I've tried tracing through the source to answer this but I get a little lost in the Flutter machinery. We pass runApp a Widget, which becomes the root widget of our app. That widget must implement build, which returns a Widget.
Does Flutter just use simple recursion to call build on that returned Widget, and so on, for the Widget returned by each subsequent child's build method?
While recursing, and it encounters a StatelessWidget, does Flutter do a type check to call createState instead of build, and then perform an extra step, calling build on the State that createState returns?
How does the recursive / chain of calls to build methods stop? For example, the Text widget has a build method that returns a RichText widget, which is a MultiChildRenderObjectWidget. Does Flutter again do type checking here and stop calling build and instead invoke createElement, which in the end generates the elements for the Element tree?
It is more complex than a simple recursion.
You can read more about the build and render process of a widget here
https://api.flutter.dev/flutter/widgets/WidgetsBinding/drawFrame.html
While the widgets are inmutable, Elements are the instantiation of them. So while StatelessWidgets and StatefulWidgets are recreated in each build, Elements are updated in the tree.
The StatefulWidget is managed by StatefulElement that keeps the state instance in its instance, so it persits while the StatefulWidget is rebuilt
The docs has a very good explanation of what is an Element too:
https://api.flutter.dev/flutter/widgets/Element-class.html
Related
In the "Performance considerations" section of StatefulWidget documentation, third point mentions the following:
If a subtree does not change, cache the widget that represents that subtree and re-use it each time it can be used. To do this, assign a widget to a final state variable and re-use it in the build method. It is massively more efficient for a widget to be re-used than for a new (but identically-configured) widget to be created. Another caching strategy consists in extracting the mutable part of the widget into a StatefulWidget which accepts a child parameter.
Its not clear that what is "state variable" here, and what to to assign, because "build" term is mentioned in that line, and there is also a build method for every widget,
I mean,
Should the user do final Widget widget = MyWidget(); or final Widget widget = MyWidget().build(context);
As they both return Widget,
Also please explain, that is there any difference between the above statements, and what is the use of Builder widget, if we have build method, or do they have different work, if so, then what ???
thanking you
Never, ever ever ever run the build method on your own, the build method is there for flutter to call for itself with an updated context, you should not be using it. Always do MyWidget()
The Builder widget is useful when you need a newer context for something inside your widget, for example:
Widget build(BuildContext context) { // context crated here
return MaterialApp( // navigator created here, after the context was defined
home: ElevatedButton(
onPressed: () => Navigator.of(context).pop()
),
);
}
This code won't work because the context was created before the navigator (which was created along with a material app), you could wrap the button inside a builder widget to get a newer context that already has a navigator
Of course, calling the build method on your own is not good because you won't be passing the correct context to the widget, instead, let flutter handle the context creating and just use normal constructors.
I have an entity object, for simplicity lets call it now Todo. I want to have a widget where I can edit multiple of these todos simultaneously, something like EditableTodoList (and this would show a list of EditableTodos).
This widget would recieve a List<Todo> and also something like Function onTodoEdited(Todo).
I am trying to use immutable data, so when an EditableTodo is being edited (and the event gets propagated to the widget which holds the EditableTodoList), I'd create a new List<Todo> which contais the updated Todo, but then EditableTodoList would rebuild with all its EditableTodos inside.
How could I optimize this, that only the relevant EditableTodo is rebuilt? I don't want to hack around with mutating the prop original List<Todo> because that hides greatly an important detail.
Background: currently the state is inside a cubit and I am context.select-ing for every EditableTodo list item the corresponding Todo in the state, this way only the edited item rebuilds. I'd like to decouple this EditableTodoList from the cubit.
Theoretically I assume its not possible, as EditableTodoList is given new properties, so it has to rebuild, and that means all subtree gets rebuilt. EditableTodo cannot be const as its prop is not a constant.
But still, what would be the most elegant way of separating EditableTodoList from the cubit? As it only needs a List<Todo> to show, it should be possible somehow to optimize the rendering of not changed Todos
You should consider how widget, element and render trees work in order to be concerned. There is a great video about Flutter's rendering mechanism. When rebuilding a tree, framework compares the two versions of widgets by using only the key and runtime type values. This means that if you change string value of the Text widget, the same render object with mutated data will be used again. New render object won't be created, which means no unnecessary rebuilds will happen on the render tree. Rebuilding the widget tree is not expensive compared to the render tree.
There is a limit on minimizing the the build scope in the ListView. ListView itself should be rebuilt in order to update an item. So you can't target a spesific item to rebuild. But items can reactively rebuild themselves (eg. an item that listens to a stream can rebuild without effecting the others). Therefore immutable collections doesn't matter in this context since the framework doesn't care the value but key and runtime type. You can either pass a completely modified list or a list with just one element changed, it will have the same effect.
The newly created State object is associated with a BuildContext. This
association is permanent: the State object will never change its
BuildContext. However, the BuildContext itself can be moved around the
tree along with its subtree.
what does this statement trying to say? i find it quite subtle. it is from the official documentation of flutter
There are a lot of core concepts here, first of all you need to understand how flutter render the widgets, ill try to make a summary.
At run time, flutter internally manage three trees in order to achieve the high performance: Widget tree, Element tree and RenderObject tree.
I'm not going to get deep into this since is complicated but basically each tree has different responsibilities:
Widget: describe the configuration for an Element. It handle
Configuration.
Element: an instantiation of a Widget at a particular location in the
tree. It manage Life cycle.
RenderObject: handles size, layout, etc. It handle render and
painting aspects.
So, for every widget, Flutter builds a corresponding Element and build the Element Tree.
For Stateless widgets, the relation between widget and the corresponding element is trivial, but for Stateful widgets the underlying Element structure looks a little different. Those elements add a state object, which holds the mutable part of the configuration, a color for example.
