Check existence of global function in Swift

Question

Is it possible to detect if some global function (not class method) is defined (in iOS)? Something like respondsToSelector in a class...

Answer 1

Swift currently does not support looking up global functions.

For C functions (most global functions from Apple's frameworks are C functions) there are at least two ways:

• using a weakly linked symbol
• the dynamic linker API: dlopen

Both check dynamically (at runtime) if a symbol can be found.

Here's an example that checks if UIGraphicsBeginImageContextWithOptions (introduced with iOS 4) is available:

void UIGraphicsBeginImageContextWithOptions(CGSize size, BOOL opaque, CGFloat scale) __attribute__((weak));

static inline BOOL hasUIGraphicsBeginImageContextWithOptions() {
    return UIGraphicsBeginImageContextWithOptions != NULL;
}

Here's the same check, using dlsym:

#import <dlfcn.h>

static inline BOOL hasUIGraphicsBeginImageContextWithOptions() {
    return dlsym(RTLD_SELF, "UIGraphicsBeginImageContextWithOptions") != NULL;
}

The advantage of using dlsym is that you don't need a declaration and that it's easily portable to Swift.