Changing font programmatically to System Semibold
Solution 1:
The system font for iOS, starting in iOS 9, is "San Fransisco." To get a semibold version, you could use something along the lines of:
Swift 2.2:
UIFont.systemFontOfSize(20, weight: UIFontWeightSemibold)
Swift 3.0:
UIFont.systemFont(ofSize: 20, weight: UIFontWeightSemibold)
Swift 4.0:
UIFont.systemFont(ofSize: 20, weight: .semibold)
For more information on this, see the UIFont Class Reference.
Solution 2:
To further assist you these are the official names of all the helvetica fonts that apple currently supports as taken directly from the Apple website: https://support.apple.com/en-us/HT202771
Helvetica
Helvetica Bold
Helvetica Bold Oblique
Helvetica Light
Helvetica Light Oblique
Helvetica Neue
Helvetica Neue Bold
Helvetica Neue Bold Italic
Helvetica Neue Condensed Black
Helvetica Neue Condensed Bold
Helvetica Neue Italic
Helvetica Neue Light
Helvetica Neue Light Italic
Helvetica Neue Medium
Helvetica Neue Medium Italic
Helvetica Neue Thin
Helvetica Neue Thin Italic
Helvetica Neue UltraLight
Helvetica Neue UltraLight Italic
Helvetica Oblique