ASRKeyboardNotificationValues.h

@import UIKit;

@interface ASRKeyboardNotificationValues : NSObject

- (instancetype)initWithNotification:(NSNotification *)notification;
@property (nonatomic, readonly) NSNotification *notification;

@property (nonatomic, readonly) NSTimeInterval animationDuration;
@property (nonatomic, readonly) UIViewAnimationCurve animationCurve;
@property (nonatomic, readonly) UIViewAnimationOptions animationOptions;

@property (nonatomic, readonly) CGRect frameBegin;
@property (nonatomic, readonly) CGRect frameEnd;

@end

ASRKeyboardNotificationValues.m

#import "ASRKeyboardNotificationValues.h"

@implementation ASRKeyboardNotificationValues

- (instancetype)initWithNotification:(NSNotification *)notification {
	self = [self init];
	if (!self) return nil;
	_notification = notification;
	return self;
}

- (NSTimeInterval)animationDuration {
	return [self.notification.userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue];
}

- (UIViewAnimationCurve)animationCurve {
	return [self.notification.userInfo[UIKeyboardAnimationCurveUserInfoKey] unsignedIntegerValue];
}

- (UIViewAnimationOptions)animationOptions {
	return [self animationCurve] << 16;
}

- (CGRect)frameBegin {
	return [self frameForKey:UIKeyboardFrameBeginUserInfoKey];
}

- (CGRect)frameEnd {
	return [self frameForKey:UIKeyboardFrameEndUserInfoKey];
}

- (CGRect)frameForKey:(NSString *)key {

	NSValue *value = self.notification.userInfo[key];

	if ([value isKindOfClass:[NSValue class]]) {
		return value.CGRectValue;
	}

	return CGRectNull;
}

@end

gistfile1.swift

class SomeViewController : UIViewController {
	@IBOutlet weak var bottomConstraint: NSLayoutConstraint!
	func keyboardWillChangeFrameNotification(notification: NSNotification) {

		let values = ASRKeyboardNotificationValues(notification: notification)
		self.bottomConstraint.constant = values.frameEnd.size.height

		UIView.animateWithDuration(values.animationDuration, delay:0, options:values.animationOptions, animations: {
			self.view.layoutIfNeeded()
		}, completion: nil)
	}
}