Updating the position of a node constantly? - ios

Question

I am experiencing some problems with my game, I am currently using the update method to create the illusion of my Hero character being "dragged" along with a moving platform. The update method works by using a bool that switches on and off whenever my Hero makes contact with the platform and then updates the position to seemingly show that the Hero is being moved. This worked nicely, but as I realize when the framerate drops, so does the character's positioning and it breaks the illusion.
Is there any sort of way to constantly update the positioning of a node without using the update method?
didBeginContact Method
-(void)didBeginContact:(SKPhysicsContact *)contact {
if (((firstBody.categoryBitMask & fPlayerCategory) != 0) && ((secondBody.categoryBitMask & fPlatformCategory) != 0))
{
[_Hero removeActionForKey:#"idleAnimation"];
[_Hero runAction:repeatWalkAnimation withKey:#"walkAnimation"];
syncMove = YES;
}
}
Update Method
-(void)update:(CFTimeInterval)currentTime {
if (gameStart == YES & gameOver == NO) {
if (syncMove == YES) {
_Hero.position = CGPointMake(_Hero.position.x - .83, _Hero.position.y);
}
}
}
Any sort of help would be appreciated.

Answer 1

You can use the currentTime argument in your update function to calculate how much _Hero should have moved. This way _Hero will always move the appropriate amount based on the time since the last update.
The steps to do this are:
Store your last update time
On each update, calculate the difference between the last update time and the current time
Calculate how far _Hero should have moved in that time
Update _Hero's position
Something like:
CFTimeInterval lastUpdateTime = 0;
CGFloat frameRate = 1.0 / 60.0; // Normal SpriteKit frame rate
CGFloat xDistanceForNormalFrameRate = 0.83;
-(void)update:(CFTimeInterval)currentTime {
if (lastUpdateTime == 0) lastUpdateTime = currentTime - frameRate; // Init lastUpdateTime to one frame back
if (gameStart && !gameOver) { // Note: the original question had & here, but it looks like it was a typo
if (syncMove) {
CFTimeInterval timeDifference = currentTime - lastUpdateTime;
CGFloat xDistance = xDistanceForNormalFrameRate * (timeDifference / frameRate);
_Hero.position = CGPointMake(_Hero.position.x - xDistance, _Hero.position.y);
}
}
lastUpdateTime = currentTime;
}
Note: If your frame rate is dropping below ~30FPS, you should probably focus more on why your frame rate is dropping than how to work around it.