How to Spawn Sprites with Increasing Speed - sprite-kit

Question

I want to know how to continuously spawn sprites with gradually increasing speed. I did see the question "How to gradually increase the rate of something?" but I'm not sure how to apply it to my code. Additionally, if there is a better way to spawn sprites regularly than using a game timer, please let me know.
I tried using a loop and having the "scheduledTimerWithTimeInterval" increase, but it ended up just causing numerous sprites to spawn at the same time.
-(void)getBalls {
//[userScore.node removeFromParent];
SKSpriteNode *ball = [SKSpriteNode spriteNodeWithImageNamed:#"ball"];
int x = arc4random() %320;
CGPoint myPoint = CGPointMake(x, 480);
ball.position = myPoint;
ball.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:ball.frame.size.width/2];
ball.physicsBody.categoryBitMask = ballCategory;
ball.physicsBody.contactTestBitMask = paddleCategory;
ball.physicsBody.collisionBitMask = ballCategory;
[self addChild:ball];
SKLabelNode *userScore = [SKLabelNode labelNodeWithFontNamed:#"Impact"];
userScore.fontColor = [SKColor blackColor];
userScore.text = [NSString stringWithFormat:#"SCORE: %i", score];
userScore.fontSize = 18;
userScore.position = CGPointMake(CGRectGetMidX(self.frame) + 110, CGRectGetHeight(self.frame)
- 30);
[self addChild:userScore];
[userScore runAction:[SKAction fadeAlphaTo:1.0 duration:1.5] completion:^{
[userScore removeFromParent];
}];
}
-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
self.backgroundColor = [SKColor whiteColor];
self.physicsWorld.gravity = CGVectorMake(0, -9.8);
self.physicsWorld.contactDelegate = self;
gameTimer = [NSTimer scheduledTimerWithTimeInterval:1.5 target:self
selector:#selector(getBalls) userInfo:nil repeats:YES];
[self addPlayer:size];
}
Thanks for your help.

Answer 1

Use a non-repeating NSTimer. Also, in the method called by the timer (getBalls) recall the timer and decrease the interval rate (linearly, exponentially, however you want).
float decTime;
decTime = 1.5f;
-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
self.backgroundColor = [SKColor whiteColor];
self.physicsWorld.gravity = CGVectorMake(0, -9.8);
self.physicsWorld.contactDelegate = self;
gameTimer = [NSTimer scheduledTimerWithTimeInterval:decTime target:self
selector:#selector(getBalls) userInfo:nil repeats:NO];
[self addPlayer:size];
}
-(void)getBalls {
//[userScore.node removeFromParent];
SKSpriteNode *ball = [SKSpriteNode spriteNodeWithImageNamed:#"ball"];
int x = arc4random() %320;
CGPoint myPoint = CGPointMake(x, 480);
ball.position = myPoint;
ball.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:ball.frame.size.width/2];
ball.physicsBody.categoryBitMask = ballCategory;
ball.physicsBody.contactTestBitMask = paddleCategory;
ball.physicsBody.collisionBitMask = ballCategory;
[self addChild:ball];
....
if (decTime > .05f) {
//linear increase
decTime -= .01f;
gameTimer = [NSTimer scheduledTimerWithTimeInterval:decTime target:self
selector:#selector(getBalls) userInfo:nil repeats:NO];
}
}