Page Flip

Introduction

Ok, so maybe you need a page flip animation. There are lots of UIView default animations, that look really good, but my UI designed really busted me and was like "NO, I WANT A BARN DOOR FLIP". Anyway, there isn't a default way that I could find to do this. So I had to take a dive into core animation land and dig deep.

Flipping Open

Aight, so to flip a view open, we take the view, and rotate it using core animation by 90 degrees in y direction around the left side of it. All makes sense right? So, get a UIView as a container and add anything you want to flip onto it.

  -(void)pageOpen {
       //Make sure the container is visible and remove any other previous animations from it
       [uiv_container.layer removeAllAnimations];
       [uiv_front setHidden:NO];
       [uiv_container setUserInteractionEnabled:NO];
      
       //Sets the anchor point to the middle of the left side
       uiv_container.layer.anchorPoint = CGPointMake(0.0f,0.5f);
      
       //Set up a new animation to describe the transform
       CABasicAnimation *ba_animation = [CABasicAnimation animationWithKeyPath:@"transform"];
      
       ba_animation.removedOnCompletion = NO;
       
       //This is the time it takes for the animation to complete, here its 2 seconds
       ba_animation.duration = 2.0;
        
       //Using different timing functions will change the animation speed at different points. Ex: kCAMediaTimingFunctionEaseIn
       ba_animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
       
       //Start from the identity matrix. If you haven't every done 3D programming, the Identity matrix is basically the normal matrix of anything
       ba_animation.fromValue = [NSValue valueWithCATransform3D:CATransform3DIdentity];
       
       //Set up the end matrix. Here we are setting it to 90 degrees in the Y direction
       CATransform3D t_end = CATransform3DMakeRotation(M_PI/2, 0.0, -1.0, 0.0);
       
       //This value controls the size of the end, modifying it like this makes it get bigger, making it more like a 3D rotation
       t_end.m14 = -0.001f;
       
       ba_animation.toValue = [NSValue valueWithCATransform3D:t_end];
      
       //Add the animation to an animation group, this lets us track the animation as it finishes
       CAAnimationGroup *ag_group = [CAAnimationGroup animation];
       ag_group.delegate = self;
       ag_group.duration = 2.0;
       
       [ag_group setValue:[NSNumber numberWithInt:1] forKey:@"Animation Tag"];
       ag_group.animations = [NSArray arrayWithObjects:ba_animation, nil];
       
       [uiv_container.layer addAnimation:ag_group forKey:@"Flip View"];
  }

Flipping Closed

Now, we have the page up, so next we need to rotate it down back to flat. Rotate it another 90 degrees in the Y direction, this time we need to rotate on the right side of it (so all the text looks right when its rotating down. This gives the illusion that there is a front and a back to the view.

  -(void)pageFlip {
  	NSLog(@"page opening");
  	 
  	uiv_container.layer.anchorPoint = CGPointMake(1.0f,0.5f);
  	
  	CABasicAnimation *ba_animation = [CABasicAnimation animationWithKeyPath:@"transform"];
  
  	ba_animation.removedOnCompletion = NO;
  	ba_animation.duration = 2.0;
  	ba_animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
  	
  	//This time we need to start from the position where the frame is 90 degrees rotated
  	CATransform3D t_start = CATransform3DMakeRotation(M_PI/2, 0.0, -1.0, 0.0);
  
   	//Need the opposite of the previous value so that the edge is sized correctly from the beginning
   	t_start.m14 = 0.001f;
  	
  	ba_animation.fromValue = [NSValue valueWithCATransform3D:t_start];
  
  	//Now rotate it down to the identity matrix
   	ba_animation.toValue = [NSValue valueWithCATransform3D:CATransform3DIdentity];
  
  	CAAnimationGroup *ag_group = [CAAnimationGroup animation];
  	ag_group.delegate = self;
  	ag_group.duration = 2.0;
  
  	[ag_group setValue:[NSNumber numberWithInt:2] forKey:@"Animation Tag"];
  	ag_group.animations = [NSArray arrayWithObjects:ba_animation, nil];
  
   	[uiv_container.layer addAnimation:ag_group forKey:@"Flip View"];
  }

Animation Did Stop

Finally, we need something to manage everything when the animation ends. So here we tell it to run the the page flip after the page open finishes

  - (void)animationDidStop:(CAAnimation *)theAnimation finished:(BOOL)flag {
  	NSLog(@"animation did stop");
      // Get the tag from the animation, we use it to find the
      // animated UIView
      NSNumber *tag = [theAnimation valueForKey:@"Animation Tag"];
  	if([tag intValue] == 1) {
  		uiv_front.hidden = YES;
  		[uiv_front removeFromSuperview];
  		[self.view addSubview:uiv_back];
  		uiv_back.hidden = NO;
  		[self pageOpen];
  	} if([tag intValue] == 2) {
  		
  	}
  }

Conclusion

Hopefully this gets you on the correct path for a flip animation. Good luck. Need, any help? Let me know!