions on when the user scrolls down on a listview.
Currently, I create the scrollcontroller and a listener in the initState. I'm doing it there, because the scroll controller is actually a PrimaryScrollController and it needs context
var _scrollController = PrimaryScrollController.of(context);
Now I've run into a problem where when my page gets rebuilt for one reason or another the listview will jump to the top.
From what I understand its because on a rebuild, everything is getting rebuild however the initState isn't being run.
SO my solution is to move the scrollcontroller creation into the build method, which seems to be working just fine. However, the listener does not work, unless I also move it into the build method.
Is this ok? Or am I creating potentially many parallel listeners which can increase each time the page gets rebuilt?
If you are looking to listen to the scroll position and do some operations based on the scroll offset you can try a builder named valueListenableBuilder
ValueListenableBuilder<int>(
builder: (BuildContext context, int value, Widget? child) {
return Row(
children: <Widget>[
Text('$value'),
child!,
],
);
},
valueListenable: scrollController.offset,
child: Container(),
)
I have this listview.builder which is supposed to show some orders from an array of Order objects based on their status.
It looks kinda like this:
The List works just fine until, for some reason, when I scroll down and the viewport can only display the order with index (the one in the listview builder function) 5 and then press another category like "New", setState() is called and the whole thing rebuilds, but the builder's index starts at 5 and the listview.builder doesn't build anything from 0 - 4. I've printed the index in the builder and caught this bug, but I still don't understand why this is happening.
This is what my listview.builder code looks like:
ListView.builder(
controller: _scrollController,
shrinkWrap: true,
itemBuilder: (BuildContext context, int index) {
print("INDEX: $index");
return _showOrder(index);
},
itemCount: orders.length,
),
This is the code for _showOrder():
Widget _showOrder(int i) {
String _currOrderStatus = orders[i].orderStatus;
/// _selectedOrderStatus is just a String which changes depending on the selected category of orders.
/// e.g. "New" or "Past"
return _currOrderStatus == _selectedOrderStatus
? ShowMyOrderWidget()
: SizedBox(
/// AND FOR SOME REALLY WEIRD REASON, THIS FIXES THE PROBLEM
/// It works with any height, as long as it's not 0, but if I have a lot of them, then the
/// layout gets spaced out and messy. With such a low number, this is highly unlikely, but
/// still seems like a stupid fix.
/// Why does this work? Why is this happening? Is there a better way to fix it?
height: 0.000000001,
);
}
And I just call setState() in the onPressed() function of those buttons on top of the screen.
Changing the items inside the ListView doesn't reset scroll position.
Since you're already assigning a ScrollController to your ListView, try calling "jumpTo(0.0)" to reset it's scroll position before calling setState().
_scrollController.jumpTo(0.0);
Let's say, I have a chat screen that looks like this.
Now, when the user clicks the "Press when ready" button, the method fetchNewQuestion() is called.
My intention is that this will make a HTTP request, and display the result using
_buildUsersReply(httpResponse);
But, the problem is that this return must be made inside the current scaffold's widget as a child under the existing children, so that it is built at the bottom with the previous ones still there. The result would be like this:
You can find my complete code here.
Is this possible to be done pro-grammatically? Or do I have to change the concept of how I do this?
[Update, I now understand that my approach above is wrong and I have to use a listview builder. CurrentStatus below shows my progress towards achieving that goal.]
Current status:
I have built a list of Widgets:
List<Widget> chatScreenWidgets = [];
And on setState, I am updating that with a new Widget using this:
setState(() { chatScreenWidgets.add(_buildUsersReply("I think there were 35 humans and one horse.")); });
Now at this point, I am not sure how to pass the widget inside the scaffold. I have written some code that does not work. For instance, I tried this:
Code in the image below and in the gist here:
Just for future reference, here is what I really needed to do:
1. Create a list of widgets
List<Widget> chatScreenWidgets = [];
2. Inside my method, I needed to use a setState in order to add elements to that list. Every widget I add to this will be displayed on ths Scaffold.
