Why does a view changes position when rotating it using CGAffineTransformMakeRotation

Question

I have the following super simple animation, I'm basically rotating a view 2 radians from its original angle/center, it rotates fine my only misunderstanding is why does the view move from its original position when the rotation occurs.

[UIView animateWithDuration:1.0 animations:^{
        self.someView.transform = CGAffineTransformMakeRotation( 2 );
    }];

Why does the view moves when rotated with the code above?

I'm currently trying to discern the information in the CGAffineTransform Reference.

Understanding the anchor point.

I found this threads but it doesn't show a concrete answer. Why rotating imageView, it changes position?

Thanks a lot

Answer 1

You need to set the anchor point of your view to rotate around.

self.somview.layer.anchorPoint = CGPointMake(0.5, 0.5);

Then start the rotation.
From Apple documentations

@property(nonatomic) CGAffineTransform transform
Changes to this property can be animated. Use the beginAnimations:context: class method to begin and the commitAnimations class method to end an animation block. The default is whatever the center value is (or anchor point if changed) Link: https://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CALayer_class/Introduction/Introduction.html#//apple_ref/occ/instp/CALayer/anchorPoint

image from here http://www.raywenderlich.com/9864/how-to-create-a-rotating-wheel-control-with-uikit
- As you see the anchor point is the point with the value from 0.0 - 1.0 for X and Y when you rotate the rotation will be around these points
NOTE: you need to import QuartzCore