Object< type>是什么？ Dart中的语法含义？ [英] What does the Object<type> syntax mean in Dart?

问题描述

在以下代码示例中，来自 flutter docs ：

In the following code example, from the flutter docs:

class RandomWords extends StatefulWidget {
  @override
  createState() => RandomWordsState();
}

class RandomWordsState extends State<RandomWords> {
  @override
  Widget build(BuildContext context) {
    final wordPair = WordPair.random();
    return Text(wordPair.asPascalCase);
  }
}

State< ; RandomWords> 语法意味着什么？

我知道您可以使用此语法为集合中包含的对象（例如列表）指定类型-列表< String>

I understand that you can specify the type for the objects contained in a collection, like lists, using this syntax - List <String>

但是我不明白 State< RandomWords>背后的动机; 。

此外，如何在 RandomWords 中引用 RandomWordsState 声明，并在 RandomWordsState 声明中引用 RandomWords 吗？

Moreover, how can you reference RandomWordsState in RandomWords declaration and also reference RandomWords in RandomWordsState declaration? Shouldn't that cause a circular reference error or something?

我来自动态类型的语言，例如python，这对我来说有点奇怪，有人可以指点我吗？

I come from dynamically typed languages like python, and this looks a little odd to me, can someone please point me to the right place?

推荐答案

< RandomWords> 是通用的类型参数传递给 State 类。

<RandomWords> is a generic type parameter passed to the State class.

State 类看起来像

abstract class State<T extends StatefulWidget> extends Diagnosticable {

和 RandomWords 将被传递到 T 类型参数，该参数具有一个约束， T 必须是 StatefulWidget的子类。

and RandomWords will be passed to the T type parameter which has a constraint that T needs to be a subclass of StatefulWidget.

State 还有一个使用类型参数的字段和获取器

State also has a field and getter where the type parameter is used

  T get widget => _widget;
  T _widget;

这会导致小部件
类型的属性，该属性提供正确的自动补全和类型检查其子类 RandomWordsState

假设您有

class RandomWords extends StatefulWidget {
  RandomWords({this.fixed});

  final WordPair fixed;

  @override
  createState() => RandomWordsState();
}

class RandomWordsState extends State<RandomWords> {
  @override
  Widget build(BuildContext context) {
    // vvvv here we can access `fixed` in a strongly typed manner
    final wordPair = widget.fixed ?? WordPair.random();
    return Text(wordPair.asPascalCase);
  }
}

另请参见 https://www.dartlang.org/guides/language/language-tour#generics

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