我注意到在网上的一些示例中，扩展 StatefulWidget 的类具有用 final 标记的实例变量。这是为什么？ 我了解 final 关键字的作用。我不明白为什么要使用扩展小部件类的每个实例变量来声明它。

最佳答案

因为StatefulWidget继承了Widget，标记为@immutable，所以StatefulWidget的任何子类也必须是不可变(即所有字段最终)。

如果你创建一个带有非最终字段的 StatefulWidget 子类，它会导致这个 Dart 分析警告:

info: This class inherits from a class marked as @immutable, and therefore should be immutable (all instance fields must be final). (must_be_immutable at [...] lib....dart:23)

以及如何使用 StatefulWidget 的说明来自 StatefulWidget documentation :

StatefulWidget instances themselves are immutable and store their mutable state either in separate State objects that are created by the createState method, or in objects to which that State subscribes, for example Stream or ChangeNotifier objects, to which references are stored in final fields on the StatefulWidget itself.

https://stackoverflow.com/questions/50108234/