Lines missing from tall UILabel when embedding NSTextAttachment

Question

I can create a multi-line NSAttributedString by using escaped new-line characters (@"\n"). With iOS 7, I can now embed a UIImage inside attributed strings (via NSTextAttachment).

I have noticed that whenever I set the attributedText of a UILabel to a multi-line attributed string with an embedded image, the number of lines actually displayed is inversely proportional to the height of the label. For example, when the height of the label is 80, two lines appear; when the height is around 100, only the second line appears; when the height is about 130, nothing appears.

This problem occurred while trying to position multiple UILabels side-by-side inside a UITableViewCell and having the labels grow (vertically) with the cell height.

Can anyone explain why this is happening? Does anyone know workarounds that don't involve making the UILabel smaller?

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Sample code:

@implementation SOViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    NSMutableAttributedString *text1 = [[NSMutableAttributedString alloc] init];
    [text1 appendAttributedString:[[NSAttributedString alloc] initWithString:@"Line 1\n"]];
    [text1 appendAttributedString:[[NSAttributedString alloc] initWithString:@"Line 2"]];

    UIImage *image = [UIImage imageNamed:@"17x10"]; //some PNG image (17px by 10px)

    NSTextAttachment *attachment = [[NSTextAttachment alloc] init];
    attachment.image = image;
    attachment.bounds = CGRectMake(0, 0, image.size.width, image.size.height);

    NSMutableAttributedString *text2 = [[NSMutableAttributedString alloc] init];
    [text2 appendAttributedString:[[NSAttributedString alloc] initWithString:@"Line 1\n"]];
    [text2 appendAttributedString:[NSAttributedString attributedStringWithAttachment:attachment]];
    [text2 appendAttributedString:[[NSAttributedString alloc] initWithString:@"Line 2"]];

    CGFloat margin = 20;

    //shows both lines when height == 80
    //shows line 2 when 90 <= height <= 120
    //shows nothing when height == 130
    CGFloat height = ???;
    CGFloat width = 200;

    UILabel *label1 = [[UILabel alloc] initWithFrame:CGRectMake(margin, margin, width, height)];
    UILabel *label2 = [[UILabel alloc] initWithFrame:CGRectMake(margin, margin + height, width, height)];
    [self.view addSubview:label1];
    [self.view addSubview:label2];
    label1.backgroundColor = [UIColor orangeColor];
    label2.backgroundColor = [UIColor blueColor];
    label2.textColor = [UIColor whiteColor];
    label1.numberOfLines = 0;
    label2.numberOfLines = 0;
    label1.attributedText = text1;
    label2.attributedText = text2;

    UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
    imageView.frame = CGRectMake(margin + width, margin + height, image.size.width, image.size.height);
    [self.view addSubview:imageView];
}

@end

... Put this in the default view controller of a "Single View Application". (You can pick your own image.)

Answer 1

I have found another workaround for this bug, which is sufficiently different from my previous answer that I'm providing it as another answer: let the label set its own height.

In this code, I'm removing the height constraint from a label with a fixed width constraint, and replacing it with a greater-than height constraint (and I'm sure there are other ways to achieve the same outcome):

[self.lab removeConstraint:self.labelHeight];
[self.lab addConstraint:
 [NSLayoutConstraint constraintWithItem:self.lab 
 attribute:NSLayoutAttributeHeight 
 relatedBy:NSLayoutRelationGreaterThanOrEqual 
 toItem:nil attribute:0 multiplier:1 constant:20]];

That label displays correctly every attributed string I throw at it! Of course, you lose the automatic vertical centering of the string, but that's the whole source of the bug, so losing it is not so terrible.

Answer 2

It seems to be a bug in UILabel. The same code works fine when UITextView is used instead of UILabel (UITextView's default font size is different, so I tested it on different heights).