I'm using 'package:reorderables/reorderables.dart'
I use the ReorderableColumn which contains a column of widgets. The 'normal' drag/drop works fine when the contents of the widgets are not changing.
The issue is that if the widgets being dragged is 'active', as in there is some activity going on that is causing rebuilds (a file is being uploaded, and there is a progress indicator, status updates etc), the system crashes with messages like:
Error: setState() called after dispose(): _FilePickerWidgetState#174b4(lifecycle state: defunct, not mounted)
This error happens if you call setState() on a State object for a widget that no longer appears in the widget tree (e.g., whose parent widget no longer includes the widget in its build). This error can occur when code calls setState() from a timer or an animation callback.
From what I understand this package removes and inserts the widget from the widget tree for a more realistic visual simulation.
Could you suggest a solution or alternative?
I was able to replace this with the Flutter build-in ReorderableListView - there are less controls available, but I no longer get any errors.
Related
I'm utterly confused regarding the question of state management in Flutter.
So far I have read that the states of widgets have to be nested as deep in the tree as possible (or rather as close to the affected widget as possible) for better performance.
But what if such a nested state widget (InheritedWidget for example) has another state widget somewhere above it? Does the state widget at the lower level not loose it's state when the state widget higher in the tree requests rebuild for all of its descendants?
With this line of thought have I came to the conclusion, that state widgets just have to be at the root in the end, but I guess I'm wrong somehow.
The first part of your question is correct -
If a widget's state changes, this might require all its children to redraw.
But this is precisely why it is important to nest state as deep down in the widget tree as possible!
Assume the contrary, that all state information is stored at the root of the widget tree, at the very top.
Now if any information changes, no matter how small, it will lead to a complete traversal of the widget tree, rebuilding everything in the worst case.
And aside from the tree traversal, your application will also become very memory intensive. If all state is stored at the root, flutter can never tell when it is okay to release some information from memory. If the user leaves some views and the views are dismissed from memory, the information for them will still be stored at the top. And the only way to check wether that information is still needed would be to once again check the whole tree - very expensive!
All of this can be mitigated by putting your state as close as possible to the widget that will consume it. Because then
If the state changes, only a small subtree of the whole widget tree has to be traversed - This is fast.
If a widget is dismissed, flutter can also release all of the state information that has been stored for it. This frees memory.
yes ! every state widget has its own state and they are all independent. if the state of widget X is updated, only widget X will be updated
let suppose that you have an application that sows a family tree. in widget A you get the gradfather from an API, when you click on it you will be redirected to widget B where you can find his childrens, when you click on one of his childrens you go to widget C which shows the childrens of the selected father in widget B, now let's supposse that you want to add one children to this father.
you call the add-children endpoint. the problem here is that widget A will not be updated.
one solution to this, and to understand the state tree logic, is to pass a functionthat updates widget A from widget A to widget B and pass it from widget B to widget C and call it when an update happens either on widget B or C or even on A so widget A gets updated and you got the updated family tree in widget A
So basically flutter have it's own state management that is called setState(() {}) itu will update the state of the screen where setState is called if i have a button class widget in it's own file if i press the button i want to change the button name to something else so the setState will update the state or variables in the button class/widget.
Now how if the button wants to update a state/variables in the different class but in same screen? Since setState only update it's own class, so you to give the button onTap property with function constructer like this
final Function onButtonTap;
then put it on onTap like
onTap:() {
widget.onButtonTap();
}
Then in the screen where you want to update the state just call onButtonTap then use setState there
Both the initState() method and didChangeDependencies() are executed before the build() method of a state object, so why is the context not available inside initState() but available inside didChangeDependencies()?
According to the official docs, didChangeDependencies() is called whenever the dependency of the State object changes. What does dependency mean here, and when does it change? (I am a beginner, so please use layman's term to explain this concept to me. I looked up other similar questions, but was not able to understand this.)
