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:

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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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19 Responses to “Dragging objects with Box2D Flash”

  1. Cool on November 20th, 2008 8:25 pm

    Very cool!

  2. Terry on November 21st, 2008 5:21 am

    Nice tutorial I was having an idea to create a game by dragging the boxes and it helps a lot. :)

  3. Colin Diam on November 21st, 2008 8:44 am

    Can you please make a turoial how to make game like mytheria

  4. Peter on November 21st, 2008 3:50 pm

    COOOOOOOOOLLLLL!!!!!! :D

  5. zap on November 21st, 2008 4:52 pm

    excelente

  6. Jacob on December 26th, 2008 11:05 am

    Won’t Work

    1046: Type was not found or was not a compile-time constant: b2Body. “public function GetBodyAtMouse(includeStatic:Boolean=false):b2Body {“

  7. vijay on December 28th, 2008 1:35 am

    hi,I’m very much interested to create games in flash can u help me

  8. zap on February 18th, 2009 1:41 pm

    Im trying to put a top limit…

    //arriba
    boxDef.SetAsBox(8.5, 14.5);
    bodyDef.position.Set(8.5, -0.5);
    boxDef.friction = 1;
    boxDef.density = 0;
    bodyDef.userData = new floor();
    bodyDef.userData.width = 8.5 * 2 * pixels_in_a_meter;
    bodyDef.userData.height = 0.5 * 2 * pixels_in_a_meter;
    addChild(bodyDef.userData);
    body = m_world.CreateBody(bodyDef);
    body.CreateShape(boxDef);

    …the limit is in a correct position but my box have a bad movement, what´s the problem?

  9. zap on February 18th, 2009 1:59 pm

    solucionado!!! Thanksssssss a lot

  10. ben on April 3rd, 2009 5:39 pm

    Fixed :
    worked well if addin my b2body a m_userData when created rather than addin userData to the body Def…
    now everything works fine….
    Is a empty b2Body always considered as null ???
    Strange ????

  11. bigLama on June 4th, 2009 4:28 pm

    Please can you fully explain the code iside the function function GetBodyAtMouse.
    Thanks

  12. Godfrey Cao on August 25th, 2009 10:27 am

    thankssssss a lot !
    regarding this function (GetBodyAtMouse), i did not understand all of them .
    especially line 112. what is it used for?

  13. Trevor on September 2nd, 2009 5:19 am

    Great!

    Now where is Box3D???

    I am SERIOUS!!!!

  14. Elf on October 17th, 2009 10:08 pm

    What faster using GetBodyAtMouse() or
    private function on_mouse_down(e:MouseEvent):void
    {
    for(var body:b2Body=world.GetBodyList();body;body=body.m_next)
    {
    if(body.m_userData==e.target) break;
    }
    if (body)
    {
    …do something…
    }
    }
    ?
    but my method depend on object count.

  15. Despecrew on November 3rd, 2009 9:42 pm

    Great !!

    I would like to know how can I delineate the area of drag and drop ?

    PS: sorry for my bad benglish.. :/

  16. Vlad on December 1st, 2009 11:28 pm

    Emanuele, might I ask why you always put everything in the same class ? is it for the simplicity of the tutorial ?

  17. Emanuele Feronato on December 2nd, 2009 11:16 am

    Having a single script to show is easier to display and understand for newbies.

  18. Jaleel on January 22nd, 2010 1:22 pm

    Emanuele, great job. thanks for u r code its really helps us to move forward with box2d. keep it up.

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  1. Goodbye AS2… : Emanuele Feronato on November 26th, 2008 1:03 pm

    [...] new cool libraries are written in AS3, just think about BOX2D and the games you can make with [...]

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