Question

所以我一直在Dart中使用typedef一段时间而不考虑它们实际上是什么。例如，在这段代码中：

new PageRouteBuilder(
  pageBuilder: (BuildContext context, Animation<double> animation1, Animation<double> animation2) => new Test(),
  transitionsBuilder: (BuildContext context, Animation<double> animation1, Animation<double> animation2, Widget child) {
    return new FadeTransition(
      child: child,
      opacity: animation1,
    );
  }
)

pageBuilder属性需要一个名为RoutePageBuilder的typedef，而transitionsBuilder属性需要一个名为RouteTransitionsBuilder的typedef。

对我来说，看起来我只是使用了一个带有typedef预定义参数的属性的函数，但我对此不确定。
另外，这里的输入实际上是什么参数？因为我例如使用Animation<double> animation1作为参数，但实际上并没有创建新的Animation，或者它是什么？如果没有，那么Animation实际上将作为参数传递？

Answer 1

typedef可用于指定我们想要的函数签名要匹配的特定功能。函数签名由a定义函数的参数（包括它们的类型）。返回类型不是功能签名的一部分。其语法如下。

在您的情况下，PageRouteBuilder作为参数：

pageBuilder RoutePageBuilder typedef - 只是一个与RoutePageBuilder typedef签名匹配的函数。
transitionsBuilder RouteTransitionsBuilder typedef - 与pageBuilder完全相同，但匹配RouteTransitionsBuilder签名。

两个typedef（RouteTransitionsBuilder和RoutePageBuilder）类似于单个方法的接口（OO编程）。在这种情况下，typedef RoutePageBuilder强制您作为参数传递一个函数，该函数返回一个Widget，它将上下文和两个Animation作为参数。

如果您想了解有关该功能中传递的参数的更多详细信息，可以在flutter的文档中进行检查，例如：

动画→动画驱动路线的动画过渡和前一路线的前向过渡。只读，继承

或

secondaryAnimation→动画路线的动画被推到这条路线之上。这个动画让这条路线与推进的路线的入口和出口过渡协调这条路线的顶部。只读，继承

PS：如果您浏览flutter代码并到达routes.dart文件，您可以找到记录此部分的注释：

/// Override this method to build the primary content of this route.
  ///
  /// The arguments have the following meanings:
  ///
  ///  * `context`: The context in which the route is being built.
  ///  * [animation]: The animation for this route's transition. When entering,
  ///    the animation runs forward from 0.0 to 1.0. When exiting, this animation
  ///    runs backwards from 1.0 to 0.0.
  ///  * [secondaryAnimation]: The animation for the route being pushed on top of
  ///    this route. This animation lets this route coordinate with the entrance
  ///    and exit transition of routes pushed on top of this route.
  ///
  /// This method is called when the route is first built, and rarely
  /// thereafter. In particular, it is not called again when the route's state
  /// changes. For a builder that is called every time the route's state
  /// changes, consider [buildTransitions]. For widgets that change their
  /// behavior when the route's state changes, consider [ModalRoute.of] to
  /// obtain a reference to the route; this will cause the widget to be rebuilt
  /// each time the route changes state.
  ///
  /// In general, [buildPage] should be used to build the page contents, and
  /// [buildTransitions] for the widgets that change as the page is brought in
  /// and out of view. Avoid using [buildTransitions] for content that never
  /// changes; building such content once from [buildPage] is more efficient.
  Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation);

Answer 2

来自docs：

在Dart中，函数是对象，就像字符串和数字是对象一样。 typedef或函数类型别名（也称为）为函数类型提供了一个名称，您可以在声明字段和返回类型时使用该名称。将函数类型分配给变量时，typedef会保留类型信息。

如果您检查RoutePageBuilder的实现，则为示例：

typedef RoutePageBuilder = Widget Function(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation);

这意味着RoutePageBuilder也称为Function，它返回一个Widget。

然后，如果您检查PageRouteBuilder的源代码，则会发现实例变量pageBuilder的类型为RoutePageBuilder，因此在创建PageRouteBuilder的新实例时您可以执行以下操作：

new PageRouteBuilder(
  pageBuilder: (BuildContext context, Animation<double> animation1, Animation<double> animation2) => new Test(),

new Test()是一个小部件。

Dart

2 个答案: