Describes the configuration for an **Element**.
描述元素的配置
Widgets are the central class hierarchyin the Flutter framework.
组件是 Flutter Framework 层次结构中的中心类
A widget is an immutable description of part of a user interface
控件是用户界面****不可变描述的一部分
Widgets can be inflated into elements, which manage the underlying****render tree.
控件能被解析到元素列表中，被底层****呈现树管理
Widgets themselves have no mutable state (all their fields must be final).
If you wish to associate mutable state with a widget, considerusing a
StatefulWidget, which creates a State object (via
StatefulWidget.createState) whenever it is inflated into an element and
incorporatedinto the tree.
控件自身拥有不可变状态（他们的属性必须是final），如果你希望关联到一个可变状态的控件，请考虑使用StatefulWidget，创建一个State 类（通过StatefulWidget.createState），不管如何，他会被解析成元素合并到树中
A given widgetcan 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.
给定的小部件可以在树中包含零次或多次。特别是给定的小部件可以多次放置在树中。每次一个小部件被放置在树中，它被解析成一个Element，这意味着一个多次合并到树中的小部件将被充气
很多次了。
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.
key属性控制一个小部件如何替换树中的另一个小部件。如果两个小部件的runtimeType和key属性分别为operator==，则新小部件依次通过更新底层元素（即通过调用element）来替换旧小部件。用新的小部件更新)。否则，将从树中删除旧元素，将新小部件解析为元素，并将新元素插入到树中。
StatefulWidget and State, for widgets that can build differentlyseveral timesover their lifetime.
StatefulWidget和State，用于在其生命周期中可以多次构建不同的小部件。
InheritedWidget, for widgets that introduce ambientstate that can be read by descendantwidgets.
这个控件引入环境 状态它能够被子控件读取
StatelessWidget, for widgets that always build the same way given a particularconfiguration and ambientstate.
StatelessWidget，用于在给定特定配置和环境状态时始终以相同方式构建的小部件。