Wednesday, May 13, 2009

Rotating view around arbitrary point the easy way

I have a UIImageView that I want to rotate around a given point. By default, the layer will rotate around the center of the UIImageView. Trying to change this anchor point involves a bit of work, either move at the beginning and reposition all the view elements, or translating the view, rotating, and translating back. The last option works fine, unless you are trying to use core animation to smoothly rotate the view.

I found out a much easier solution to the whole problem. You could call it a workaround, but it works well without disturbing the view you want rotated.

Basically, you create a new UIView. Place the center point where you want the rotation to occur. Change the size of the view so that the view you want rotated will fit inside. Make your view (you want rotated) a subview of the new view. Now, just rotate the new view, and your subview will be rotated correctly.

Example: lets say we have a UIImageView that is 320x480 (the size of the whole iPhone screen). We want it to rotate from the bottom center instead of its middle. Create a new UIView that is 0,0,320,960. This is basically double the height of the screen, positioned so the bottom half is off the screen, which places the center point right where we want it. Now place your UIImageView inside the new UIView in the top half (visible on the screen.) Now when you rotate the UIView, it will pivot on the bottom center of the screen, and your UIImageView will rotate along with it, as expected.


Dean said...

I wanted to know if you can help me figure out a solution to a custom map UIView I am working on. The map itself is just a small image.

After reading your custom UIView rotation scheme maybe you can help me come up with a scheme cause I have been struggling for a few days.

I have described it here

Basically I am creating a custom map (cartoon map) with pins on top.

I want the pins to stay the same size when the zoom is happening and also stay in the same spot when the map is being zoomed.

KiwiBastard said...

Genius. I had been struggling with this for a while, then read your post and it was obvious. Thanks again.