how can I apply gravity to my gameobject in flutters flame library - flutter

Question

I am having trouble applying gravity to a Rect object I want to fall freely along the Y-axis by updating Rect's position In my code with the body.position.y. Here is a snippet of my code :
import 'dart:ui';
import 'package:box2d_flame/box2d.dart';
import 'package:flame/sprite.dart';
import 'package:mimo/mimo-game.dart';
class Mimo {
final MimoGame game;
CircleShape shape;
Body body;
List<Sprite> mimoSprite;
List<Sprite> deadSprite;
double flyingSpriteIndex = 0;
Rect mimoRect;
bool isDead = false;
bool isOffScreen = false;
Mimo(this.game, double x, double y) {
shape = CircleShape(); //build in shape, just set the radius
shape.p.setFrom(Vector2.zero());
shape.radius = .1; //10cm ball
BodyDef bd = BodyDef();
bd.position = new Vector2(x,y);
bd.type = BodyType.DYNAMIC;
body = game.world.createBody(bd);
body.userData = this;
FixtureDef fd = FixtureDef();
fd.restitution = 0.5;
fd.density = 0.05;
fd.friction = 0;
fd.shape = shape;
body.createFixtureFromFixtureDef(fd);
mimoSprite = List();
mimoSprite.add(Sprite('mimo/mimo-1.png'));
mimoSprite.add(Sprite('mimo/mimo-2.png'));
mimoSprite.add(Sprite('mimo/mimo-3.png'));
mimoSprite.add(Sprite('mimo/mimo-4.png'));
mimoSprite.add(Sprite('mimo/mimo-5.png'));
deadSprite = List();
mimoRect = Rect.fromLTWH(body.position.x, body.position.y, game.mimoSize, game.mimoSize);
}
void render(Canvas c) {
mimoSprite[flyingSpriteIndex.toInt()].renderRect(c, mimoRect.inflate(2));
}
void update(double t) {
mimoRect = mimoRect.translate(body.position.x, body.position.y);
}
}
in the update method I use the line mimoRect = mimoRect.translate(body.position.x, body.position.y); so the body.position can update my Rect object in real-time. but nothing happens as my spawned object is always fixed at a position and does not move. I decided to log body.position.y to the console and noticed it does not change.
In my main class, I create a world object like this :
//Needed for Box2D
static const int WORLD_POOL_SIZE = 100;
static const int WORLD_POOL_CONTAINER_SIZE = 10;
//Main physic object -> our game world
World world;
//Zero vector -> no gravity
final Vector2 _gravity = Vector2(0, 4.0);
Body body;
CircleShape shape;
//Scale to get from rad/s to something in the game, I like the number 5
double sensorScale = 5;
//Draw class
Paint paint;
//Initial acceleration -> no movement as its (0,0)
Vector2 acceleration = Vector2.zero();
MimoGame() {
world = new World.withPool(_gravity,
DefaultWorldPool(WORLD_POOL_SIZE, WORLD_POOL_CONTAINER_SIZE));
initialize();
}
Please any idea on what I can do?

Answer 1

You need to use the Box2DGame (flame 0.22.0^) class in flame for your game and the object that you want to apply gravity to needs to extend BodyComponent, then you create an instance of your BodyComponent in your Box2DGame and use the add function to add it to the game loop which will make sure that update and render is called on it every iteration.
There is a good example in the flame source here.
Also remember that if your gravity is positive, your object will be flying upwards.