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| Describes the configuration for an [Element].
Widgets are the central class hierarchy in the Flutter framework. A widget
is an immutable description of part of a user interface. Widgets can be
inflated into elements, which manage the underlying render tree.
Widgets themselves have no mutable state (all their fields must be final).
If you wish to associate mutable state with a widget, consider using a
[StatefulWidget], which creates a [State] object (via
[StatefulWidget.createState]) whenever it is inflated into an element and
incorporated into the tree.
A given widget can be included in the tree zero or more times. In particular
a given widget can be placed in the tree multiple times. Each time a widget
is placed in the tree, it is inflated into an [Element], which means a
widget that is incorporated into the tree multiple times will be inflated
multiple times.
The [key] property controls how one widget replaces another widget in the
tree. If the [runtimeType] and [key] properties of the two widgets are
[operator==], respectively, then the new widget replaces the old widget by
updating the underlying element (i.e., by calling [Element.update] with the
new widget). Otherwise, the old element is removed from the tree, the new
widget is inflated into an element, and the new element is inserted into the
tree.
See also:
* [StatefulWidget] and [State], for widgets that can build differently
several times over their lifetime.
* [InheritedWidget], for widgets that introduce ambient state that can
be read by descendant widgets.
* [StatelessWidget], for widgets that always build the same way given a
particular configuration and ambient state.
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