Sprite Kit: create centred sprites in grid - ios

Question

I want to create a new sprite in a grid if the field is empty. I also want this new sprite to be centred in the empty field. Let's say the field is:
1 1 1
1 1 0
1 1 1
Whenever I click on one of the fields that is filled with a 1, the sprite disappears (this is what I want and this works fine).
Whenever I click on the 0, a new sprite is added with the following code:
UITouch *touch = [touches anyObject];
CGPoint positionInScene = [touch locationInNode:self];
SKSpriteNode *touchedNode = (SKSpriteNode *)[self nodeAtPoint:positionInScene];
if(touchedNode.position.x == 0) { // x is 0 when I click an empty field, aka background
[self addThing:positionInScene];
}
And:
-(void)addThing:(CGPoint)newLocation{
SKSpriteNode *thing = [SKSpriteNode spriteNodeWithImageNamed:#"thing"];
thing.location = newLocation;
[self addChild:thing];
thing.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:thing.size.width/2];
thing.physicsBody.dynamic = NO;
thing.physicsBody.categoryBitMask = somethingCategory;
thing.physicsBody.contactTestBitMask = somethingElseCategory;
thing.physicsBody.collisionBitMask = 0;
}
However, this sprite is centred in the exact coordinates I click, rather than the centre point of the empty field. How can I make this happen?

Answer 1

The following creates an N x M grid with a sprite at each row and column. When the user touches one of the sprites, it rotates the sprite and then removes it from the scene. If a sprite is not found at the touch location, a sprite is added there.
#define kBoardMinX 25
#define kBoardMinY 25
#define kNumCols 3
#define kNumRows 3
#define kSpacing 40
-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
/* Setup your scene here */
self.backgroundColor = [SKColor colorWithRed:0.15 green:0.15 blue:0.3 alpha:1.0];
[self addPiecesToScene];
}
return self;
}
// Convert row and col to the appropriate point (in scene coordinates) in the grid
- (CGPoint) pointFromRow:(NSInteger)row andCol:(NSInteger)col
{
return CGPointMake(kBoardMinX+col*kSpacing, kBoardMinY+(kNumRows-row-1)*kSpacing);
}
// If a sprite is found at (row, col) rotate it, else add a sprite there
- (void) rotatePieceAtRow:(NSInteger)row andCol:(NSInteger)col
{
SKSpriteNode *node = (SKSpriteNode *)[self nodeAtPoint:[self pointFromRow:row andCol:col]];
if (node && (SKNode *)node != self) {
SKAction *rotate = [SKAction rotateByAngle:M_PI*2 duration:2];
SKAction *remove = [SKAction removeFromParent];
SKAction *rotateThenRemove = [SKAction sequence:#[rotate,remove]];
[node runAction:rotateThenRemove];
}
else {
[self addPieceAtRow:row andCol:col];
}
}
- (void) togglePieceAtTouchLocation:(CGPoint)location
{
// Convert touch location to row and column
NSInteger col = (NSInteger)roundf((location.x - kBoardMinX) / kSpacing);
NSInteger row = kNumRows - (NSInteger)roundf((location.y - kBoardMinY) / kSpacing) - 1;
// Check if the touch was within the grid
if (col >= 0 && col < kNumCols && row >= 0 && row < kNumRows) {
[self rotatePieceAtRow:row andCol:col];
}
}
- (void) addPieceAtRow:(NSInteger)row andCol:(NSInteger)col
{
CGSize size = CGSizeMake(32, 32);
SKColor *color = [SKColor whiteColor];
SKSpriteNode *node = [SKSpriteNode spriteNodeWithColor:color size:size];
node.position = [self pointFromRow:row andCol:col];
[self addChild:node];
}
- (void) addPiecesToScene
{
for (int row=0;row<kNumRows;row++) {
for (int col=0;col<kNumCols;col++) {
[self addPieceAtRow:row andCol:col];
}
}
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
/* Called when a touch begins */
for (UITouch *touch in touches) {
CGPoint location = [touch locationInNode:self];
[self togglePieceAtTouchLocation:location];
}
}