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// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'package:flutter/foundation.dart';
import 'simulation.dart';
// Examples can assume:
// late AnimationController _controller;
/// A simulation that applies a constant accelerating force.
/// Models a particle that follows Newton's second law of motion. The simulation
/// ends when the position exceeds a defined threshold.
/// {@tool snippet}
/// This method triggers an [AnimationController] (a previously constructed
/// `_controller` field) to simulate a fall of 300 pixels.
/// ```dart
/// void _startFall() {
/// _controller.animateWith(GravitySimulation(
/// 10.0, // acceleration, pixels per second per second
/// 0.0, // starting position, pixels
/// 300.0, // ending position, pixels
/// 0.0, // starting velocity, pixels per second
/// ));
/// }
/// ```
/// {@end-tool}
/// This [AnimationController] could be used with an [AnimatedBuilder] to
/// animate the position of a child as if it was falling.
/// The end distance threshold (the constructor's third argument) must be
/// specified as a positive number but can be reached in either the positive or
/// negative direction. For example (assuming negative numbers represent higher
/// physical positions than positive numbers, as is the case with the normal
/// [Canvas] coordinate system), if the acceleration is positive ("down") the
/// starting velocity is negative ("up"), and the starting distance is zero, the
/// particle will climb from the origin, reach a plateau, then fall back towards
/// and past the origin. If the end distance threshold is less than the height
/// of the plateau, then the simulation will end during the climb; otherwise, it
/// will end during the fall, after the particle travels below the origin by
/// that distance.
/// See also:
/// * [Curves.bounceOut], a [Curve] that has a similar aesthetics but includes
/// a bouncing effect.
class GravitySimulation extends Simulation {
/// Creates a [GravitySimulation] using the given arguments, which are,
/// respectively: an acceleration that is to be applied continually over time;
/// an initial position relative to an origin; the magnitude of the distance
/// from that origin beyond which (in either direction) to consider the
/// simulation to be "done", which must be positive; and an initial velocity.
/// The initial position and maximum distance are measured in arbitrary length
/// units L from an arbitrary origin. The units will match those used for [x].
/// The time unit T used for the arguments to [x], [dx], and [isDone],
/// combined with the aforementioned length unit, together determine the units
/// that must be used for the velocity and acceleration arguments: L/T and
/// L/T² respectively. The same units of velocity are used for the velocity
/// obtained from [dx].
double acceleration,
double distance,
double endDistance,
double velocity,
) : assert(endDistance >= 0),
_a = acceleration,
_x = distance,
_v = velocity,
_end = endDistance;
final double _x;
final double _v;
final double _a;
final double _end;
double x(double time) => _x + _v * time + 0.5 * _a * time * time;
double dx(double time) => _v + time * _a;
bool isDone(double time) => x(time).abs() >= _end;
String toString() => '${objectRuntimeType(this, 'GravitySimulation')}(g: ${_a.toStringAsFixed(1)}, x₀: ${_x.toStringAsFixed(1)}, dx₀: ${_v.toStringAsFixed(1)}, xₘₐₓ: ±${_end.toStringAsFixed(1)})';