Simple UITextView subclass for vertically aligning text in middle.

JHAVerticalTextView.h

//
//  JHAVerticalTextView.h
//  Quotes background
//
//  Created by Jens Andersson on 23/02/14.
//  Copyright (c) 2014 Jens Andersson. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface JHAVerticalTextView : UITextView

@end

JHAVerticalTextView.m

//
//  JHAVerticalTextView.m
//  Quotes background
//
//  Created by Jens Andersson on 23/02/14.
//  Copyright (c) 2014 Jens Andersson. All rights reserved.
//

#import "JHAVerticalTextView.h"

@implementation JHAVerticalTextView

- (id)initWithFrame:(CGRect)frame {
    
    self = [super initWithFrame:frame];
    
    if (self) {
        [self setupObserver];
    }
    
    return self;
}

- (id)initWithCoder:(NSCoder *)aDecoder {
    
    self = [super initWithCoder:aDecoder];
    
    if (self) {
        [self setupObserver];
        [self alignVertically];
    }
    
    return self;
}

- (void)setupObserver {
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(alignVertically)
                                                 name:UITextViewTextDidChangeNotification
                                               object:self];
}

- (void)setText:(NSString *)text {
    [super setText:text];
    
    [self alignVertically];
}

- (void)alignVertically
{
    CGSize size = [self sizeThatFits:self.frame.size];
    
    CGFloat offsetY = (CGRectGetHeight(self.frame) - size.height) / 2;
    self.contentInset = UIEdgeInsetsMake(offsetY, 0, 0, 0);
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

@end

Thanks so much.
I tried to replace the text size calculation with self.contentSize. It works in iOS8, but fails in iOS9. What's the problem of using "contentSize"?