CTFontGetAdvancesForGlyphs工作机制及单字形调用性能疑问

Great question! Let's break this down clearly:

First, let's talk about how CTFont works under the hood. A CTFont object caches most glyph-related metrics (including advance widths) once they're first calculated. When you call CTFontGetAdvancesForGlyphs with a single glyph, it's almost always a fast lookup from the font's internal cache, not a full recalculation every time—assuming the font itself (size, style, etc.) hasn't changed between calls.

That said, even with internal caching, there's still a tiny overhead to calling the CoreText function each time. If you're repeatedly fetching the same glyph's advance width in a performance-sensitive context (like rendering a large amount of text in a loop), storing the result as a constant is absolutely a good idea. It eliminates the function call overhead entirely, which can add up with enough iterations.

To confirm the behavior: when your array has only one glyph, CTFontGetAdvancesForGlyphs skips the "sum all advances" logic and just returns that single glyph's width directly from the cache (after the first call, that is). The first call will trigger the initial calculation and cache the value, but every subsequent call for the same glyph/font combo is just a quick lookup.

Quick Recommendations: #

• If the glyph and font are fixed (no changes to size, weight, or the font itself), cache the advance width as a constant—this is a safe, performance-friendly choice.
• If the font or glyph might change dynamically (e.g., switching fonts at runtime), you'll need to call the function each time to get the up-to-date value.

内容的提问来源于stack exchange，提问作者MassMover