Why do we use the dispose() method in Flutter Dart code?

Question

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();
  }

Answer 1

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.

Answer 2

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();
}

Answer 3

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.

Answer 4

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;
  }());
}

Answer 5

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.

Answer 6

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();
}

Answer 7

We uses dispose() method in order to stop/avoid memory linkage when state object is removed from the widget tree.

Answer 8

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.).

Answer 9

Well the answer is in the word dispose. so imagine you are party and there is table where glasses of water are place which are plastic glasses. Now you will get one glass use it and dispose (throw to dustbin). if you don't do like that you place the same glass on the table then there will be no place for other new glasses to put(memory error). Because table is full now. Second thing is if you put glass on table it's possible there might some drink is left. so if someone else use 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 screen where inline editing was needed means you click 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 use of it. I hope this real life example will help little bit. Thankyou

  void dispose() {
    super.dispose();
    _editButtonAnimationController.dispose();
    _ageController.dispose();
  }