SpriteKit: moving sprite infinitely cause the frame sometimes incontinuous - objective-c

Question

all.
I am now developing an ios game using sprite kit.The game mechanics include: infinitely scroll sprites from top to bottom.
My game scene holds just 3 sprites,top,middle,bottom.
I have dozens of sprite images,so my solution is dynamic create 3 sprites ( so the sprites can fill the whole screen),when the bottom sprite is off screen,then destroy it;when the top sprite
goes down the top frame,create a new sprite node,then exchange the bottom,middle,top sprite.
The pseudo code:
in the interface file:
- GameScene:SKScene
{
...
SKSpriteNode *_topSprite;
SKSpriteNode *_middleSprite;
SKSpriteNode *_bottomSprite;
...
}
in the implementation file:
- (void)update:(CFTimeInterval)currentTime
{
// 1 compute time interval
// 2 update sprite node
_topSprite.position move down 100*timeInterval
_middleSprite.position move down 100*timeInterval
_bottomSprite.position move down 100*timeInterval
// 3 crop the off-screen bottom sprite node
if (_bottomSprite is offScreen)
{
[_bottomSprite removeFromParent]
}
// 4 check the highest sprite position visible
// if the highest position is below the frame top,then create new sprite
_bottomSprite = _middleSprite;
_middleSprite = _topSprite;
_topSprite = [self createNewNode]; // random a sprite image
}
the game fps is 60,and the nodes counts is not increasing.
All seems good,but i found sometimes the game just suddenly has a very shot choke causing the moving frame not continuous.
In my instinct,I thought the main reason is caused by step 4.
But I don't have any idea to solve it.
Any suggestions will be appreciated.
Here is the createNewNode method:
- (SKSpriteNode *)createNewNode {
NSString *blockName = [self randomImageName];
SKSpriteNode *sprite = [[SKSpriteNode alloc] initWithImageNamed:blockName];
sprite.position = _highestPosition;
sprite.anchorPoint = CGPointMake(0.5, 0);
NSLog(#"putAnotherNodeOnTop %#", blockName);
[self addChild:sprite];
return sprite;
}

Answer 1

A couple of suggestions.
Since you are using the same three images, you don't need to re-create the sprites each time. It should be less expensive to set the hidden value to YES, move the sprite somewhere, set the texture and then show it again with hidden to NO. SpriteKit doesn't draw hidden nodes, or ones off screen.
Your code basically asks for a PNG on disk every frame in update when createNewNode: is called. It should be more performant to load the textures in memory and assign them to sprites as needed. If you have just a few, making them SKTexture instance variables would be an easy way to do it.
Use spritesheets (texture atlases), it is much faster to draw an image from memory at mapped coordinates than to load it from disk each time. For example with three images in a spritesheet this can be a single draw call. From disk this is three separate draw calls.
You are returning a sprite and adding it to the scene in the createNewNode: method. Maybe that is what you want for some reason. It stood out as possible duplicate effort.