When I use "StreamSubscription" to instance a value.
StreamSubscription? _webViewListener;
It always show the problem
Cancel instances of dart.async.StreamSubscription.
I promise that there has dispose function.
#override
void dispose() {
_webViewListener!.cancel();
_webViewListener = null;
topController!.dispose();
super.dispose();
}
I have seen the offical methods. However, it does not work for me.
At last, I want to know the function of StreamSubscription. And how to solve this problem.
The most important feature of StreamSubscription is the ability to dispose it. If you don't do this, even though you don't need the subscription anymore, e.g. after leaving a page in Flutter, the stream or the subscription will run forever until the whole program is closed.
If you dispose all subscriptions in the dispose function of your state, you should be able to ignore this warning. Sometimes such warnings are displayed incorrectly.
I'm working on an app that accesses two api's that contain location data that updates regularly (every 5 seconds or so). I want to utilize flutter and flutter_bloc to manage my state, but I'm not sure where the interval would come into play. I understand bloc and how the ui interacts with it with BlocBuilder and BlocProvider, as well as providing a repository that handles the api calls. What I'm not sure about is where to put the interval/timer. My idea was to do a normal bloc setup:
class DataBloc extends Bloc<DataEvent, DataState> {
//constructor
#override
Stream<DataState> mapEventToState(DataEvent event) async* {
if (event is FetchData) {
var data = repository.getData();
yield* _mapFetchDataToState(data);
}
}
}
In the ui:
class MainPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
final dataBloc = BlocProvider.of<DataBloc>(context);
Timer.periodic(fiveSeconds, (Timer t) => dataBloc.add(FetchData()));
return ui stuff
}
}
But I'm not sure if this is the right way to leverage BLoCs and/or timers. But I basically need to call the api on an interval and then have the data update in my ui without a full refresh of all the widgets. They're going to be icons on a google map, and I want the icons to update their locations when the data I pull is updated. How can I accomplish this? For reference, I've looked at the flutter_bloc docs for the timer app here and I thought it was a little overkill and that my idea above was simpler, just not sure if this is the right way. I'm pretty new to flutter so any help would be appreciated.
I'd recomend not to put initializing code inside build. Make it a StatefulWidget and add a dispose method to clear the timer. And inside initState setup the timer.
There's no right way in programming. There are best practices, so as long as it work for your use case is ok.
I would either put the timer into the bloc/cubit because its part of your logic and not of your ui.
Or if you use a repository I would probably use a stream listen to it in your bloc and periodically update the stream in your repo.
But any better suggestions are more than welcome, because I am also looking for some kind of 'the way to do'...
I'm involved with the FlutterSound project which is shipped as a package containing an api that, for the purposes of this question, doesn't contain a widget.
The api needs to handle events when the application changes its state (AppLifecycleState.pause/resume). (we need to stop/resume audio when the app is paused/resumed).
I can see how to do this in a widget using WidgetsBindingObserver but the api needs this same info without having to rely on a widget.
The SchedulerBinding class has a method handleAppLifecycleStateChanged which seems to provide the required info but its unclear how to implement this outside of a widget.
Below is a code sample that can listen to AppLifecycleState change events without directly involving a widget:
import 'package:flutter/material.dart';
class MyLibrary with WidgetsBindingObserver {
AppLifecycleState _state;
AppLifecycleState get state => _state;
MyLibrary() {
WidgetsBinding.instance.addObserver(this);
}
/// make sure the clients of this library invoke the dispose method
/// so that the observer can be unregistered
void dispose() {
WidgetsBinding.instance.removeObserver(this);
}
#override
void didChangeAppLifecycleState(AppLifecycleState state) {
this._state = state;
}
void someFunctionality() {
// your library feature
}
}
Now you can instantiate the library in a flutter widget, for instance, from which point it will start listening to any change in AppLifecycleState.
Please note that, the above code doesn't take care of redundant bindings. For example, if the client of your library is meant to initialize the library more than once in different places, the didChangeAppLifecycleState() method will be triggered multiple times per state change (depending on the number of instances of the library that were created). Also I'm unsure whether the solution proposed conforms to flutter best practices. Nevertheless it solves the problem and hope it helps!
Why are we using dispose() method? I'm little confused about it.
what will be issue occurs If we don't use it and what's the benefit of using it?
#override
void dispose() {
// TODO: implement dispose
super.dispose();
}
dispose method used to release the memory allocated to variables when state object is removed.
For example, if you are using a stream in your application then you have to release memory allocated to the stream controller. Otherwise, your app may get a warning from the PlayStore and AppStore about memory leakage.
dispose() method called automatically from stateful if not defined.
In some cases dispose is required for example in CameraPreview, Timer etc.. you have to close the stream.