The other thing that you should know is that BuildContext is actually a Element.
With that in mind, the meaning of this:
The newly created State object is associated with a BuildContext. This
association is permanent: the State object will never change its
BuildContext. However, the BuildContext itself can be moved around the
tree along with its subtree.
Is trying to say that when you build a Stateful widget, flutter is going to build a BuildContext (an element that hold the widget position, among other properties) and that contexts will hold the mutable State object.
Then the buildContext (element) itself can change (moved on the tree for example), but the thing that never is going to happen is that changing the state object will change the BuildContext. And that's why you can change, for example, the widget color or any mutable property on State object, and it will never change the element position in the tree.
Is a really interesting topic but is not simple. I highly recommend you to check this video and this article that have a deep explanation into the this.
Hope it helps!
I am confused about when to put my code in initState() compared to build() in a stateful widget. I am doing a quiz on flutter's udacity course which has this todo item, which was to move a block of code from build() to initState(). But I don't know the purpose or advantage of doing that. Why not just put all the code in build()?
Is it that build() is called only once while initState() is called on every state change?
Thank you.
This is actually the opposite.
build can be called again in many situations. Such as state change or parent rebuild.
While initState is called only one time.
build should be used only for layout. While initState is usually used for variable initialization.
It's in the comment within the build state of the link you provided.
Widget build(BuildContext context) {
// TODO: Instead of re-creating a list of Categories in every build(),
// save this as a variable inside the State object and create
// the list at initialization (in initState()).
// This way, you also don't have to pass in the list of categories to
// _buildCategoryWidgets()
final categories = <Category>[];
...
Creating Categories List in the Build State will lead to the list being created on every build. This is necessary since you only want it to be created once, so the best place to do this is in initState() since it will only be called once when the state object is created, Thereby eliminating the cost of re-creating the categories on each build.
According to flutter doc:
InitState
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)
build
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).
What does BuildContext do, and what information do we get out of it?
https://docs.flutter.dev/flutter/widgets/BuildContext-class.html is just not clear.
https://flutter.dev/widgets-intro/#basic-widgets on the 9th instance of the term BuildContext there is an example, but it's not clear how it is being used. It's part of a much larger set of code that loses me, and so I am having a hard time understanding just what BuildContext is.
Can someone explain this in simple/very basic terms?
BuildContext is, like it's name is implying, the context in which a specific widget is built.
If you've ever done some React before, that context is kind of similar to React's context (but much smoother to use) ; with a few bonuses.
Generally speaking, there are 2 use cases for context :
Interact with your parents (get/post data mostly)
Once rendered on screen, get your screen size and position
The second point is kinda rare. On the other hand, the first point is used nearly everywhere.
For example, when you want to push a new route, you'll do Navigator.of(context).pushNamed('myRoute').
Notice the context here. It'll be used to get the closest instance of NavigatorState widget above in the tree. Then call the method pushNamed on that instance.
Cool, but when do I want to use it ?
BuildContext is really useful when you want to pass data downward without having to manually assign it to every widgets' configurations for example ; you'll want to access them everywhere. But you don't want to pass it on every single constructor.
You could potentially make a global or a singleton ; but then when confs change your widgets won't automatically rebuild.
In this case, you use InheritedWidget. With it you could potentially write the following :
class Configuration extends InheritedWidget {
final String myConf;
const Configuration({this.myConf, Widget child}): super(child: child);
#override
bool updateShouldNotify(Configuration oldWidget) {
return myConf != oldWidget.myConf;
}
}
And then, use it this way :
void main() {
runApp(
new Configuration(
myConf: "Hello world",
child: new MaterialApp(
// usual stuff here
),
),
);
}
Thanks to that, now everywhere inside your app, you can access these configs using the BuildContext. By doing
final configuration = context.inheritFromWidgetOfExactType(Configuration);
And even cooler is that all widgets who call inheritFromWidgetOfExactType(Configuration) will automatically rebuild when the configurations change.
Awesome right ?
what is the BuildContext object/context?
Before we knowing about BuildCotext, We have to know about the Element object.
What is Element object
(note: As a flutter developer we never worked with Element object, but we worked with an object(known as BuildContext object) that's similar to Element object)
The Element object is the build location of the current widget.
What's really mean by "build location" ?
when the framework builds a widget object by calling its constructor will correspondingly need to create an element object for that widget object.
And this element object represents the build location of that widget.
This element object has many useful instance methods.
Who uses the Element object and its methods ?
They are 02 parties that use the Element object and its methods.
Framework (To create RenderObject tree etc)
Developers (Like us)
What is BuildContext object ?
BuildContext objects are actually Element objects. The BuildContext interface is used to discourage direct manipulation of Element objects.
So BuildContext object = discouraged element object (That contains less number of instance methods compared to the original Element object)
Why framework discouraged the Element object and pass it to us ?
Because Element object has instance methods that must only be needed by the framework itself.
but what happens when we access these methods by us, It's something that should not be done.
So that the reason why framework discouraged the Element object and pass it to us
Ok Now let's talk about the topic
What does BuildContext object do in Flutter ?
BuildContext object has several useful methods to easily perform certain tasks that need to be done in the widget tree.
findAncestorWidgetOfExactType().
Returns the nearest ancestor widget of the given type T.
findAncestorStateOfType().
Returns the State object of the nearest ancestor StatefulWidget.
dependOnInheritedWidgetOfExactType().
Obtains the nearest widget of the given type T, which must be the type of a concrete InheritedWidget subclass, and registers this build context with that widget such that when that widget changes.
[Used by Provider package]
The above methods are mostly used instance methods of BuildContext object if you want to see all the methods of that BuildContext object visit this LINK + see #remi Rousselot's answer.