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.
Related
I have seen in various files about init method under some classes, why we use this method and please also describe other useful methods if you know, Thanks
I have initialized this method but it didn't work properly.
To understand the methods and structure of flutter widgets, you need to understand the lifecycle of flutter screen or flutter application.
So, basically flutter's stateful widgets have really good methods which are
CreateState()
initState()
didChangeDependencies()
build()
deactivate()
dispose()
You can learn more about this on medium
The #initState method is called by Flutter when a widget is inserted into the tree. It's important to note that the initState method is only called once, when the widget is inserted into the tree, and not every time the widget is updated. If you need to perform an operation every time the widget is updated, you can use the didUpdateWidget method instead.
dispose, build, setState, build and much more method are interesting in Flutter, you can try to understand what each method do in this amazing "cookbook" in flutter documentation.
https://docs.flutter.dev/cookbook
I don't completely understand the difference between the both methods and when to use them, anyone help me please?
dispose: 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.
You can find more about it here.
removeListener: Remove a previously registered closure from the list of closures that are notified when the object changes.
removeListener must not be called after dispose has been called.
listener is a widget that calls callbacks in response to common pointer events.
More and ref : dispose, removeListener, listener and SO Question on dispose.
I am trying to understand the method didChangeDependencies and according to the definition
It is Called when a dependency of this State object changes.
what do they mean by this. My guess is when properties in the state you manage per screen change. Am I right. Please I would love to understand better.
Try to think of it the same as initState but just a little later.
initstate is called before the state loads its dependencies and for that reason no context is available and you get an error for that if u use context in initstate. However, didChangeDependencies is called just few moments after the state loads its dependencies and context is available at this moment so here you can use context.
In case BuildContext.dependOnInheritedWigetOfExactType called, or in case the widget is been moved inside of the element tree, didChangeDependencies will is always called.
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 Flutter, StatefulWidget has dispose() and deactivate(). How are they different?
dispose is definitive. deactivate is not.
deactivate is called when a widget may be disposed. But that is not guaranteed.
A typical situation where deactivate is called but not dispose,is when moving widgets in the widget tree using a GlobalKey.
deactivate:
Called when object is removed from the tree. In some cases, the framework will reinsert the State object into another part of the tree (e.g., if the subtree containing this State object is grafted from one location in the tree to another). source
dispose:
Called when this object is removed from the tree permanently. source
By understanding both sentences, you'll see that deactivate will be called for widgets that are removed from the tree, temporarily or permanently, whereas dispose will only be called for widgets being removed permanently.
Thanks to Antonio Oliveira for the links, now I understand.
When a widget is removed from the tree, deactivate is called. However, the framework may decide to insert it again in another part of the widget tree. A good example would be when you reorder a list!
#override
void deactivate() {
super.deactivate();
}
Dispose is called when a widget is permanently removed from the tree. This has reached the end of its life cycle.
#override
void dispose() {
super.dispose();
}
"deactivate" refers to the process of removing a widget from the tree and calling its deactivate method. This happens when the widget's parent changes and the widget is removed from the tree.
"dispose" refers to the process of releasing resources used by a widget. This is usually done in the dispose method of the widget, which is called when the widget is removed from the tree and is no longer needed.
In summary, "deactivate" is a step in the process of removing a widget from the tree, while "dispose" is a method that is called when the widget is removed and is used to release resources.