Unrelated to this topic, I understand that flutter creates elements and renderObjects for each widget it encounters in the widget tree and if that widget is a statefulWidget, then a state object is also created and a reference to that state object is held inside the element. My question is when are the element, renderObject and state object associated with a widget completely removed from their corresponding trees? Does this happen when the widget is permanently removed from the widget tree, or are they kept even after the widget is removed from the widget tree and some other widget is displayed in its place? For example, let's say that I have a floating action button on my homepage and on clicking that I'm pushing a namedPage onto the stack using the pushNamed method of the Navigator class and displaying a new page. Let's say that the new page is a StatefulWidget which has a state object associated with it, so when I go back to my home page again using the back button, that StatefulWidget is removed from the widget tree, right? Are the State object, element and renderObject associated with that widget destroyed immediately, or does flutter keep them around? If flutter does keep them around for some time, then how does it decide when to destroy those objects?
Firstly, you have to understand what is BuildContext that usually name as context.
buildContext is: the place in the widget tree in which this widget is placed.
All widgets have a bool this.mounted property. It is turned true
when the buildContext is assigned. It is an error to call setState
when a widget is unmounted.
why unsafe to use context in initState is because the widget is unmounted. means that the widget doesn't have place in the widget tree.
didChangeDependencies(): This method is called immediately after initState on the first time the widget is built.
its safe now because property mounted is true.
for the 3rd, you may know about deactivate() method. here
this is another answer for full explanation of flutter widget lifecylce :Life cycle in flutter
In my flutter app, i use a list view inside a HookConsumerWidget, this list has a filter and once it filters the list tiles have big gaps, and i see this error:
This error happens if you call setState() on a State object for a widget that no longer appears in the widget tree (e.g., whose parent widget no longer includes the widget in its build). This error can occur when code calls setState() from a timer or an animation callback.
i couldn't solve it because its a HookConsumerWidget and its not flexible
try to show some code snippets of the problem or the log messages,
my concern is when you're using HookConsumerWidget you don't need to use setState as you could manage the UI changes directly with listeners or watcher with 'StateProvider' for example.
I've got a parent widget that has a Stack with two children.
When using setState in the parent, without actually changing something (setState with empty body) why are the build methods of the children being called?
Is it not the case that flutter rebuilds only the widgets with dirty params?
Flutter will run the build function for all children (exception being const Widgets). It will not rerender them though. If Flutter finds that nothing changed in the widget, it will just reuse the previous render.
The program has no way of knowing if state has changed, unless it checks by running build.
I noticed that the build method gets called often in a flutter app.
I know that if the states of the page change in a statefulWidget, the build method gets triggered. But I also noticed that the build method is called even if nothing is changed in the app.
Considering the case where you leave the app to itself, is it normal for the build method to get called frequently? If so, why and how often?
Why
The build method is called any time you call setState, your widget's dependencies update, or any of the parent widgets are rebuilt (when setState is called inside of those).
Your widget will depend on any InheritedWidget you use, e.g. Theme.of(context), MediaQuery.of(context) etc.
This means that if the theme changes for example or the screen orientation swaps, your widget will also be rebuilt.
When you use widgets like MaterialApp, Scaffold etc. that are provided by the framework, your widget will be rebuilt a lot because these parent widgets depend on many InheritedWidget's and then are rebuilt, which causes your widget to be rebuilt as well.
How often
There is no number for how many rebuilds are "normal" as this completely depends on your tree size and most importantly widgets are in that tree. If you were to run runApp(Container()), there would be no rebuilds.
Just keep in mind that all of these rebuilds probably have a good reason to occur and Flutter is built for this, so you do not need to worry about this.
The only point you should start worrying is when you have constant rebuilds that are probably caused by some builder (which calls setState internally) you are using incorrectly.
Exactly when
The documentation lists all specific cases when rebuilds can occur:
After calling initState.
After calling didUpdateWidget.
After receiving a call to setState.
After a dependency of this State object changes (e.g., an InheritedWidget referenced by the previous build changes).
After calling deactivate and then reinserting the State object into the tree at another location.
Rebuilds from parent widgets
If you want to understand how InheritedWidget works, see this answer. It also touches when a rebuild in a parent widget causes the subtree to rebuild.
After calling initState.
After calling didUpdateWidget.
After receiving a call to setState.
After a dependency of this State object changes (e.g., an InheritedWidget referenced by the previous build changes).
After calling deactivate and then reinserting the State object into the tree at another location.
Read this for more info https://api.flutter.dev/flutter/widgets/State/build.html