In Flutter, how do you give one pushed route a custom slide-up transition with PageRouteBuilder, and what do its two animations drive?

## What PageRouteBuilder is A **route** is one entry on a `Navigator` stack; a **page route** (`PageRoute<T>`) is a route that covers the whole screen. `MaterialPageRoute` and `CupertinoPageRoute` pick their animation for you. **`PageRouteBuilder<T>`** is the page route you reach for when one screen needs its own animation and you do not want to write a `PageRoute` subclass: you hand it two callbacks and a few settings, and it is pushed like any other route with `Navigator.of(context).push(...)`. The type argument `T` is the type of the result the route pops with. ## The two callbacks - **`pageBuilder`** — a `RoutePageBuilder`: `(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation)`. It returns the screen itself. The route builds this page once and caches it, so the page is not rebuilt on every animation frame. - **`transitionsBuilder`** — a `RouteTransitionsBuilder`: the same three arguments plus `Widget child`, where `child` is the cached page. The route rebuilds this callback whenever either animation ticks, so it is where the moving parts go: a `SlideTransition`, `FadeTransition`, `ScaleTransition` or any combination of them. - **The default `transitionsBuilder` returns `child` unchanged.** The route's animation still runs for its duration, but nothing reads it, so the new screen simply appears. Supplying only `pageBuilder` is the usual way to get an instant, animation-free push. ## The two animations | Argument | Runs 0.0 to 1.0 when | Runs 1.0 to 0.0 when | |---|---|---| | `animation` | this route is pushed | this route is popped | | `secondaryAnimation` | another route is pushed on top of this one | that covering route is popped | `animation` animates the route **in and out**. `secondaryAnimation` lets the route react while it is **covered** — for example sliding slightly left while the next screen arrives. The covered route only receives a running `secondaryAnimation` when the two routes agree to coordinate: a `MaterialPageRoute` underneath animates only when the route above is also a Material-transition route or offers a delegated transition. A `PageRouteBuilder` offers neither by default, so the screen below it stays still. ## A slide-up in a dating app In a dating app, tapping a match card opens the full profile, which should rise from the bottom of the screen: ```dart Navigator.of(context).push( PageRouteBuilder<void>( pageBuilder: (context, animation, secondaryAnimation) => MatchProfileScreen(match: match), transitionsBuilder: (context, animation, secondaryAnimation, child) { final Animation<Offset> offset = Tween<Offset>( begin: const Offset(0, 1), end: Offset.zero, ).chain(CurveTween(curve: Curves.easeOutCubic)).animate(animation); return SlideTransition(position: offset, child: child); }, ), ); ``` What happens, in order: 1. `push` installs the route and starts its internal controller forward. 2. Each frame, `animation` advances and `transitionsBuilder` returns a `SlideTransition` at the new offset; the cached profile page rides inside it. 3. When the user taps back, `pop` runs the same controller in reverse over `reverseTransitionDuration`, so the profile slides back down. ## Defaults and knobs - `transitionDuration` and `reverseTransitionDuration` — both **300 ms** by default. - `opaque` — **true**: routes underneath stop painting once the transition finishes. Set it to false for a translucent overlay whose background still shows. - `barrierDismissible` — **false**; `barrierColor` and `barrierLabel` — null. - `maintainState` — **true**: the route keeps its state while covered. - `fullscreenDialog` — **false**; `allowSnapshotting` — **true**. ## What you give up The platform transitions carry behaviour, not just visuals. **iOS edge swipe-back** is a gesture detector built into the Cupertino page transition, and **Android predictive back** is a gesture listener built into `PredictiveBackPageTransitionsBuilder`. A `PageRouteBuilder` replaces the whole transition with your callback, so neither exists on that route: iOS users cannot swipe the profile away, and on Android a back gesture pops it only after the user releases, with your reverse animation instead of a live preview. Use `PageRouteBuilder` for a genuinely one-off route; for an app-wide look, a custom `PageTransitionsBuilder` in `ThemeData.pageTransitionsTheme` keeps routes on `MaterialPageRoute` and lets you choose per platform. ## Common mistakes - **Creating an `AnimationController` for the route.** The route already owns one and exposes it to you as `animation`; `transitionsBuilder` only reads it. - **Doing expensive work in `transitionsBuilder`.** It runs on every animation frame, so keep it to transition widgets wrapped around `child`, and build the screen in `pageBuilder`. - **Animating the exit with `secondaryAnimation`.** Popping reverses `animation`; `secondaryAnimation` only describes being covered by another route. - **A transparent background with `opaque: true`.** Once the transition completes the routes underneath stop painting, so the see-through area goes blank. - **Forgetting `reverseTransitionDuration`.** A long, expressive entrance usually wants a shorter exit; the two durations are independent.