A helper to animate a flutter_map's MapController

animated_map_controller.dart

import 'package:flutter/material.dart';
import 'package:flutter_map/flutter_map.dart' show MapController;
import 'package:latlong/latlong.dart';

// ////////////////////////////////////////////////////////////////////////////

class AnimatedMapController {
  /// Creates an animated MapController
  AnimatedMapController({
    @required this.mapController,
    @required TickerProvider tickerProvider,
    this.duration = const Duration(milliseconds: 2500),
    this.curve = Curves.fastOutSlowIn,
  }) {
    _animationController = AnimationController(
      vsync: tickerProvider,
    )..addListener(onAnimationTick);
  }

  /// Holds a reference to the [MapController]
  final MapController mapController;

  /// Holds the default [Duration] for the animations
  final Duration duration;

  /// Holds the easing [Curve] for the animation
  final Curve curve;

  AnimationController _animationController;

  /// Provides the [AnimationController]
  AnimationController get animationController => _animationController;

  Animation<LatLng> _centerAnimation;
  Animation<double> _zoomAnimation;

  bool _willUpdateProp = false;

  /// Signifies if a prop is being updated
  bool get willUpdateProp => _willUpdateProp;

  bool _needsAnimation = false;

  /// Signifies if this instance needs to animate
  bool get needsAnimation => _needsAnimation;

  /// Animates a mapController.move to a given center (and zoom)
  TickerFuture move(LatLng center, [double zoom]) {
    assert(center != null);
    return animate(() {
      this.center = center;
      this.zoom = zoom ?? mapController.zoom;
    });
  }

  /// Release the resources used by this object. The object is no longer usable
  /// after this method is called.
  void dispose() {
    if (animationController.isAnimating) {
      animationController.stop();
    }
    animationController.dispose();
  }

  /// Animates the changed properties
  ///
  /// Changes can be made prior calling the method or within the [animator]
  /// callback.
  TickerFuture animate([Function animator]) => animateIn(duration, animator);

  /// Animates the changed properties, overriding the default [Duration]
  ///
  /// Changes can be made prior calling the method or within the [animator]
  /// callback.
  ///
  /// NOTE: Calling this method will stop any ongoing animation.
  /// Its [orCancel] future will be (silently) rejected.
  TickerFuture animateIn(Duration duration, [Function animator]) {
    assert(duration != null);
    if (animator != null) {
      animator();
    }
    if (needsAnimation) {
      _needsAnimation = false;
      return animationController.animateTo(
        animationController.upperBound,
        duration: duration,
      );
    }
    return TickerFuture.complete();
  }

  /// A convenience getter to get the map's center
  LatLng get center => mapController.center;

  /// Sets the map's center
  ///
  /// Must be set within an [animate] or [animateIn] callback, or the desired
  /// method must be called after the set to run the animation.
  set center(LatLng value) {
    assert(value != null);
    updateProp(() {
      _centerAnimation = animationFor(LatLngTween(
        begin: center,
        end: value,
      ))
        ..onEnd(() => _centerAnimation = null);
      _needsAnimation = true;
    });
  }

  /// A convenience getter to get the map's zoom level
  double get zoom => mapController.zoom;

  /// Sets the map's zoom level
  ///
  /// Must be set within an [animate] or [animateIn] callback, or the desired
  /// method must be called after the set to run the animation.
  set zoom(double value) {
    assert(value != null);
    updateProp(() {
      _zoomAnimation = animationFor(Tween<double>(
        begin: zoom,
        end: value,
      ))
        ..onEnd(() => _zoomAnimation = null);
      _needsAnimation = true;
    });
  }

  /// Returns an [Animation] by chaining the [tween] with the curve and
  /// attaching to the [AnimationController]
  @protected
  Animation<T> animationFor<T>(Tween<T> tween) {
    final curveTween = CurveTween(curve: curve);
    return tween.chain(curveTween).animate(animationController);
  }

