How to delete nodes that fall below the screen in sprite kit?

Question

i'm making a game where random obstacles fall from the top of the screen and the player has to avoid them. i'm having trouble implementing a method to delete the nodes (obstacles) that fall below the screen. although it may seem simple the way I'm spawning the obstacles has made it confusing. here is how i spawn the obstacles:

- (void)spawnNewObstacles
{
    //spawn obstacles
    SKSpriteNode *obstacle = [SKSpriteNode spriteNodeWithImageNamed:@"black.png"];

    int position = arc4random() % 320 + 1;
    obstacle.position = CGPointMake(position, 800);
    obstacle.size = CGSizeMake(200, 20);

    obstacle.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:obstacle.frame.size];
    obstacle.physicsBody.dynamic = YES;

    [self addChild:obstacle];
    [obstacles addObject:obstacle]; //obstacles is an NSMutableArray

    [obstacle.physicsBody applyImpulse:CGVectorMake(0, -80.0f)];

    [self performSelector:@selector(spawnNewObstacles) withObject:nil afterDelay:0.25];
}

and here is how i'm trying (but failing) to delete the ones that fall below the screen:

-(void)update:(CFTimeInterval)currentTime
{ 
    for(int i=0;i<obstacles.count;i++)
    {
        NSLog(@"Removed an Obstacle"); //this log isn't showing so i concluded that it wasn't working
        SKSpriteNode *obstacle = [obstacles objectAtIndex:i];
        if(obstacle.position.y<0)
        {
            [obstacle removeFromParent];
        }
    }
}

how would u guys suggest i do this?

p.s. plz explain with code...

Answer 1

From the sound of it, I'm guessing you haven't initialized the obstacles array with a line like this:

obstacles = [NSMutableArray array];

Set a breakpoint and check if the obstacles variable is non-nil.

Other than that the code looks okay. Except you should prefer to use fast enumeration - it's safer (index out of bounds can not occur), faster and easier to read:

for (SKSpriteNode* obstacle in obstacles)
{
    NSLog(@"Removed an Obstacle");
    if (obstacle.position.y < 0)
    {
        [obstacle removeFromParent];
    }
}