When designing an application for iOS, am I tied to the San Francisco typography guidelines offered by Apple?
To better put this, do I have to follow them strictly?
I am new to this and I would like to better understand the iOS environment.
Apple's iOS developer guidelines on typography are pretty clear (emphasis mine):
San Francisco (SF) is the system typeface in iOS. The fonts of this typeface are optimized to give your text unmatched legibility, clarity, and consistency...
If possible, use a single typeface. Mixing several different typefaces can make your app seem fragmented and sloppy. Consider using one typeface and just a few font variants and sizes.
Use built-in text styles whenever possible. The built-in text styles let you express content in ways that are visually distinct, while retaining optimal legibility. These styles are based on the system fonts and allow you to take advantage of key typographic features, such as Dynamic Type, which automatically adjusts tracking and leading for every font size.
Further we find (again, emphasis mine):
Make sure custom fonts are legible. Custom typefaces are supported on iOS, but are often tough to read. Unless your app has a compelling need for a custom font, such as for branding purposes or to create an immersive gaming experience, it’s usually best to stick with the system fonts. If you do use a custom font, make sure it’s easily readable, even at small sizes.
Implement accessibility features for custom fonts. System fonts automatically react to accessibility features like bold text and larger type. Apps using custom fonts should implement the same behavior by checking whether accessibility features are enabled and registering for notifications when they change.
What we can tell from all this:
Apple has spent somewhere between millions and billions of person-hours developing their UI and typography standards, and have a peerless track record of phenomenal results (regardless of how you may feel about any of their business practices or choices regarding open coding, open source, blackboxing etc) for product design, legibility and typography, and are loathe to allow anyone to goober it up easily, and are more than happy to provide easy access to their iOS inbuilt resources, and have provided super-clear direction for how to address type styling within that use case easily.
They also provide information on the use case in which you choose not to use their inbuilt system font, and use custom fonts, both to help you maintain an experience which feels native to their device and ecosystem and which helps you provide both legibility and design consistency, which aide you in presenting your work as being professional in quality.
So - no, you are not locked in, but yes, they have clear guidelines, and yes they, and we the broad user-base, expect you to follow them as best practice.
From what I've seen over time in this ecosystem, apps whose designers tend towards unique-just-to-differentiate typography tend to either disappear or shift to finding better, more usability-oriented UI design areas to express the developer's unique design sensibilities.
Hope that helps.