How do I measure the real height of a rendered string? - ios

Question

I’m trying to measure a string to be drawn. I have tried sizeWithAttributes:, but that doesn’t make a difference between different characters:
NSLog(#"A: %#", NSStringFromCGSize([#"." sizeWithAttributes:labelAttributes]));
NSLog(#"B: %#", NSStringFromCGSize([#"X" sizeWithAttributes:labelAttributes]));
A: {6, 23}
B: {12, 23}
Note that the reported height is the same for both characters. Is there a call that would honour the height difference between . and X?

Answer 1

There's no such thing as "real height". Font metrics are a complex topic.
A string (consisting of characters) is converted to a bunch of glyphs. Those glyphs define what will be visible (rendered). This process involves endless subtleties that might change what you define as the "real height".
Examples:
Some characters extend very far over the normal line height.
Some (graphical) rendering options stretch out beyond the bounding box of all glyphs (like shadows or outlines).
Font metrics contain a myriad of different values for what could be considered the bounding box of a character (or glyph).
This image shows the string "abc f 123" rendered using the font Zapfino (in TextEdit.app on a Mac). The f character is selected (green box). The visual character stretches far beyond of what the selection considers the bounding box.
The red text has a shadow applied to it. It's not easy to calculate the bounding box of this part of the text either.
Your case sounds like you consider a "." character to have a smaller size than a "X". But has a space character a zero size then? It's really difficult to define the "real size" without exact knowledge of the use case. And even then there are often many undefined edge cases.
I had a use case where I wanted to convert strings to PNG images (stupid, I know). In that case the best option was to render the string into a large bitmap context and then search through the raw data bitplanes from the outside inwards until finding first non-transparent pixels.

Answer 2

Agree with Nikolai. I think the best you can do is to get smallest possible size like following.
self.myLabel.text = #".";
[self.myLabel setNeedsLayout];
[self.myLabel layoutIfNeeded];
NSLog(#"A: %#", NSStringFromCGSize([self.myLabel systemLayoutSizeFittingSize:UILayoutFittingCompressedSize]));
self.myLabel.text = #"X";
[self.myLabel setNeedsLayout];
[self.myLabel layoutIfNeeded];
NSLog(#"B: %#", NSStringFromCGSize([self.myLabel systemLayoutSizeFittingSize:UILayoutFittingCompressedSize]));
A: {5, 21}
B: {11, 21}
If you want the smallest possible size, pass UILayoutFittingCompressedSize; for the largest possible size, pass UILayoutFittingExpandedSize.