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
Related
I am getting a serious issue that I failed solve so need help.
I was working on a app and there is a provider called CategoryProvider which I used on main screen and there are some forms and a timer so for those things I had to use disposed method (on different screen) and some stateful widgets.
I used MaterialApp inside MultiProvider.
In one of my projects, I am using flutter_bloc package.
In that example, I instantiated bloc inside didChangeDependencies(). So bloc lives during the lifecycle of a screen.
However, my colleague told me not to initialize there. He said bloc should be initialized in app.dart where It can live over the lifecycle of the app. Because, in cases where there should be multiple screens on tablet, it will break the code.
For example, I have ScreenA, ScreenB and ScreenC. you can go to ScreenB and ScreenC through ScreenA. ScreenB and ScreenC use the same bloc, and bloc is initialized sepearately in each screens. He is saying that if the requirement is to show ScreenB and ScreenC simultaneously on a tablet, it might break the code. How?
Could you suggest me what is best? Or does it depend on the usecase?
It is the best practice to initiate bloc instance during initState as it runs only once per widget lifecycle. didChangeDependencies() may be called multiple times for example if widget is moved in the widget tree or widget gets notified of InheritedWidget change, so bloc will get new instance like that and you dont want that. What your collegue is talking is more about BlocProviders scope and sure is normal practice, but for some small-usecase-bloc might be redundant. That is up to you to decide does your whole app needs to be in scope of this bloc. Keep in mind that if lazy property is not set to false, your bloc will be created with the first call to it.
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.
I understand the general approach to building UI layouts using Flutter. However, I'm still unclear which classes or UI widgets require a .build() method when I'm creating my own vs. using the defaults generated by the project.
I haven't found a clear explanation yet - even in the Flutter tutorials. They all seem to just gloss over how "the build method takes a BuildContext" and then go on to the next subject without explaining further.
Does anyone have a succinct explanation of the build method and when it is or isn't needed? And more specifically: what does it actually do?
build() method describes the part of the user interface represented by this widget.
The framework calls this method in a number of different situations:
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.
You can find more Here
TLDR: The build methode is used to create a new widget tree by placing the Widget reurned in the page tree. This method is essentially called when you create or update the widget (by calling setState((){})
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.