Q:

Not doing CG drawings correctly

Just started working with Core Graphics and I probably got no idea what's going on. 
In the following code I'm trying to create a small rounded translucent black square overlaid on top of the UINavigationController, but so far nothing showed up...
UIView *notificationView = [[UIView alloc] initWithFrame:[[[self navigationController] view] frame]];

CGRect rect = CGRectMake(self.view.frame.size.width / 2 - 50, self.view.frame.size.height / 2, 100, 100);
UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0.0);
[[UIColor colorWithWhite:0 alpha:0.5] setFill];

UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:rect cornerRadius:10];
[path fill];

[notificationView setNeedsDisplay];

[[[self navigationController] view] addSubview:notificationView];

A:

Option 1 (the most design-friendly one)
UIViews are not really meant for other objects to draw into them.  It makes much more design sense to subclass UIView and let it do its own drawing in drawRect.  That way, you don't have to paste so much code every time you want to use a notification view. 
Option 2 (the easiest one, and probably best)
If you just want a translucent black rounded rectangle (I'm assuming for a loading indicator), you can do it much more easily by creating the UIView at the size you want and centering it in the view.  Set its background color to the translucent color, [UIColor colorWithWhite:0.0 alpha:0.5].  Finally, add the line
notificationView.layer.cornerRadius = 10.0;

You may also need to put #import <QuartzCore/QuartzCore.h> in your header file, since this is a CALayer trick. 
Option 3 (the roundabout one)
If you really want to do it the way you're already doing, change the notificationView to a UIImageView, then set the frame of the view to be the size of the black rounded rect.  Then add this after you fill the path:
UIImage *indicatorImage = UIGraphicsGetImageFromCurrentImageContext();
notificationView.image = indicatorImage;

You don't need to call setNeedsDisplay anymore. 
Hopefully one of these sounds good to you!