View is flipped when being added as subview of UIImageView

Question

I am drawing a triangle on a UIView via drawRect and subsequently add this UIView as subview of the main view ( UIView ).

Very easy and it shows.

The problem occurs when I use an UIImageView .

UIView (main view) adding UIImageView as subview and with this UIImageView adding the UIView as subview onto which I draw the triangle.

Hierarchy:

[UIView] (main view; i.e. self.view)
      |
    [UIImageView] 
            |
         [UIView] (onto which drawing is)

View creations:

self.view = [[UIView alloc] initWithFrame:(CGRect){{0.0, 0.0}, 320.0, 480.0}];
imageView = [[UIImageView alloc] initWithImage:...];

[self.view addSubview:imageView];

viewWithTriangle = [[MyView alloc] initWithFrame:imageView.bounds];
[imageView addSubview:viewWithTriangle];

The triangle is flipped vertically. The easy fix would be to flip the UIView vertically again, but I would like to know why adding the UIView as subview of UIImageView would cause this flip.

This doesn't occur if I add the view where the drawing is directly as subview of the main view.

Answer 1

The UIImage's orientation likely bleeds into UIImageView's coordinate system at times. UIImageView is highly optimized to work with drawing UIImage's to the screen. UIImageView doesn't even invoke drawRect , for eg

Using a breakpoint, when you see this behavior, check the value of imageView.image.imageOrientation . Is it equal to anything other than UIImageOrientationUp ? If so, you likely need to work with the fact that the UIImageView is "translating" some properties into the UIImage's "human preferred" coordinate system, instead of the image data's actual coodinate system.

I haven't personally tested this code or approach, as I do not add subviews to UIImageView's, but it's a likely problem. Please let us know if this helps diagnose it -- I'm very curious!