what is the difference between UIViewContentModeScaleAspectFit and UIViewContentModeScaleToFill...?
If you are talking about UIViewContentMode, the following is from the Doc.
UIViewContentModeScaleToFill
Scales the content to fit the size of itself by changing the aspect ratio of the content if necessary.
UIViewContentModeScaleAspectFit
Scales the content to fit the size of the view by maintaining the aspect ratio. Any remaining area of the view’s bounds is transparent.
UIViewContentModeScaleAspectFill
Scales the content to fill the size of the view. Some portion of the content may be clipped to fill the view’s bounds.