UITextView content offset changes after setting frame

Question

I'm building a view that's very similar to the messages app - I have a subview at the bottom of the page with a UITextView in it and as the user types and reaches the end of the line the text view as well as the view containing it should expand upward.

The way I have it working is that in the textViewDidChange: method I call my layout function, and that does

CGFloat textViewWidth = 200;
CGFloat textViewHeight = [self.textView sizeThatFits:CGSizeMake(textViewWidth, 2000)].height;
[self resizeParentWithTextViewSize:CGSizeMake(textViewWidth, textViewHeight)];

// And then the resize parent method eventually calls
textView.frame = CGRectMake(10, 10, textViewWidth, textViewHeight);

The problem is that when typing at the end of line and the view expands, I end up with an arbitrary contentOffset.y of something like 10.5 on the text view so the text is all shifted up to the top of the view. Weirdly, it's alternating on every other line, so expanding the first time leaves the y content offset shifted up, then at the next line it's close to zero, then back to 10.5 on the next line, etc. (not sure if that's helpful or just a strange artifact of my values). I can set it back to zero afterwards but it looks terrible because there's a brief flash where the text has the offset value and then it gets shifted back to the middle.

I've read that it's usually better to use content insets for scroll views rather than changing the frame, but I don't get how to do that because I do need to change the frame size as well.

How can I resize the UITextView without this happening? I think I can get by with setting the text view not to be scrollable and that fixes the issue, but I'd like to understand what's going on.

Answer 1

Setting textView.scrollable = NO lets me resize the text view without any strange offsets, that's the only way I've been able to figure out. And I guess it's not too much of a limitation for common scenarios, if you want the text view to be scrollable you probably don't need to resize it on the fly since the user can scroll around as the content changes.

Answer 2

The problem is that UITextView's scroll animation and your frame setting action were happened at the same time. UITextView internally scrolls the texts you currently typing to visible when typed one more character at the end of the line or typed the new line character. But the scroll animation does not need because you are expanding the textview. Unfortunately we can't control textview's internal scroll action so the text scrolls to the top of the expanded textview weirdly. And that weird scroll makes unnecessary bottom padding too.

You can avoid this weird action very simply with overriding UITextView's setContentOffset:animated: like this.

Objective-C

- (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated {
    [super setContentOffset:contentOffset animated:NO];
}

Swift

override func setContentOffset(_ contentOffset: CGPoint, animated: Bool) {
    super.setContentOffset(contentOffset, animated: false)
}

This code avoids the auto sizing UITextView's unnecessary scroll animations and you can expand the size of the text view freely.