CGContext does not update immediately - ios

Question

The Problem: I have a small game where u can scratch a image above another image. The upper image becomes invisible where you are scratching. The whole thing is placed into a SpriteKit view. The problem is, that on weaker devices (iPhone4) the scratching image is only updated when the user stops scratching.
I assume that the image is only updated when the user does not scratch until the image is completely rendered and displayed.
Can someone suggest a better way to see the scratching immediately. I am aware that the scratching consumes a lot of performance but I dont mind if it lags a little.
Here the code:
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:self.view];
UIGraphicsBeginImageContext(self.view.frame.size);
[_scratchImage drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 25.0f );
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.0, 0.0, 0.0, 1.0);
CGContextSetBlendMode (UIGraphicsGetCurrentContext(), kCGBlendModeClear);
CGContextStrokePath(UIGraphicsGetCurrentContext());
_scratchImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
_lastPoint = currentPoint;
SKTexture* scratchTexture = [SKTexture textureWithImage:_scratchImage];
_scratchSprite.texture = scratchTexture;
}
UPDATE
I ended up recording the points as suggested in the correct answer. But other than updating the image in a CADisplayLink callback I updated the screen in a
-(void)update(float currentTime)
callback from SpriteKit (Which is kind of the same for SpriteKit use cases)
My code in the update method looks like the following:
-(void)displayUpdated
{
UIGraphicsBeginImageContext(self.view.frame.size);
[_scratchImage drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGMutablePathRef linePath = nil;
linePath = CGPathCreateMutable();
CGPathMoveToPoint(linePath, NULL, _lastPoint.x, _lastPoint.y);
for (NSValue* val in _recordedPoints) {
CGPoint p = [val CGPointValue];
CGPathAddLineToPoint(linePath, NULL, p.x, p.y);
NSLog(#"%f, %f", p.x, p.y);
_lastPoint = p;
}
//flush array
[_recordedPoints removeAllObjects];
CGContextAddPath(UIGraphicsGetCurrentContext(), linePath);
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 25.0f );
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.0, 0.0, 0.0, 1.0);
CGContextSetBlendMode (UIGraphicsGetCurrentContext(), kCGBlendModeClear);
CGContextStrokePath(UIGraphicsGetCurrentContext());
_scratchImage = UIGraphicsGetImageFromCurrentImageContext();
CGPathRelease(linePath);
UIGraphicsEndImageContext();
SKTexture* scratchTexture = [SKTexture textureWithImage:_scratchImage];
_scratchSprite.texture = scratchTexture;
}
A more performant code would also hold a reference to the context but I could not manage to get that working. If someone can suggest a solution I would be happy. Nevertheless my Framerate on a iPhone 4 went up to 25-30 fps from about 5 fps.

Answer 1

I would try the following:
Measure in Instruments.app Time Profiler.
Speed up drawing by creating a CGBitmapContext as an instance variable. Then you can skip the [_scratchImage drawInRect:...] in each step.
Separate the drawing and the event handling. Right now, you will always create a new UIImage for each touch event. It could be better to coalesce the stroked lines so the creation of a UIImage does not happen as often. E.g. put the line segments in a queue and draw in a callback from CADisplayLink.

Answer 2

I don't know much about SpriteKit, In UIView, you should put your drawing code in drawRect method. Is it the same as SpriteKit ?