  /// Runs on every animation tick to adjust the map's center and/or zoom
  @protected
  void onAnimationTick() {
    if (willUpdateProp) {
      // NOTE: ignore the value reset
      return;
    }
    if (_centerAnimation == null && _zoomAnimation == null) {
      return;
    }
    mapController.move(
      _centerAnimation?.value ?? mapController.center,
      _zoomAnimation?.value ?? mapController.zoom,
    );
  }

  /// Resets the [AnimationController], if needed
  ///
  /// It ensures the [AnimationController] is reset, but retain
  /// the [MapController]'s center and zoom.
  @protected
  void updateProp(Function callback) {
    _willUpdateProp = true;
    if (animationController.value != 0.0 || animationController.isAnimating) {
      animationController.value = 0.0;
    }
    try {
      callback();
    } finally {
      _willUpdateProp = false;
    }
  }
}

// ////////////////////////////////////////////////////////////////////////////

/// An interpolation between two LatLng instances.
///
/// See [Tween] for a discussion on how to use interpolation objects.
class LatLngTween extends Tween<LatLng> {
  /// Creates a [LatLng] tween.
  ///
  /// The [begin] and [end] properties may be null; the null value
  /// is treated as an empty LatLng.
  LatLngTween({LatLng begin, LatLng end}) : super(begin: begin, end: end);

  /// Returns the value this variable has at the given animation clock value.
  @override
  LatLng lerp(double t) {
    assert(t != null);
    if (begin == null && end == null) return null;
    double lat, lng;
    if (begin == null) {
      lat = end.latitude * t;
      lng = end.longitude * t;
    } else if (end == null) {
      lat = begin.latitude * (1.0 - t);
      lng = begin.longitude * (1.0 - t);
    } else {
      lat = lerpDouble(begin.latitude, end.latitude, t);
      lng = lerpDouble(begin.longitude, end.longitude, t);
    }
    return LatLng(lat, lng);
  }

  @protected
  double lerpDouble(double a, double b, double t) {
    if (a == null && b == null) return null;
    a ??= 0.0;
    b ??= 0.0;
    return a + (b - a) * t;
  }
}

// ////////////////////////////////////////////////////////////////////////////

extension EndListener<T> on Animation<T> {
  /// Adds a one-off `completed/dismissed` listener that is automatically
  /// removed
  Function onEnd(Function callback) {
    AnimationStatusListener wrapper;
    wrapper = (AnimationStatus status) {
      if (status == AnimationStatus.completed ||
          status == AnimationStatus.dismissed) {
        removeStatusListener(wrapper);
        callback();
      }
    };
    addStatusListener(wrapper);
    return () {
      removeStatusListener(wrapper);
    };
  }
}

Thanks for response.

FYI: I'm trying to animate the route, traveling along the route and changing the color as I step thru each straight line segment along the route and step thru corners. I have the animation running. It moves smoothly along the route based on the numbers generated by the animator. No problem there.

Ordinarily everything would be fine at this point. But I noticed that there is a jitter on the screen along the green route as though google maps is trying to re-render parts of the green route that has already being drawn, while the remainder of the green route is being rendered. So maybe google maps needs something I have missed, or there is a bug in the rendering or somewhere in the calculation is incorrect or their is some kind of rounding error. I captured the points generated by a regular Animator() in Flutter (that I am using) and the subsequent LatLngs that I generate and plotted them on a map. You can see the errors below. Tiny but apparently enough to cause jitter on the mobile screen.

The original route (from Google's Drive API?) is in purple. The points generated by animator (and my calculations) are in green. I even tried your lerp formula a + (b - a) * t. Also tried a 'snap to line' approach and rounding. All attempts result in a tiny error as shown.

Maybe I'll try it on another mobile device.

Let me know if you run into something like this in the future and figure out how to solve it.

While it's up you can see the full map at https://maps.co/map/5f3fef8911764419538679e9be34

Anyway, fun times :)