/// <summary>
/// Gets the list of grid positions that a game object occupies (either 1, 2, or 4 different positions).
/// </summary>
/// <param name="gameObject">The game object to check.</param>
/// <returns></returns>
private List<int> GetGridPositions(GameObject gameObject)
{
// Create new list of grid positions
var positions = new List<int>();
// Get the bounding rectangle of the game object
var boundBox = gameObject.BoundingBox;
// Get the grid position for each corner of the game object's bounding box
// Bottom left
int gridPos = GetGridPosition(boundBox.MinX, boundBox.MinY);
positions.Add(gridPos);
// Top left
gridPos = GetGridPosition(boundBox.MinX, boundBox.MaxY);
if (!positions.Contains(gridPos)) positions.Add(gridPos);
// Top right
gridPos = GetGridPosition(boundBox.MaxX, boundBox.MaxY);
if (!positions.Contains(gridPos)) positions.Add(gridPos);
// Bottom right
gridPos = GetGridPosition(boundBox.MaxX, boundBox.MinY);
if (!positions.Contains(gridPos)) positions.Add(gridPos);
// Return the list of positions
return positions;
}
public CollisionEventArgs(GameObject sourceObject, GameObject hitObject)
{
SourceObject = sourceObject;
HitObject = hitObject;
}
/// <summary>
/// Updates the grid position of the specified game object.
/// </summary>
/// <param name="gameObject">The game object to update.</param>
public void UpdateLocation(GameObject gameObject)
{
// We're finished if there's no game object to update (could happen for an object that has had a collision)
if (gameObject == null) return;
// Get the grid positions that the game object occupies
var gridPositions = GetGridPositions(gameObject);
// Add the game object to each grid position list that it occupies
foreach (var gridPos in gridPositions)
{
// Initialize the list of game objects for the grid position if necessary
if (!_gridBins.ContainsKey(gridPos)) _gridBins.Add(gridPos, new List<GameObject>());
// Get the list of game objects associated with the grid position
var gameObjectList = _gridBins[gridPos];
// Add the game object to this position's game object list if it's not already in there
if (!gameObjectList.Contains(gameObject)) gameObjectList.Add(gameObject);
}
// Add the list of positions to the game object bins list
if (_gameObjectBins.ContainsKey(gameObject)) _gameObjectBins.Remove(gameObject);
_gameObjectBins.Add(gameObject, gridPositions);
}
/// <summary>
/// Checks for a collision for the specified game object.
/// </summary>
/// <param name="sourceObject">The game object to check.</param>
public void CheckCollision(GameObject sourceObject)
{
// We're finished if there's no game object to check (could happen for an object that has had a collision)
if (sourceObject == null) return;
// Get the grid positions for the specified game object
var gridPositions = _gameObjectBins[sourceObject];
// Check each game object list for the grid positions and see if there are any possible collisions
foreach (var position in gridPositions)
{
// Get the list of game objects at this position
var gameObjectList = _gridBins[position];
// Only check if there's more than one game object in a grid position
if (gameObjectList.Count > 1)
{
foreach (var go in gameObjectList)
{
// Obviously, checking collisions against the same object is right out
if (go == sourceObject) continue;
// Same type objects don't collide with each other (e.g. Enemy with enemy)
if (go.Type == sourceObject.Type) continue;
// Ship and ship bullets can't collide with each other
if ((go.Type == GameObjectType.Ship && sourceObject.Type == GameObjectType.ShipBullet) || (go.Type == GameObjectType.ShipBullet && sourceObject.Type == GameObjectType.Ship))
continue;
// Enemy ships and enemy bullets can't collide with each other
if ((go.Type == GameObjectType.Enemy && sourceObject.Type == GameObjectType.EnemyBullet) || (go.Type == GameObjectType.EnemyBullet && sourceObject.Type == GameObjectType.Enemy))
continue;
// All other collisions are valid, so check for a real collision (Cocos2d-XNA will give you the point of collision, but I don't
// care about it here)
CCPoint collisionPoint;
if (sourceObject.CollidesWith(go, out collisionPoint))
{
// Let any subscribers know that a collision has occurred
if (Collision != null)
Collision(this, new CollisionEventArgs(sourceObject, go));
return;
}
}
}
}
}
