didBeginContact entering too early - sprite-kit

Question

I am trying to stick a ball to a spinner when they collide at the contact point. However, it seems as though the didBeginContact is being called before the contact starts. Image is below showing them both spinning together but there is a big space.
Code is below:
#import "GameScene.h"
#implementation GameScene
#synthesize _flowIsON;
NSString *const kFlowTypeRed = #"RED_FLOW_PARTICLE";
const float kRED_DELAY_BETWEEN_PARTICLE_DROP = 0.01; //delay for particle drop in seconds
static const uint32_t kRedParticleCategory = 0x1 << 0;
static const uint32_t kSpinnnerCategory = 0x1 << 1;
NSString *const kStartBtn = #"START_BTN";
NSString *const kLever = #"Lever";
NSString *const START_BTN_TEXT = #"Start Game";
CFTimeInterval lastTime;
-(void)didMoveToView:(SKView *)view {
_bkgNode = (SKSpriteNode *)[self.scene childNodeWithName:#"Background"];
[self initializeScene];
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
for (UITouch *touch in touches) {
CGPoint location = [touch locationInNode: self];
SKNode *node = [self nodeAtPoint:location];
if ([node.name isEqualToString:kStartBtn]) {
[node removeFromParent];
//initalize to ON
_flowIsON = YES;
//[self initializeScene];
}
}
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
}
-(void)update:(CFTimeInterval)currentTime {
float deltaTimeInSeconds = currentTime - lastTime;
//NSLog(#"Time is %f and flow is %d",deltaTimeInSeconds, _flowIsON);
if ((deltaTimeInSeconds > kRED_DELAY_BETWEEN_PARTICLE_DROP)) {
//TBD
SKAction *rotation = [SKAction rotateByAngle: M_PI/8.0 duration:0];
[_spinner runAction:rotation];
//only if its been past 1 second do we set the lasttime to the current time
lastTime = currentTime;
}
}
- (void) initializeScene {
self.physicsWorld.contactDelegate = self;
//create ball
SKSpriteNode *ball = [SKSpriteNode spriteNodeWithImageNamed:#"Ball"];
ball.size = CGSizeMake(50, 50);
ball.position = CGPointMake(320, 1050);
ball.zPosition = 1;
ball.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:25];
ball.physicsBody.restitution = 0.0;
ball.physicsBody.categoryBitMask = kRedParticleCategory;
ball.physicsBody.contactTestBitMask = kSpinnnerCategory;
ball.physicsBody.collisionBitMask = kSpinnnerCategory;
ball.name = #"Ball";
NSLog(#"Ball size is %f",ball.size.width);
[self addChild:ball];
//Create spinner
_spinner = [SKSpriteNode spriteNodeWithImageNamed:#"Spinner"];
_spinner.size = CGSizeMake(300, 300);
_spinner.position = CGPointMake(320, 500);
_spinner.zPosition = 1;
_spinner.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:150];
_spinner.physicsBody.affectedByGravity = NO;
_spinner.physicsBody.allowsRotation = YES;
_spinner.physicsBody.dynamic = NO;
_spinner.physicsBody.restitution = 0.0;
_spinner.physicsBody.categoryBitMask = kSpinnnerCategory;
_spinner.physicsBody.contactTestBitMask = kRedParticleCategory;
_spinner.physicsBody.collisionBitMask = kRedParticleCategory;
_spinner.name = #"Spinner";
[self addChild:_spinner];
//create pipe
// CGPoint center = CGPointMake(400, 600) ;
//
// UIBezierPath *bezierPath = [UIBezierPath bezierPath];
// [bezierPath addArcWithCenter:center radius:400 startAngle:1.825777 endAngle:2.011118 clockwise:YES];
// [bezierPath addLineToPoint:center];
// [bezierPath closePath];
//
// SKShapeNode *shapeNode = [SKShapeNode shapeNodeWithPath:bezierPath.CGPath];
// shapeNode.strokeColor = [UIColor whiteColor];
// shapeNode.fillColor = [UIColor whiteColor];
// [self addChild:shapeNode];
}
# pragma mark -- SKPhysicsContactDelegate Methods
- (void)didBeginContact:(SKPhysicsContact *) contact {
if ([contact.bodyA.node.name isEqualToString:#"Ball"] && [contact.bodyB.node.name isEqualToString:#"Spinner"]) {
[self connectNode1:(SKSpriteNode *)contact.bodyA.node toNode2:(SKSpriteNode *)contact.bodyB.node withContact:contact];
}
}
- (void)didEndContact:(SKPhysicsContact *) contact {
//NSLog(#"didEndContact called");
}
- (void) connectNode1:(SKSpriteNode *)node1 toNode2:(SKSpriteNode *)node2 withContact: (SKPhysicsContact *)contact
{
SKPhysicsJointFixed *joint = [SKPhysicsJointFixed jointWithBodyA:node1.physicsBody
bodyB:node2.physicsBody
anchor:node2.position];
[self.physicsWorld addJoint:joint];
}
#end
If I comment out the did begin contact method you can see the images are correctly sized becuase when they collide they rest on each other perfectly.
How come the contact.contactPoint is not the same as the point at which both bodys are colliding when I comment out the didEnterContact method? Any idea how to fix?

Answer 1

'didBeginContact' is not called too early.. the problem is that actions are evaluated earlier in the scene cycle: first 'didEvaluateActions()' then 'didSimulatePhysics()'. So, even though your SKactions may look correct, there comes a physics evaluation afterwards.. I suggest not to use actions to make rotation corrections when working with the physics engine.. Perhaps use constraints, those come after the physics update..