Struggling with Scrollview - ios

Question

I am an elderly attempting to learn new-by. This works fine.
#implementation MESViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.view= [[QuartzLineView alloc] initWithFrame:CGRectZero];}
-(void)Play{
for(int i=0;i<2000;i++){
short int lineno=function result;
CGRect myRect=CGRectMake(0,lineno, 1024,lineno+235);
[self.view setNeedsDisplayInRect:myRect];}
However using a scrollview the view does not seem to redraw at all.
Scrollview displays and takes touch input ok.
#import <UIKit/UIKit.h
#interface MESViewController : UIViewController<UIScrollViewDelegate>
#property (nonatomic, strong) IBOutlet UIScrollView *scrollView;
#end
#implementation MESViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.scrollView=[[UIScrollView alloc] initWithFrame: CGRectMake(0, 0, x, y)];
self.scrollView.contentSize=CGSizeMake(x,y);
UIView *myview= [[QuartzLineView alloc] initWithFrame: CGRectMake(0, 0, x, y)];
[self.scrollView addSubview:myview];
self.view=self.scrollView;}
-(void)Play{...as above}
#implementation QuartzLineView
- (instancetype)initWithFrame:(CGRect)r{
self = [super initWithFrame:CGRectMake(0, 0, x, y];
if (self) {
self.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:1.0];
self.multipleTouchEnabled = YES;
}
return self;}

Answer 1

The issue is that when you set self.view to be the scrollview, you are still calling [self.view setNeedsDisplay] which will tell the UIScrollView that it needs display, and you want to tell the QuartzLineView that it needs display.
Saving a reference to the QuartzLineView, and calling setNeedsDisplay on that should do the trick:
#implementation MESViewController{
// save a reference to the QuartzLineView
QuartzLineView* lineView;
}
- (void)viewDidLoad {
[super viewDidLoad];
self.scrollView=[[UIScrollView alloc] initWithFrame: CGRectMake(0, 0, x, y)];
self.scrollView.contentSize=CGSizeMake(x,y);
// saving the reference here
lineView= [[QuartzLineView alloc] initWithFrame: CGRectMake(0, 0, x, y)];
[self.scrollView addSubview:lineView];
self.view=self.scrollView;
}
-(void)Play{
for(int i=0;i<2000;i++){
short int lineno=function result;
CGRect myRect=CGRectMake(0,lineno, 1024,lineno+235);
// now tell the _lineView_ to setNeedsDisplay, instead of self.view
[lineView setNeedsDisplayInRect:myRect];
}
}