从Flutter源代码注释中：

/// Signature for the [PageRouteBuilder] function that builds the route's
/// transitions.
///
/// See [ModalRoute.buildTransitions] for complete definition of the parameters.
typedef Widget RouteTransitionsBuilder(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child);

并进一步：

  /// We've used [PageRouteBuilder] to demonstrate the [buildTransitions] method
  /// here. The body of an override of the [buildTransitions] method would be
  /// defined in the same way.
  ///
  /// When the [Navigator] pushes a route on the top of its stack, the
  /// [secondaryAnimation] can be used to define how the route that was on
  /// the top of the stack leaves the screen. Similarly when the topmost route
  /// is popped, the secondaryAnimation can be used to define how the route
  /// below it reappears on the screen. When the Navigator pushes a new route
  /// on the top of its stack, the old topmost route's secondaryAnimation
  /// runs from 0.0 to 1.0.  When the Navigator pops the topmost route, the
  /// secondaryAnimation for the route below it runs from 1.0 to 0.0.
  ///
  /// The example below adds a transition that's driven by the
  /// [secondaryAnimation]. When this route disappears because a new route has
  /// been pushed on top of it, it translates in the opposite direction of
  /// the new route. Likewise when the route is exposed because the topmost
  /// route has been popped off.
  ///
  /// ```dart
  ///   transitionsBuilder: (
  ///       BuildContext context,
  ///       Animation<double> animation,
  ///       Animation<double> secondaryAnimation,
  ///       Widget child,
  ///   ) {
  ///     return new SlideTransition(
  ///       position: new AlignmentTween(
  ///         begin: const Offset(0.0, 1.0),
  ///         end: Offset.zero,
  ///       ).animate(animation),
  ///       child: new SlideTransition(
  ///         position: new TweenOffset(
  ///           begin: Offset.zero,
  ///           end: const Offset(0.0, 1.0),
  ///         ).animate(secondaryAnimation),
  ///         child: child,
  ///       ),
  ///     );
  ///   }
  /// ```
  ///
  /// In practice the `secondaryAnimation` is used pretty rarely.
  ///
  /// The arguments to this method are as follows:
  ///
  ///  * `context`: The context in which the route is being built.
  ///  * [animation]: When the [Navigator] pushes a route on the top of its stack,
  ///    the new route's primary [animation] runs from 0.0 to 1.0. When the [Navigator]
  ///    pops the topmost route this animation runs from 1.0 to 0.0.
  ///  * [secondaryAnimation]: When the Navigator pushes a new route
  ///    on the top of its stack, the old topmost route's [secondaryAnimation]
  ///    runs from 0.0 to 1.0.  When the [Navigator] pops the topmost route, the
  ///    [secondaryAnimation] for the route below it runs from 1.0 to 0.0.
  ///  * `child`, the page contents.
  ///
  /// See also:
  ///
  ///  * [buildPage], which is used to describe the actual contents of the page,
  ///    and whose result is passed to the `child` argument of this method.

我个人的建议是文档模糊不清或缺少代码；）

最好