On Jun 18, 2009, at 6:39 PM, David Duncan wrote:
On Jun 17, 2009, at 10:45 PM, WT wrote:
If the center is in the superview's coordinate system, how can it
not be changed by a translation?
Because the center is only 1 part in the chorus of things that
determines where your view is. All of this really translates to Core
Animation under the covers, and what UIKit reports to you as the
center of a view is the view's layer's position. So lets go strictly
to layers for a moment.
A layer has 4 properties that affect its area on screen – position,
bounds.size, anchorPoint and transform. All of these properties
together produce the final bounding rect in 3D space for the layer,
which is then flattened into the frame rect that we all know and love.
So the position property of the layer never changes when you
transform a layer, regardless of the transform. The layer moves when
you apply a translation transform to it because you've changed the
bounding rect that was calculated from the transform along with the
other 3 properties I mentioned.
In the view's own coordinate system, yes, I can see that that's
true, because then the center is half-way the bounds' width and
height, and those don't change under a translation.
The view's center property is the point around which the view bounds
are located. If you change the view's layer's anchorPoint, then the
view's center will have a different meaning. It defaults to half way
between, but really its saying to place half of the bounds on either
side of the position.
--
David Duncan
Apple DTS Animation and Printing
Hi David,
I very much appreciate that you spent the time to explain all that
(and not just for my benefit, but for everyone else's as well), but
I'm still very confused by what you said, even more so now.
Ultimately, I have some rectangular area on the 2D screen of an iPhone
or a Mac, and it has a center point (which, I understand, doesn't
*have* to be halfway through the bounds), whose coordinates are
measured in the coordinate system of the rectangle's superview. I then
apply a translation to this rectangle and it ends up somewhere else on
that screen. Assuming that the rectangle's superview was not
translated along as well, I cannot see how the coordinates of the
rectangle's center would not have changed.
Now, I fully realize that my lack of understanding is entirely due to
not having worked with layers, meaning that I have not spent any
serious time reading the docs on them (meaning that I should!).
I would love to continue this discussion off-list but, realistically,
I am not prepared enough for that yet, so I won't impose on you any
more than I already have. I am soon going to dive into core animation
for an app I'm working on and maybe then I'll understand what you
wrote above. And, if not, at least I'll be ready to ask more specific
questions.
Regardless, thank you for trying to make sense of it for me (and
others).
Wagner_______________________________________________
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