`setState(() {
chatScreenWidgets.add(_buildUsersReply("Some Text"));
});`
3. And then, load that inside my Scaffold, I used an itemBuilder in order to return a list of widgets to my ListView. I already had that ListView (where I was manually adding children). Now this just returns them through the setState method inside my business logic method (in this case, fetchNewQuestion()).
body: Stack(
children: <Widget>[
Padding(
padding: EdgeInsets.only(bottom: 0),
child: new ListView.builder(
physics: BouncingScrollPhysics(),
padding: EdgeInsets.symmetric(horizontal: 25),
itemCount: chatScreenWidgets.length,
itemBuilder: (BuildContext context, int itemCount) {
return chatScreenWidgets[itemCount];
}
),
),
],
),
);`
I hope this helps future flutter engineers!
forget about the scaffold the idea is about what you really want to change, lets say it is
a list and your getting the data from an array if you update the array, then the list will update,if it is another type widgets then you can handle it in a different way i will edit this answer if you clarify what each part does in your widget as i cant read your full code.
first you have to create an object with two attributes one is the type of the row(if it is a user replay or other messages) and the second attribute is the string or the text.
now create a global list in the listview class from the above object, so you get the data from the user or even as a news and you create a new object from the above class and add your data to it and add it to the list.
item builder returns a widget so according to the the widget that you return the row will be set , so according to the data in the object call one of your functions that return the views like _buildUsersReply(string text).
if you have any other question you can ask :) if this what you need please mark it as the answer.
I am running into a globalKey error after I navigate from Screen A to Screen B and click a "Cancel" button to go back to Screen A.
It seems like the issue is that Screen B is either
A) Not being disposed of correctly
B) Is not doing something that it otherwise could
And I don't actually know:
What bad things are happening if I just remove the use of a globalKey? (as to get a better understanding of the fundamentals)
How can I correctly resolve this issue?
StatefulWidget documentation states:enter link description here
A StatefulWidget keeps the same State object when moving from one
location in the tree to another if its creator used a GlobalKey for
its key. Because a widget with a GlobalKey can be used in at most one
location in the tree, a widget that uses a GlobalKey has at most one
associated element. The framework takes advantage of this property
when moving a widget with a global key from one location in the tree
to another by grafting the (unique) subtree associated with that
widget from the old location to the new location (instead of
recreating the subtree at the new location). The State objects
associated with StatefulWidget are grafted along with the rest of the
subtree, which means the State object is reused (instead of being
recreated) in the new location. However, in order to be eligible for
grafting, the widget must be inserted into the new location in the
same animation frame in which it was removed from the old location.
Console Error Output:
══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown while finalizing the widget tree:
Duplicate GlobalKey detected in widget tree.
The following GlobalKey was specified multiple times in the widget tree. This will lead to parts of
the widget tree being truncated unexpectedly, because the second time a key is seen, the previous
instance is moved to the new location. The key was:
- [LabeledGlobalKey<FormFieldState<String>>#3c76d]
This was determined by noticing that after the widget with the above global key was moved out of its
previous parent, that previous parent never updated during this frame, meaning that it either did
not update at all or updated before the widget was moved, in either case implying that it still
thinks that it should have a child with that global key.
The specific parent that did not update after having one or more children forcibly removed due to
GlobalKey reparenting is:
- Column(direction: vertical, mainAxisAlignment: start, crossAxisAlignment: center, renderObject:
RenderFlex#7595c relayoutBoundary=up1 NEEDS-PAINT)
A GlobalKey can only be specified on one widget at a time in the widget tree.
So this part of the error output:
previous parent never updated during this frame, meaning that it
either did not update at all or updated before the widget was moved
makes me think there was some opportunity for my old Stateful widget to do something (either reposition itself or release something as to be disposed correctly.
This seems to be failing in framework.dart on assert(_children.contains(child)):
#override
void forgetChild(Element child) {
assert(_children.contains(child));
assert(!_forgottenChildren.contains(child));
_forgottenChildren.add(child);
}
In my case, it likes a hot reload bug. Just restart debugging works for me.
Remove the static and final type from the key variable so if
static final GlobalKey<FormState> _abcKey = GlobalKey<FormState>();
change it to
GlobalKey<FormState> _abcKey = GlobalKey<FormState>();
Thanks to Gunter's commments, I determined that this is because the Screens are not being properly disposed.
