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
