Why does setting the anchorPoint of the layer's bounds rectangle behave so strange?

Question

I try to set the anchorPoint property, in order to rotate a view by a well defined axis. But:

myView.layer.anchorPoint = CGPointMake(myView.layer.anchorPoint.x - 1.0, myView.layer.anchorPoint.y);

Wenn I shift it by -1.0, it will not just move 1 unit to left. Instead, my whole view moves by the width of the view to right.

What kind of coordinate system is that? It seems inverted. But also the units don't match with those of for example myView.frame.size.width ?

Answer 1

anchorPoint is a normalized position within your layer. That is, (0.0, 0.0) is the upper-left corner of your layer (in the iPhone's flipped UIView layer coordinate system) and (1.0, 1.0) is the lower-right corner. (0.5, 0.5) is the middle of your layer. These positions are size-independent, and thus merely relative locations.

This makes it possible to scale about that anchorPoint, which would be confusing to do if anchorPoint was an absolute coordinate.