how to set animation curve when using keyframe animation in ios?

Actually, you can use the same curve flags that you use for animateWithDuration:delay:options:animations:completion:. Just "binary or" them in with the other options in your call to animateKeyframesWithDuration:delay:options:animations:completion:

The documentation is confusing, but it does work.

The curve values you can use are:

UIViewAnimationOptionCurveEaseInOut = 0 << 16,
UIViewAnimationOptionCurveEaseIn    = 1 << 16,
UIViewAnimationOptionCurveEaseOut   = 2 << 16,
UIViewAnimationOptionCurveLinear    = 3 << 16,

The default animation curve you get is 0, or ease-in, ease-out.

Swift 2.0 will not allow casting of UIViewAnimationOptions as UIViewKeyframeAnimationOptions. It will also not allow |ing them together.

However, there is an initializer for UIViewKeyframeAnimationOptions that takes a rawValue. So, the following code works to put UIViewAnimationOptions into UIViewKeyframeAnimationOptions:

let animationOptions: UIViewAnimationOptions = .CurveEaseOut
let keyframeAnimationOptions: UIViewKeyframeAnimationOptions = UIViewKeyframeAnimationOptions(rawValue: animationOptions.rawValue)

Then I can pass keyframeAnimationOptions to animateKeyframesWithDuration and everything works great.

For Swift 4:

let animationOptions = AnimationOptions.curveEaseOut
let options = KeyframeAnimationOptions(rawValue: animationOptions.rawValue)