how to synchronise removal of b2bodies when firing in game - iphone

Question

In my game i have some monsters and my hero. when i fire some bullets i remove bodies on collision either with monster or ground body, and also added a timer event to remove the bullet as
-(void) removeProjectile:(CCPhysicsSprite*)projectile{
dispatch_time_t removeProjectile = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1* NSEC_PER_SEC));
dispatch_after(removeProjectile, dispatch_get_main_queue(), ^(void){
int i;
for(i=0;i<deadBodyCount;i++)
{
if(deadBodies[i] == projectile.b2Body)
{
break;
}
}
if(i==deadBodyCount)
{
deadBodies[deadBodyCount]=projectile.b2Body;
deadBodyCount++;
}
});
}
but what happens my code crashes, don't know. please help me. If you have any approach to do this.
i am removing deadbodies in
-(void)removeDeadBodies{
for (int i=0; i<deadBodyCount; i++) {
if(deadBodies[i]){
if([self getPSTag:deadBodies[i]]==3){
CCPhysicsSprite *temp=(CCPhysicsSprite *)deadBodies[i]->GetUserData();
NSLog(#"%#",temp);
[[self getChildByTag:kTagParentNode]removeChild:temp cleanup:YES];
}
else if([self getPSTag:deadBodies[i]]==2){
for (int j=0; j<monsterCount; j++) {
if (monsters[j].b2Body==deadBodies[i]) {
[[self getChildByTag:kTagParentNode]removeChild:monsters[j] cleanup:YES];
monsters[j]=NULL;
}
}
}
_world->DestroyBody(deadBodies[i]);
deadBodies[i]=NULL;}
}
deadBodyCount=0;
}
-(int)getPSTag:(b2Body *)body{ //physics sprite tag value from a b2Body
CCPhysicsSprite *sprite=(CCPhysicsSprite*)body->GetUserData();
//Here is the error------------------------------------
if(sprite)
return (int)sprite.tag;
else
return 0;
}
and this is my begin contact method
-(void)beginContact:(b2Contact *)contact{
b2Body *bodyA=contact->GetFixtureA()->GetBody();
b2Body *bodyB=contact->GetFixtureB()->GetBody();
int collideCheck=[self checkForCollisionGroups:bodyA And:bodyB];
if(!collideCheck)
return;
switch (collideCheck) {
case 1:
for (int i=0; i<monsterCount; i++) {
if(monsters[i].isVisible){
//Checking if Projectile hitting the monster
//bodyA is monster and bodyB is projectile
if([self getPSTag:bodyA]==2&&[self getPSTag:bodyB]==3){
[monsters[i] monsterHit];
[self doExplosionAtPoint:monsters[i].position];
deadBodies[deadBodyCount++]=monsters[i].b2Body;
CCPhysicsSprite *x=(CCPhysicsSprite *)bodyB->GetUserData();
for (int i=0; i<10; i++) {
if(deadBodies[i]==x.b2Body)
return;
}
deadBodies[deadBodyCount++]=x.b2Body;
[self.top updateScore:100];
}//bodyA is Projectile and bodyB is Monster
else if([self getPSTag:bodyA]==3&&[self getPSTag:bodyB]==2){//if one of the body is projectile
[monsters[i] monsterHit];
[self doExplosionAtPoint:monsters[i].position];
CCPhysicsSprite *x=(CCPhysicsSprite *)bodyA->GetUserData();
for (int i=0; i<10; i++) {
if(deadBodies[i]==x.b2Body)
return;
}
deadBodies[deadBodyCount++]=x.b2Body;
deadBodies[deadBodyCount++]=monsters[i].b2Body;
[self.top updateScore:100];
}
}
}
break;
case 2:
if (jumping==YES) {
jumping=NO;
}
break;
case 3: //bullet hitting with any object
if([self getPSTag:bodyA]==3){//if one of the body is projectile
CCPhysicsSprite *x=(CCPhysicsSprite *)bodyA->GetUserData();
for (int i=0; i<10; i++) {
if(deadBodies[i]==x.b2Body)
return;
}
deadBodies[deadBodyCount++]=x.b2Body;
}
else if ([self getPSTag:bodyB]==3){
CCPhysicsSprite *x=(CCPhysicsSprite *)bodyB->GetUserData();
for (int i=0; i<10; i++) {
if(deadBodies[i]==x.b2Body)
return;
}
deadBodies[deadBodyCount++]=x.b2Body;
}
break;
case 4:
for (int i=0; i<monsterCount; i++) {
if(monsters[i].isVisible){
if( (bodyA==monsters[i].b2Body&&bodyB==_heroBody)||(bodyA==_heroBody&&bodyB==monsters[i].b2Body)) {
_heroBody->SetLinearVelocity(b2Vec2(-3*hero.scaleX, 5));
lifeCount=[_top updateHealthMeterWithDamage:10];
NSLog(#"collide");
moving=false;
}
}
}
break;
/*case 6:if([self getPSTag:bodyA]==33){
movingTile.position=((CCSprite*)bodyA->GetUserData()).position;
}
else
movingTile.position=((CCSprite*)bodyB->GetUserData()).position;*/
default:
break;
}
}

Answer 1

I looked at your code. I dont understand the use of removeProjectile: CCPhysicsSprite*)projectile{} method while you indirectly actually adding the projectiles to the deadBodies[] in the beginContact(). So i guess here you have two pointers pointing to same body. And when you run the removeDeadBodies() for loop, it actually first deletes the body and then in next iteration when again it gets the dangling pointer with no body then it crashes.
I commented the line [self removeProjectile:projectile]; in the createProjectileAt() method. And now everything is working fine.
I know it might be late to implement but a suggestion, to have better hold on your b2Bodies and safe deletion from world, you can take a struct datatype with data members as a Sprite and a BOOL variable say isDelete. And use this struct as the userdata of the body. So now you can just set this bool variable to YES inside beginContact() call back. And in you update: tick method after the world->step is over just iterate through the world of bodies and check the BOOL var that can be deleted. HEre you can safely delete the bodies. No need to maintain an array of deadBodies etc. Hence there will not be any synchronization issue.