UITextView contentSize changes and NSLayoutManager

问题:

The problem: UITextView silently changes it's contentSize in some situations.

The simplest case textView with large text and keyboard. Just add UITextView outlet and set - viewDidLoad as:

- (void)viewDidLoad {
    [super viewDidLoad];
    // expand default "Lorem..."
    _textView.text = [NSString stringWithFormat:@"1%@\n\n2%@\n\n3%@\n\n4%@\n\n5", _textView.text, _textView.text, _textView.text, _textView.text];
    _textView.keyboardDismissMode = UIScrollViewKeyboardDismissModeInteractive;
    _textView.contentInset = UIEdgeInsetsMake(0, 0, 216, 0);
}

Now showing and hiding keyboard will cause text jumps in some cases.

I've found the reason of jumping by subclass UITextView. The only method in my subclass is:

- (void)setContentSize:(CGSize)contentSize {
    NSLog(@"CS: %@", NSStringFromCGSize(contentSize));
    [super setContentSize:contentSize];
}

And it show contentSize shrinks and expands on keyboard hide. Something like this:

013-09-16 14:40:27.305 textView-bug2[11087:a0b] CS: {320, 651}
2013-09-16 14:40:27.313 textView-bug2[11087:a0b] CS: {320, 885}
2013-09-16 14:40:27.318 textView-bug2[11087:a0b] CS: {320, 902}

Looks like behavior of UITextView was changed a lot in iOS7. And some things are broken now.

Discovering further I've found that new layoutManager property of my textView changes too. There some interesting info in log now:

2013-09-16 14:41:59.352 textView-bug2[11115:a0b] CS: {320, 668}
<NSLayoutManager: 0x899e800>
    1 containers, text backing has 2129 characters
    Currently holding 2129 glyphs.
    Glyph tree contents:  2129 characters, 2129 glyphs, 3 nodes, 96 node bytes, 5440 storage bytes, 5536 total bytes, 2.60 bytes per character, 2.60 bytes per glyph
    Layout tree contents:  2129 characters, 2129 glyphs, 532 laid glyphs, 13 laid line fragments, 4 nodes, 128 node bytes, 1048 storage bytes, 1176 total bytes, 0.55 bytes per character, 0.55 bytes per glyph, 40.92 laid glyphs per laid line fragment, 90.46 bytes per laid line fragment

And next line with contentSize = {320, 885} contains Layout tree contents: ..., 2127 laid glyphs, 51 laid line fragments. So it looks like some kind of autolayout tries to re-layout textView on keyboard show and changes contentSize even if layout is not finished yet. And it runs even if my textView is not changes between keyboard show/hide.

The question is: how to prevent contentSize changes?

回答1:

Looks like the problem is in default layoutManager of UITextView. I've decided to subclass it and see, where and why re-layout being initiated. But simple creation of NSLayoutManager with default settings solved the problem.

Here is code (not perfect) from my demo project (see in question). The _textView there was an outlet, so I remove it from superview. This code is placed in - viewDidLoad:

NSTextStorage* textStorage = [[NSTextStorage alloc] initWithString:_textView.text];
NSLayoutManager* layoutManager = [NSLayoutManager new];
[textStorage addLayoutManager:layoutManager];
_textContainer = [[NSTextContainer alloc] initWithSize:self.view.bounds.size];
[layoutManager addTextContainer:_textContainer];
[_textView removeFromSuperview];    // remove original textView
_textView = [[MyTextView alloc] initWithFrame:self.view.bounds 
                                textContainer:_textContainer];
[self.view addSubview:_textView];

MyTextView here is a subclass of UITextView, see question for details.

For more info see:

• About Text Handling in iOS
• Using Text Kit to Draw and Manage Text
• NSLayoutManager Class Reference for iOS

回答2:

I met a similar situation as urs. Mine shows with a different bug but due to the same reason: the contentSize property is silently changed by iOS7 incorrectly. Here is how I work around it. Its kinda a ugly fix. Whenever I need to use textView.contentSize, I calculate it by myself.

-(CGSize)sizeOfText:(NSString *)textToMesure widthOfTextView:(CGFloat)width withFont:(UIFont*)font
{
    CGSize size = [textToMesure sizeWithFont:font constrainedToSize:CGSizeMake(width-20.0, FLT_MAX) lineBreakMode:NSLineBreakByWordWrapping];
    return size;
}

then you can just call this function to get the size:

CGSize cont_size =   [self sizeOfText:self.text widthOfTextView:self.frame.size.width withFont:[UIFont systemFontOfSize:15]];

then, don't do the following:

self.contentSize = cont_size;// it causes iOS halt occasionally.

so, just use cont_size directly. I believe it's bug in iOS7 for now. Hopefully apple will fix it soon. Hope this is helpful.

回答3:

Seems the bug in iOS7. At the time of input text content area behavior is wired in iOS7, It works fine with lower iOS7 version.

I have added below delegate method of UITextView to resolved this issue :

- (void)textViewDidChange:(UITextView *)textView {
CGRect line = [textView caretRectForPosition:
    textView.selectedTextRange.start];
CGFloat overflow = line.origin.y + line.size.height
    - ( textView.contentOffset.y + textView.bounds.size.height
    - textView.contentInset.bottom - textView.contentInset.top );
if ( overflow > 0 ) {
// We are at the bottom of the visible text and introduced a line feed, scroll down (iOS 7 does not do it)
// Scroll caret to visible area
    CGPoint offset = textView.contentOffset;
    offset.y += overflow + 7; // leave 7 pixels margin
// Cannot animate with setContentOffset:animated: or caret will not appear
    [UIView animateWithDuration:.2 animations:^{
        [textView setContentOffset:offset];
    }];
}