Dragging objects with Box2D Flash

This is ideally the next part after Understanding pixels and meters with Box2D and how to select an object with mouse – part 2, but it’s so important understanding how to drag objects with Box2D that I decided to change the title.

Anyway, in this uncommented example (a commented one will follow) you can select any crate with the mouse and drag it.

Changing at line 89 the mouse_joint.maxForce value will affect gameplay.

This is the source code:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	import Box2D.Dynamics.Joints.*;
	public class cratetest extends Sprite {
		public var m_world:b2World;
		public var m_iterations:int = 10;
		public var m_timeStep:Number = 1/30;
		public var mousePVec:b2Vec2 = new b2Vec2();
		public var real_x_mouse:Number;
		public var real_y_mouse:Number;
		public var pixels_in_a_meter = 30;
		public var worldAABB:b2AABB = new b2AABB();
		public var gravity:b2Vec2 = new b2Vec2(0.0, 10.0);
		public var mouseJoint:b2MouseJoint;
		public function cratetest() {
			addEventListener(Event.ENTER_FRAME, Update, false, 0, true);
			stage.addEventListener(MouseEvent.MOUSE_DOWN, on_mouse_down);
			stage.addEventListener(MouseEvent.MOUSE_UP, on_mouse_up);
			worldAABB.lowerBound.Set(-100.0, -100.0);
			worldAABB.upperBound.Set(100.0, 100.0);
			m_world = new b2World(worldAABB, gravity, true);
			var body:b2Body;
			var bodyDef:b2BodyDef;
			var boxDef:b2PolygonDef;
			bodyDef = new b2BodyDef();
			bodyDef.position.Set(8.5, 13.5);
			boxDef = new b2PolygonDef();
			var ground_width = 8.5;
			var ground_height = 0.5;
			boxDef.SetAsBox(ground_width, ground_height);
			boxDef.friction = 0.3;
			boxDef.density = 0;
			bodyDef.userData = new floor();
			bodyDef.userData.width = ground_width * 2 * pixels_in_a_meter;
			bodyDef.userData.height = ground_height * 2 * pixels_in_a_meter;
			addChild(bodyDef.userData);
			body = m_world.CreateBody(bodyDef);
			body.CreateShape(boxDef);
			body.SetMassFromShapes();
			for (var i:int = 1; i <=5; i++) {
				bodyDef = new b2BodyDef();
				bodyDef.position.x = Math.random() * 15 + 1;
				bodyDef.position.y = Math.random();
				var crate_width:Number = 1.5;
				var crate_height:Number = 1.5;
				boxDef = new b2PolygonDef();
				boxDef.SetAsBox(crate_width, crate_height);
				boxDef.density = 1.0;
				boxDef.friction = 0.5;
				boxDef.restitution = 0.2;
				bodyDef.userData = new crate();
				bodyDef.userData.width = crate_width * 2 * pixels_in_a_meter;
				bodyDef.userData.height = crate_height * 2* pixels_in_a_meter;
				body = m_world.CreateBody(bodyDef);
				body.CreateShape(boxDef);
				body.SetMassFromShapes();
				addChild(bodyDef.userData);
			}
		}
		public function Update(e:Event):void {
			m_world.Step(m_timeStep, m_iterations);
			if (mouseJoint) {
				var mouseXWorldPhys = mouseX/pixels_in_a_meter;
				var mouseYWorldPhys = mouseY/pixels_in_a_meter;
				var p2:b2Vec2 = new b2Vec2(mouseXWorldPhys, mouseYWorldPhys);
				mouseJoint.SetTarget(p2);
			}
			for (var bb:b2Body = m_world.m_bodyList; bb; bb = bb.m_next) {
				if (bb.m_userData is Sprite) {
					bb.m_userData.x = bb.GetPosition().x * pixels_in_a_meter;
					bb.m_userData.y = bb.GetPosition().y * pixels_in_a_meter;
					bb.m_userData.rotation = bb.GetAngle() * (180/Math.PI);
				}
			}
		}
		public function on_mouse_down(evt:MouseEvent):void {
			var body:b2Body = GetBodyAtMouse();
			if (body) {
				var mouse_joint:b2MouseJointDef = new b2MouseJointDef;
				mouse_joint.body1 = m_world.GetGroundBody();
				mouse_joint.body2 = body;
				mouse_joint.target.Set(mouseX/pixels_in_a_meter, mouseY/pixels_in_a_meter);
				mouse_joint.maxForce = 10000;
				mouse_joint.timeStep = m_timeStep;
				mouseJoint = m_world.CreateJoint(mouse_joint) as b2MouseJoint;
			}
		}
		public function on_mouse_up(evt:MouseEvent):void {
			if (mouseJoint) {
				m_world.DestroyJoint(mouseJoint);
				mouseJoint = null;
			}
		}
		public function GetBodyAtMouse(includeStatic:Boolean=false):b2Body {
			real_x_mouse = (stage.mouseX)/pixels_in_a_meter;
			real_y_mouse = (stage.mouseY)/pixels_in_a_meter;
			mousePVec.Set(real_x_mouse, real_y_mouse);
			var aabb:b2AABB = new b2AABB();
			aabb.lowerBound.Set(real_x_mouse - 0.001, real_y_mouse - 0.001);
			aabb.upperBound.Set(real_x_mouse + 0.001, real_y_mouse + 0.001);
			var k_maxCount:int = 10;
			var shapes:Array = new Array();
			var count:int = m_world.Query(aabb, shapes, k_maxCount);
			var body:b2Body = null;
			for (var i:int = 0; i < count; ++i) {
				if (shapes[i].m_body.IsStatic() == false || includeStatic) {
					var tShape:b2Shape = shapes[i] as b2Shape;
					var inside:Boolean = tShape.TestPoint(tShape.m_body.GetXForm(), mousePVec);
					if (inside) {
						body = tShape.m_body;
						break;
					}
				}
			}
			return body;
		}
	}
}

And this is the result:

Enjoy dragging and dropping crates with your mouse.

Download the source code.

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