问题
I want to use the MouseJoint in Java for Android. I am new in box2d and cocos2d. I don't know how to use the mouse joint.
回答1:
i recomend you to see this tutorial example. Copy from it (sorry i don't like broken links :) ) ...
you drag your body with the help of the MouseJoint, it will collide with the other bodys in the world and apply force to them.
Box2d - Manual - http://www.box2d.org/manual.html#_Toc258082974
8.10 Mouse Joint
The mouse joint is used in the testbed to manipulate bodies with the mouse. It attempts to drive a point on a body towards the current position of the cursor. There is no restriction on rotation.
The mouse joint definition has a target point, maximum force, frequency, and damping ratio. The target point initially coincides with the body’s anchor point. The maximum force is used to prevent violent reactions when multiple dynamic bodies interact. You can make this as large as you like. The frequency and damping ratio are used to create a spring/damper effect similar to the distance joint.
Many users have tried to adapt the mouse joint for game play. Users often want to achieve precise positioning and instantaneous response. The mouse joint doesn’t work very well in that context. You may wish to consider using kinematic bodies instead.
So let's start..
- You have to create your PhysicWorld and at least one body in it. ( Checkout the PhysicExample how to.. )
- MouseJoint method
public MouseJoint createMouseJoint(AnimatedSprite box , float x, float y)
{
final Body boxBody =
this.mPhysicsWorld.getPhysicsConnectorManager().findBodyByShape(box);
Vector2 v = boxBody.getWorldPoint(
new Vector2(x/pixelToMeteRatio, y/pixelToMeteRatio)
);
MouseJointDef mjd = new MouseJointDef();
mjd.bodyA = groundBody;
mjd.bodyB = boxBody;
mjd.dampingRatio = 0.2f;
mjd.frequencyHz = 30;
mjd.maxForce = (float) (200.0f * boxBody.getMass());
mjd.collideConnected = true;
mjd.target.set(v);
return (MouseJoint) this.mPhysicsWorld.createJoint(mjd);
}
- Touching Body
we have to override our onAreaTouched method to create an MouseJoint anchor-point on the touch position.
MouseJoint mjActive = null;
private float pixelToMeteRatio = PhysicsConstants.PIXEL_TO_METER_RATIO_DEFAULT;
@Override
public boolean onAreaTouched(
final TouchEvent pSceneTouchEvent,
final ITouchArea pTouchArea ,
final float pTouchAreaLocalX,
final float pTouchAreaLocalY )
{
if(pSceneTouchEvent.getAction() == MotionEvent.ACTION_DOWN) {
this.runOnUpdateThread(new Runnable() {
@Override
public void run() {
final AnimatedSprite face = (AnimatedSprite)pTouchArea; //The touched body
//If we have a active MouseJoint, we are just moving arround don't create an 2nd one.
if( mjActive == null)
{
Vector2 vector = new Vector2(pTouchAreaLocalX/pixelToMeteRatio,pTouchAreaLocalY/pixelToMeteRatio);
//=====================================
// GROUNDBODY - Used for the MouseJoint
//=====================================
BodyDef groundBodyDef = new BodyDef();
groundBodyDef.position.set(vector);
groundBody = mPhysicsWorld.createBody(groundBodyDef);
//====================================
// CREATE THE MOUSEJOINT
//====================================
mjActive = PhysicsJumpExample.this.createMouseJoint(face, pTouchAreaLocalX, pTouchAreaLocalY);
}
}});
return true;
}
return false;
}
- Moving the body
We are moving our finger over the scene, so we have to move the MouseJoint too. If we release the finger.. we must destroy the MouseJoint..
@Override
public boolean onSceneTouchEvent(final Scene pScene, final TouchEvent pSceneTouchEvent) {
if(this.mPhysicsWorld != null) {
if(pSceneTouchEvent.getAction() == MotionEvent.ACTION_MOVE) {
this.runOnUpdateThread(new Runnable() {
@Override
public void run() {
if( mjActive != null ){ //If the MJ is active move it ..
// =========================================
// MOVE THE MOUSEJOINT WITH THE FINGER..
// =========================================
Vecotr2 vec = new Vector2(pSceneTouchEvent.getX()/pixelToMeteRatio, pSceneTouchEvent.getY()/pixelToMeteRatio);
mjActive.setTarget(vec);
}
}});
return true;
}
//===========================================
// RELEASE THE FINGER FROM THE SCENE..
//===========================================
if( pSceneTouchEvent.getAction() == MotionEvent.ACTION_UP ||
pSceneTouchEvent.getAction() == MotionEvent.ACTION_CANCEL
) {
this.runOnUpdateThread(new Runnable() {
@Override
public void run() {
if( mjActive != null )
{
//======================================
// DESTROY OUR MOUSEJOINT
//======================================
PhysicsJumpExample.this.mPhysicsWorld.destroyJoint(mjActive);
PhysicsJumpExample.this.mPhysicsWorld.destroyBody(groundBody);
mjActive = null;
}
}});
return true;
}
return false;
}
FYI: To fit your needs, you have to play with this settings ( in the createMouseJoint method )
mjd.dampingRatio = 0.2f;
mjd.frequencyHz = 30;
mjd.maxForce = (float) (200.0f * boxBody.getMass());
来源：https://stackoverflow.com/questions/7594620/how-to-use-the-mouse-joint-in-android