Dragging objects with Box2D Flash
- November 20, 2008 by Emanuele Feronato
- Filed under Actionscript 3, Box2D, Flash, Game design | 23 Comments
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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 | 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.
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(20 votes, average: 4.40 out of 5)

Very cool!
Nice tutorial I was having an idea to create a game by dragging the boxes and it helps a lot. :)
Can you please make a turoial how to make game like mytheria
COOOOOOOOOLLLLL!!!!!! :D
excelente
[...] new cool libraries are written in AS3, just think about BOX2D and the games you can make with [...]
Won’t Work
1046: Type was not found or was not a compile-time constant: b2Body. “public function GetBodyAtMouse(includeStatic:Boolean=false):b2Body {“
hi,I’m very much interested to create games in flash can u help me
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?
solucionado!!! Thanksssssss a lot
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 ????
Please can you fully explain the code iside the function function GetBodyAtMouse.
Thanks
thankssssss a lot !
regarding this function (GetBodyAtMouse), i did not understand all of them .
especially line 112. what is it used for?
Great!
Now where is Box3D???
I am SERIOUS!!!!
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.
Great !!
I would like to know how can I delineate the area of drag and drop ?
PS: sorry for my bad benglish.. :/
Emanuele, might I ask why you always put everything in the same class ? is it for the simplicity of the tutorial ?
Having a single script to show is easier to display and understand for newbies.
Emanuele, great job. thanks for u r code its really helps us to move forward with box2d. keep it up.
I am trying to integrate this into my own Box2D project where i have about 20 objects that are just falling to the ground, but when I click on an object that is falling it will just STOP – it won’t let me drag it…do you know why that might be?
The Vector class doesn’t work with Flash CS3 and Flash player 9 :/
i get this error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at HelloBoxWorld/GetBodyAtMouse()
the line that flash says is wrong is this one:
var count:int = m_world.Query(aabb, shapes, k_maxCount);
can someone help
I met a very strange problem!
If I compile this source in Flex Builder with Flex SDK without any change except embed those necessary images with meta tag, then everything stops in the begining.
I tried other sources too, they all failed! Have no idea why. Can everybody help me on this?
Thanks in advanced.