When closing the stream is required you have to use it in dispose method.
dispose() is used to execute code when the screen is disposed. Equal to onDestroy() of Android.
Example:
#override
void dispose() {
cameraController?.dispose();
timer.cancel();
super.dispose();
}
The main purpose is to get a callback where in you can free-up all your resources.
If you have initialized any resource in a State, it is important that you close or destroy that resource when that state is disposed.
For e.g: If you are creating a stream in initState of your StatefullWidget, then it is important that you close that stream in dispose method of that state or else it will cause memory leak.
For more details you can refer following comments which I got from the source code of the dispose method of the StatefulWidget:
Called when this object is removed from the tree permanently. The
framework calls this method when this [State] object will never build
again. After the framework calls [dispose], the [State] object is
considered unmounted and the [mounted] property is false. It is an
error to call [setState] at this point. This stage of the lifecycle is
terminal: there is no way to remount a [State] object that has been
disposed. Subclasses should override this method to release any
resources retained by this object (e.g., stop any active animations).
{#macro flutter.widgets.subscriptions} If you override this, make sure
to end your method with a call to super.dispose(). See also: *
[deactivate], which is called prior to [dispose].
Or you can refer the docs: https://api.flutter.dev/flutter/widgets/State/dispose.html
So basically dispose is called when that current state will never be used again. So, if you are having any listeners that are active in that state then they can cause memory leaks and so you should close them.
dispose() method is called when this object is removed from the tree permanently.
For more information, you can refer official Docs: https://api.flutter.dev/flutter/widgets/State/dispose.html
Implementation Example:
#protected
#mustCallSuper
void dispose() {
assert(_debugLifecycleState == _StateLifecycle.ready);
assert(() {
_debugLifecycleState = _StateLifecycle.defunct;
return true;
}());
}
In general terms dispose means freeing the resources before the related object is removed from the focus. By focus, I mean from widget tree or navigation stack whichever is relevant.
When you call dispose on a widget state, the associated widget is supposed to be unmounted which means the widget will never rebuild.
Called when this object is removed from the tree permanently.
The framework calls this method when this State object will never build again.
After the framework calls dispose, the State object is considered unmounted and the mounted
property is false. It is an error to call setState at this point.
This stage of the lifecycle is terminal: there is no way to remount a State object that has been
disposed.
When you call dispose on a bloc, the bloc is supposed to be closing the event and state stream.
Update: New Bloc does not have dispose, instead it has close.
So, This is what dispose basically means.
TL;DR
In general, it means releasing the resources held by the associated instance/object.
The specific meaning of dispose, however, changes with the types of the object on which this method is called.
Well, the answer is in the word dispose of. so imagine you are at a party and there is a table where glasses of water are placed which are plastic glasses. Now you will get one glass use it and dispose (throw it into the dustbin). if you don't do that you place the same glass on the table then there will be no place for other new glasses to be put(memory error). Because the table is full now. The second thing is if you put a glass on the table it's possible there might some drink is left. so if someone else uses that glass then there will be something in it already. (controller already having some value if you don't dispose and the same form or animation controller on the same screen).
happy scene:- all take glasses of drink and dispose them into dustbin so all table will also get space and everybody will get new drink not the old drink of someone.
Practical where I got to know the accurate use- I made a screen where inline editing was needed means you click the button all text converts into text form fields and you change the required values and press that button again to submit(icon change of the same button on editing),
So the word dispose expose everything about its use I hope this real-life example will help a little bit. Thank you
void dispose() {
super.dispose();
_editButtonAnimationController.dispose();
_ageController.dispose();
}
You should not only override dispose method to free up some memory but also to dispose those objects that would otherwise be visible on the screen like a BannerAd.
Say, you have a BannerAd and you don't dispose the instance of bannerAd and navigate back to the previous page, your ad would still be visible on the screen which you don't want. So, you must dispose it like:
#override
void dispose() {
bannerAd?.dispose();
super.dispose();
}
Given all the answers above regarding garbage collection and examples of what to put in dispose(), it seems like the general idea is to dispose/cancel anything that has a future or is tied to hardware access, because you don't want to return to a non-existent object or to block other apps from using that resource (ex.camera, file system, microphone, etc.).
We uses dispose() method in order to stop/avoid memory linkage when state object is removed from the widget tree.
I want to handle a user interaction globally.
what solution is best for it ?
Copy and paste following code in init state your app's base view that has implemented GestureBinding.
#override
void initState() {
GestureBinding.instance.pointerRouter.addGlobalRoute(_globalUserInteractionHandler);
super.initState();
}
_globalUserInteractionHandler(PointerEvent event) {
print("user interaction detected. $event");
}
One way of handling actions anywhere in the app is by using provider. This guide has a complete sample on app state management using provider.