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.
Related
This question already has answers here:
What is a Future and how do I use it?
(6 answers)
Closed 1 year ago.
I have some code I need to run in my initState() method as it initialises a late variable widgets in the build method depend on. The problem is the method I want to run is asynchronous as I am loading data from the disk using the Shared Preferences package. I tried making the method asynchronous but I get an error in my IDE saying that the initState() method can't be asynchronous. I tried looking up solutions and they said that you can run them without awaiting in the initState() method. I tried this but the problem is that it doesn't await the initialisation so I get the red error screen saying the variable hasn't been initialised. I originally was going to run this in a constructor of a custom class I had that deals with the data but I also get an error saying that the constructor method can' t be async.
My IDE suggested using the didChangeDependencies() as it can be async, however, the build method doesn't await it.
I tried all the answers in this question (Is there a way to load async data on InitState method?) that didn't suggest a StreamBuilder as I am not trying to load a stream that has to do with Authentication. I also tried to change around the order calling super.initState(); before and after the code, etc. and it didn't work.
make a function that is async and perform your asynchronous work in it ..
then call it in initState without async await keywords but like this
#override
void initState() {
checkLoggedIn().then((user) {
// do anything with your data here
});
super.initState();
}
I was reading on how to use dispose on flutter but I cannot figure it out. If I call the below pagedispose function after Navigator.pushReplacementNamed, I am always getting an error. If I call this function from another class page, I then get errors that it doesn't exist etc.
Would appreciate your guidance on how to clear all running functions etc on a page when transitioning to another Flutter page.
#override
void pagedispose(){
vtimer.cancel();
vcontroller.dispose();
super.dispose();
}
Thanks in advance.
According to the documentation
dispose method
Called when this object is removed from the tree permanently.
Regarding a page, the dispose method is called when the page is removed from the navigation stack. Here is a good explanation of Navigation.
When your widget (page) extends StatefulWidget, it's not mandatory but you can override the dispose method to execute additional instructions depending on your need. The method is called automatically when the page is being removed from navigation tree. Override the method as following
#override
void dispose() {
// your desired instructions here
super.dispose(); // This will free the memory space allocated to the page
}
Nonetheless, the method void pagedispose() cannot be overridden as it ain't a known method of StatefulWidget
How can I end the real-time geolocation service when I close the activity and move on to another one, for example MapReceiver to AppHome, since when I do so the GPS is still active and continues to update my database in Firebase realtime until I close the app completely
You can use dispose method when you leave the page or if you want to close the service. Note that dispose method is only supported inside Stateful widget only.
#override
void dispose(){
super.dispose();
// write code to close your streams,services, etc. here
}
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.
In the documentation it is written but I am not able to understand it.
Called when this object is inserted into the tree.
The framework will call this method exactly once for each State object it creates.
Override this method to perform initialization that depends on the location at which this object was inserted into the tree (i.e., context) or on the widget used to configure this object (i.e., widget).
If a State's build method depends on an object that can itself change state, for example a ChangeNotifier or Stream, or some other object to which one can subscribe to receive notifications, then the State should subscribe to that object during initState, unsubscribe from the old object and subscribe to the new object when it changes in didUpdateWidget, and then unsubscribe from the object in dispose.
You cannot use BuildContext.inheritFromWidgetOfExactType from this method. However, didChangeDependencies will be called immediately following this method, and BuildContext.inheritFromWidgetOfExactType can be used there.
If you override this, make sure your method starts with a call to super.initState().
But I'm not sure about its meaning. Can you explain it?
Credit to #Remi, initState() is a method which is called once when the stateful widget is inserted in the widget tree.
We generally override this method if we need to do some sort of initialisation work like registering a listener because, unlike build(), this method is called once.
And to unregister your listener (or doing some post work), you override dispose()method.
From here
A subclass of State can override initState to do work that needs to happen just once. For example, override initState to configure animations or to subscribe to platform services. Implementations of initState are required to start by calling super.initState
When a state object is no longer needed, the framework calls dispose() on the state object. Override the dispose function to do cleanup work. For example, override dispose to cancel timers or to unsubscribe from platform services. Implementations of dispose typically end by calling super.dispose
Uses of initState()
initState() is a method of class State and it is considered as an important lifecycle method in Flutter. initState() is called only Once and we use it for one time initializations.
Example :
To initialize data that depends on the specific BuildContext.
To initialize data that needs to executed before build().
Subscribe to Streams.
Thank you for the answers, but I will also reiterate what the guys above have state
#overrride
initState() { // this is called when the class is initialized or called for the first time
super.initState(); // this is the material super constructor for init state to link your instance initState to the global initState context
}
Please allow me to quote the content written by others. I think his explanation is excellent.
https://www.geeksforgeeks.org/flutter-initstate/
There are two types of widgets provided in Flutter.
The Stateless Widget
The Stateful Widget
As the name suggests Stateful Widgets are made up of some ‘States’. The initState() is a method that is called when an object for your stateful widget is created and inserted inside the widget tree. It is basically the entry point for the Stateful Widgets. initState() method is called only and only once and is used generally for initializing the previously defined variables of the stateful widget. initState() method is overridden mostly because as mentioned earlier it is called only once in its lifetime. If you want to trigger it again you have to shift the control to an entirely new screen and a new state.