Flutter's pushReplacement makes a call to Route.dispose which will ultimately dispose the screen.
I am still unsure as to this comes into play:
widget must be inserted into the new location in the same animation
frame
I'm not sure what situation would benefit from such trickery. However, my problem is solved. I just need to make a call to pop or replace.
Here are the available options:
Use push from A to B and just Navigator.pop from B
Use pushReplacement from A to B and from B to A
I've recently started playing with Fluro for routing and there are a few more ways to to handle these situations (Note the optional argument replace):
Use router.navigateTo(context, route, replace: false) from A to B and Navigator.pop from B
Use router.navigateTo(context, route, replace: true) from A to B the same from B to A (the key is replace: true)
Make sure that you don't have a Form parent and a Form child with the same key
I had this issue too.
I had a four screen bottom tabbed application and a 'logout' method.
However, that logout method was calling a pushReplacementNamed.
This prevented the class that held the global keys (different from the logout function) from calling dispose.
The resolution was to change pushReplacementNamed with popAndPushNamed to get back to my 'login' screen.
Best way to solve that, which worked for me:
class _HomeScreenState extends State<HomeScreen> {
GlobalKey<FormState> _homeKey = GlobalKey<FormState>(debugLabel: '_homeScreenkey');
#override
Widget build(BuildContext context) {
return Container(
key: _homeKey,
);
}
}
In my case I wanted to use the static GlobalKey<ScaffoldState> _scaffoldKey but when I used the same widget multiple times it gave this duplicate error.
I wanted to give it a unique string and still use this scaffold state.
So I ended up using:
static GlobalObjectKey<ScaffoldState> _scaffoldKey
and in the initState:
_scaffoldKey = new GlobalObjectKey<ScaffoldState>(id);
Edit:
Actually, silly me. I just simply removed the static and made it GlobalKey again :)
please take SingleChildScrollview:
and after if you use the bloc pettern then use strem with broadcast
code is here:
body: Container(
decoration: BoxDecoration(
image: DecorationImage(
image: AssetImage('assets/images/abcd.jpg'),
fit: BoxFit.cover,
),
),
child: Container(child:Form(key: _key,
child: Padding(
padding: EdgeInsets.symmetric(vertical: 100.0, horizontal: 20.0),
child: SingleChildScrollView(child:Column(
children: <Widget>[
Padding(
padding: const EdgeInsets.all(10.0),
child: Image.asset('assets/images/logo.png', height: 80, width:80,),
),
email(),
Padding(
padding: EdgeInsets.all(5.0),
),
password(),
row(context),
],
),
),
),
),
),
),
resizeToAvoidBottomPadding: false,
);
}
and the bloc pettern code is here:
final _email = StreamController<String>.broadcast();
final _password = StreamController<String>.broadcast();
Stream<String> get email => _email.stream.transform(validateEmail);
Stream<String> get password=> _password.stream.transform(validatepassword);
Function(String) get changeEmail=> _email.sink.add;
Function(String) get changePassword => _password.sink.add;
dispose(){
_email.close();
_password.close();
}
}
final bloc=Bloc();
I had similar issue on a StatelessWidget class, Converted it to StatefulWidget and error is gone.
If you have multiple forms with different widgets, you must use separate GlobalKey for each form. Like I have two forms, one with Company signup & one with Profile. So, I declared
GlobalKey<FormState> signUpCompanyKey = GlobalKey<FormState>();
GlobalKey<FormState> signUpProfileKey = GlobalKey<FormState>();
This happened to me, what I did was enclosed the whole view into a navigator using an extension I made
Widget addNavigator() => Navigator(
onGenerateRoute: (_) => MaterialPageRoute(
builder: (context2) => Builder(
builder: (context) => this,
),
),
);
I also got this error. There was a static bloc object in a class and I removed the static keyword which fixed the error.
Events should be added by using the BlocProvider anyway.
I also had a similar error. My answer was that after I updated Flutter some widgets no longer had child or children properties. In my case it was the CircleAvatar. The build doesn't error out initially, but when navigating back and forth through the app it will fail.
*Please review all widgets that require a child then review the updated documentation and make sure you're parameters are still correct.