Simple combo multiplier in sprite-kit

I am making a reaction game, where you can destroy enemys and earn points. Now I would like to have combo points if you destroy them fast and if there is a specific time gap the combo multiplier should go to zero again.

I would like to multiple the points like this: 2 * 2 = 4 * 2 = 8 * 2 = 16 * 2... (you get 2 points if you destroy an enemy).

I add the points here:

if (CGRectIntersectsRect(enemy.frame, player.frame)) {
        points = points + 1;
        [enemy removeFromParent];
    }

I could always multiply the current points with 2, but I want to reset the combo multiplier if there is specific amount of time without getting points.

I hope someone can help me. (code in objective c please)

Solution

It seems no more complicated than recording the time the last enemy was destroyed and then in the update: method deciding if the combo has elapsed as no more enemies were hit in whatever timeout period you allow.

I am not familiar with Sprite kit, but the update appears to pass the current time; excellent. You will need to record the following:

timeout (time): The current timeout. This will reduce as the game progresses, making it harder.
lastEnemyKillTime (time): the time the last enemy was killed.
comboPoints (integer): How many points the user gets per hit. This will increase as the combo extends.
points (integer): The current score.

So, something like this:

@interface MyClass ()
{
    NSTimeInterval _timeout;
    NSTimeInterval _lastEnemyKillTime;
    BOOL _comboFactor;
    NSUInteger _points;

}
@end

I guess Sprite Kit uses an init: method; use it to initialize the variables:

- (id)init
{
    self = [super init];
    if (self != nil) {
        _timeout = 1.0;
        _lastEnemyKillTime = 0.0;
        _points = 0;
        _comboPoints = 1;
    }
}

The update: method would be something like:

- (void)update:(NSTimeInterval)currentTime
{
    BOOL withinTimeout = currentTime - _lastEnemyKillTime <= _timeout;
    if (CGRectIntersectsRect(enemy.frame, player.frame)) {
        _inCombo = withinTimeout;
        if (_inCombo)
            _comboPoints *= 2;
        _points += _comboPoint;
        _lastEnemyKillTime = currentTime;
        [enemy removeFromParent];
    } else if (_comboPoints > 1 && !withinTimeout) {
        _lastEnemyKillTime = 0.0;
        _comboPoints = 1;
    }
}