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19dfde6989
* Remove unnecessary null checks in flutter/foundation * Remove unnecessary null checks in flutter/services * Remove unnecessary null checks in flutter/physics
96 lines
3.7 KiB
Dart
96 lines
3.7 KiB
Dart
// Copyright 2014 The Flutter Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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import 'package:flutter/foundation.dart';
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import 'simulation.dart';
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// Examples can assume:
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// late AnimationController _controller;
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/// A simulation that applies a constant accelerating force.
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///
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/// Models a particle that follows Newton's second law of motion. The simulation
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/// ends when the position exceeds a defined threshold.
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///
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/// {@tool snippet}
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///
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/// This method triggers an [AnimationController] (a previously constructed
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/// `_controller` field) to simulate a fall of 300 pixels.
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///
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/// ```dart
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/// void _startFall() {
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/// _controller.animateWith(GravitySimulation(
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/// 10.0, // acceleration, pixels per second per second
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/// 0.0, // starting position, pixels
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/// 300.0, // ending position, pixels
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/// 0.0, // starting velocity, pixels per second
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/// ));
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/// }
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/// ```
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/// {@end-tool}
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///
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/// This [AnimationController] could be used with an [AnimatedBuilder] to
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/// animate the position of a child as if it was falling.
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///
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/// The end distance threshold (the constructor's third argument) must be
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/// specified as a positive number but can be reached in either the positive or
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/// negative direction. For example (assuming negative numbers represent higher
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/// physical positions than positive numbers, as is the case with the normal
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/// [Canvas] coordinate system), if the acceleration is positive ("down") the
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/// starting velocity is negative ("up"), and the starting distance is zero, the
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/// particle will climb from the origin, reach a plateau, then fall back towards
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/// and past the origin. If the end distance threshold is less than the height
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/// of the plateau, then the simulation will end during the climb; otherwise, it
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/// will end during the fall, after the particle travels below the origin by
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/// that distance.
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///
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/// See also:
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///
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/// * [Curves.bounceOut], a [Curve] that has a similar aesthetics but includes
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/// a bouncing effect.
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class GravitySimulation extends Simulation {
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/// Creates a [GravitySimulation] using the given arguments, which are,
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/// respectively: an acceleration that is to be applied continually over time;
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/// an initial position relative to an origin; the magnitude of the distance
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/// from that origin beyond which (in either direction) to consider the
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/// simulation to be "done", which must be positive; and an initial velocity.
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///
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/// The initial position and maximum distance are measured in arbitrary length
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/// units L from an arbitrary origin. The units will match those used for [x].
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///
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/// The time unit T used for the arguments to [x], [dx], and [isDone],
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/// combined with the aforementioned length unit, together determine the units
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/// that must be used for the velocity and acceleration arguments: L/T and
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/// L/T² respectively. The same units of velocity are used for the velocity
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/// obtained from [dx].
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GravitySimulation(
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double acceleration,
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double distance,
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double endDistance,
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double velocity,
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) : assert(endDistance >= 0),
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_a = acceleration,
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_x = distance,
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_v = velocity,
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_end = endDistance;
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final double _x;
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final double _v;
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final double _a;
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final double _end;
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@override
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double x(double time) => _x + _v * time + 0.5 * _a * time * time;
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@override
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double dx(double time) => _v + time * _a;
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@override
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bool isDone(double time) => x(time).abs() >= _end;
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@override
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String toString() => '${objectRuntimeType(this, 'GravitySimulation')}(g: ${_a.toStringAsFixed(1)}, x₀: ${_x.toStringAsFixed(1)}, dx₀: ${_v.toStringAsFixed(1)}, xₘₐₓ: ±${_end.toStringAsFixed(1)})';
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}
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