Entity moving after setting motion to zero

[charsmud]

I am trying to keep all entities within a certain bounds from moving.  Currently, this is working for mobs only, and all other entities (Item drops, falling sand/gravel, prime TNT, etc) keep falling at a slower rate than normal.

 

public void updateEntity()
{
	World world = this.worldObj;
	int x = this.xCoord;
	int y = this.yCoord;
	int z = this.zCoord;

	updateWalls(world, x, y, z);

	if(world.isBlockIndirectlyGettingPowered(x, y, z))
	{
		List<Entity> entitiesToFreeze = world.getEntitiesWithinAABB(Entity.class, AxisAlignedBB.getBoundingBox(this.xCoord,  0, this.zCoord, xcoord, 128, zcoord));
		if(entitiesToFreeze.size() > 0)
		{
			for(int i = 0; i < entitiesToFreeze.size(); i++)
			{
				Entity e = entitiesToFreeze.get(i);
				System.out.println(e);
				if(e instanceof EntityPlayer)
				{

				}
				else
				{
					e.motionX = 0.0D;
					e.motionY = 0.0D;
					e.motionZ = 0.0D;
					e.setPosition(e.posX, e.posY, e.posZ);
				}
			}
		}
	}
}

 

Any ideas why this might be happening or how to fix it?

[chimera27]

Did you make sure the part of the code stopping it is actually running? If it's actually being run, then you might wanna try e.setVelocity(0, 0, 0); that's what I normally use to modify velocities

[charsmud]

That code is indeed actually running, and I have tried with e.setVelocity(0.0D, 0.0D, 0.0D), and all other entities besides mobs move. 

[charsmud]

Any other ideas why this isn't working?

[MrCaracal]

A quick glance at EntityItem's class reveals the following line (118), in the onUpdate method:

 

this.motionY -= 0.03999999910593033D;

 

I believe this is to simulate gravity. This is the reason why items fall "slower" in your bounds - their acceleration is being effectively zeroed by your code setting their Y motion to zero. They are not stopped because EntityItem has it's own ideas and sometime after you set the entity's velocity to zero, the onUpdate method is called and suddenly they're moving again.

 

Fortunately, EntityItem does check the noClip bounding box before actually moving the items, perhaps you could take advantage of this?

[charsmud]

Could I add that value to motionY to counteract it?

[MrCaracal]

You could certainly try that, but would it work in every case?

 

For instance, what would happen to an entityItem within your bounding box that has a different onUpdate method?

[charsmud]

Hm, I tried this and it did not fix the problem, the entity is still falling. 

[brandon3055]

I think you just need to set entity.velocityChanged to true when you set the motion to 0.

[charsmud]

That did not work either, same behavior.  Is it possible that the block's update loop is being called before the entity, and therefore is updating but being overridden?

[brandon3055]

I just checked some code i used to ass velocity to an entity and i used

entity.addVelocity(d2 / d4 * 8.0D, 5.20000000298023224D, d3 / d4 * 8.0D);
entity.velocityChanged = true;

so maby try

entity.setVelocity(0D, 0D, 0D);
entity.velocityChanged = true;

 

Edit: i just threw that into my LivingUpdateEvent handler and it froze every entity in the world so if it dosnt work you you it must be as you said "the block's update loop is being called before the entity"

[charsmud]

Yep, they're still falling at the same rate. 

[MrCaracal]

I suppose I wasn't overt enough before. I apologize if this comes off as rude.

 

-You are applying a velocity to things that have an acceleration. This is why you're seeing a "slowing" effect but not a "stopping" effect, you're only removing one dimension of the object's displacement. Acceleration is displacement over time squared, if you remove one dimension of this, you're left with a displacement over time, a velocity.

 

-By adding universal "set velocity of this entity" rule, you may encounter odd behavior in the future if ever your mod is installed alongside another that has different movement rules for their entities. There is a nonzero chance of this happening, because Forge provides ways for modders to accomplish precisely this.

 

Instead, I would suggest picking around in the net.minecraft.entity superclass, which is the point from which all the objects you care about extend. There are a few functions in there that may catch your eye, namely setPosition, setVelocity (as brandon3055 suggested), moveEntity, and even some public fields dealing with this stuff such as isCollided (and the ones you've found, motionX, motionY, motionZ). You might be able to trick the entity into thinking it has collided with something while it is in your AABB, thereby halting it's movement, or just repeatedly reset the position of the entity, etc.

 

Failing all that, you could even try to write your own EntityItem that doesn't move, and have that object temporarily replace all the ones in your AABB.

 

What's the scope of "all entities movement", if I might ask? Does it include blocklike entities such piston heads, blocks moved by pistons, TNT and other moving entities like arrows, snowballs, particles, etc? Movement-like block state changes, like lever flips, button pushes, doors being closed?

 

Unrelated: I noticed you're using a List, and lists are iterable. Have you yet opened your heart to the Church of the For-Each-Loop? I love them.

 

for (Entity iteratedEntity : entitiesToFreeze) { // For each element iteratedEntity in the list entitiesToFreeze,
iteratedEntity.extinguish(); // call the extinguish method on it.
}