CALayer gets redrawn frequently despite shouldRasterize --> why? - ios

Question

I have some fairly complex layers in a part of my app and I want them to rasterize as their content changes close to never. The frame of the view these layers are in can be changed by dragging a "spacerbar" around with your finger.
I made a simple test-app to visualise my problem and show you some code:
#interface ViewController () {
UIView* m_MainView;
UIView* m_TopView;
UIView* m_BottomView;
UIView* m_MidView;
int m_Position;
}
#end
#implementation ViewController
- (void)loadView {
m_MainView = [[UIView alloc] init];
m_TopView = [[UIView alloc] init];
m_TopView.backgroundColor = UIColor.blueColor;
m_TopView.translatesAutoresizingMaskIntoConstraints = false;
m_BottomView = [[UIView alloc] init];
m_BottomView.backgroundColor = UIColor.blueColor;
m_BottomView.translatesAutoresizingMaskIntoConstraints = false;
m_MidView = [[UIView alloc] init];
m_MidView.backgroundColor = UIColor.blackColor;
m_MidView.translatesAutoresizingMaskIntoConstraints = false;
[m_MainView addSubview:m_TopView];
[m_MainView addSubview:m_BottomView];
[m_MainView addSubview:m_MidView];
CALayer* targetLayer = [[CALayer alloc] init];
targetLayer.frame = CGRectMake (100, 200, 500, 50);
targetLayer.backgroundColor = UIColor.yellowColor.CGColor;
targetLayer.shouldRasterize = true;
[m_BottomView.layer addSublayer:targetLayer];
UIPanGestureRecognizer* recognizer = [[UIPanGestureRecognizer alloc]
initWithTarget:self action:#selector(handlePan:)];
[m_MainView addGestureRecognizer:recognizer];
m_Position = 400;
super.view = m_MainView;
}
- (void)handlePan:(UIPanGestureRecognizer*)sender {
float touchY = [sender locationInView:m_MainView].y;
m_Position = (int)touchY;
[m_MainView setNeedsLayout];
}
- (void)viewWillLayoutSubviews {
CGRect bounds = super.view.bounds;
m_TopView.frame = CGRectMake (0, 0, bounds.size.width, m_Position - 10);
m_MidView.frame = CGRectMake (0, m_Position - 10, bounds.size.width, 20);
m_BottomView.frame = CGRectMake (0, m_Position + 10, bounds.size.width,
bounds.size.height - m_Position - 10);
}
#end
.
Now ... By using instruments and activating "Color Hits Green and Misses Red" one can see when a layer is redrawn (is then highlighted in red).
In this example the bar (targetLayer) is green (cached) most of the time, but if I start dragging the spacer (which in fact changes the frame of the bottom-view) the layer turns red some times ... especially if I reverse the direction of the drag or if I drag slowly.
In my app this causes flickering as redrawing these layers is expensive.
Why does this happen??
I never change any property of the layer but it gets redrawn anyways?
I am sure I am missing something :-)
As a workaround I could make a snapshot of the layers and use the image ... that would work I think and if there is no solution I will have to use this approach but I would like to understand what is the problem here.
Anyone?