SpriteKit update function

update.m

-(void)update:(CFTimeInterval)currentTime {
    static CFTimeInterval lastObstacleAt = 1;
    static CFTimeInterval pausedLoopAt;
    if (self.paused) {
        if (!pausedLoopAt) {
            pausedLoopAt = currentTime;
        }
        return;
    } else if (pausedLoopAt) {
        lastObstacleAt += currentTime - pausedLoopAt;
        pausedLoopAt = 0;
    }

    // Corgi sometimes gets stuck hovering...
    if (self.corgi.isJumping && self.corgi.physicsBody.isResting) {
        // ...so we nudge it a bit so the physics simulator comes back to life.
        [self.corgi.physicsBody applyImpulse:CGVectorMake(0, -1)];
    }

    // Update score
    _gameState.score += roundf(0.5 * _gameState.speed);
    self.scoreLabel.text = [NSString stringWithFormat:@"%lupts", (unsigned long)_gameState.score];

    // Base movement rate for background & obstacles
    float xMovement = _gameState.movementRate * _gameState.speed;

    // Move background
    [_bgSprites enumerateObjectsUsingBlock:^(SKSpriteNode *bgSprite, NSUInteger idx, BOOL *stop) {
        // There are two copies of each background layer, offset by 0.5 their width to allow for continuous scrolling
        NSUInteger layerIdx = floor(idx / 2.0); // This converts array index to logical layer index
        CGPoint bgPos = bgSprite.position;
        // The background layers move at different rates to give a parallax effect
        bgPos.x -= xMovement * [_layerProcessionRates[layerIdx] floatValue];
        if (bgPos.x + bgSprite.size.width / 2.0 <= 0) {
            // If the layer has moved off to the left, put it back on the right side so we can keep scrolling
            bgPos.x += bgSprite.size.width * 2;
        }
        bgSprite.position = bgPos;
    }];

    // Move extant obstacles
    [self.foreground enumerateChildNodesWithName:kObstacleNodeName usingBlock:^(SKNode *obs, BOOL *stop) {
        obs.position = CGPointMake(obs.position.x - xMovement, obs.position.y);
        if (obs.position.x < 0) {
            [obs removeFromParent];
        }
    }];

    // Spawn new obstacles
    if (currentTime > lastObstacleAt + ((2 + arc4random() % 5) / _gameState.speed)) {
        SKSpriteNode *obstacle = [sharedObstacle copy];
        obstacle.position = CGPointMake(CGRectGetMaxX(self.frame) - obstacle.size.width / 2,
                                        _groundTop + obstacle.size.height);
        [self.foreground addChild:obstacle];
        lastObstacleAt = currentTime;
    }
}