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...

Are you specifically looking for a Swift function—or a C function imported from Apple's frameworks? — Nikolai Ruhe, Jul 13 '16 at 14:00
if its global function, so why do you care about checking its existence. Just curious — Hossam Ghareeb, Jul 13 '16 at 14:12
@HossamGhareeb That's commonly needed to check availability of functions from Apple's frameworks. Some functions are supported only on later OS versions. — Nikolai Ruhe, Jul 13 '16 at 14:30
@NikolaiRuhe In Swift we are supposed to use `#available` for that. — Sulthan, Jul 13 '16 at 14:42
@Sulthan There's a slight difference: `#available` can only check for OS versions. The question is explicitly about checking the existence of a global function. Anyway, for the common use case (Apple frameworks) the result is the same. — Nikolai Ruhe, Jul 13 '16 at 15:27

score 6 · Accepted Answer · answered Jul 13 '16 at 14:19

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.

Now I remember I actually have used the weak linking myself before. I am not really sure if that would work for Swift globals though. — Sulthan, Jul 13 '16 at 14:42
@Sulthan It wouldn't, because Objective-C cannot see Swift globals. But perhaps that is not what the OP is asking. — matt, Jul 13 '16 at 14:46
@Sulthan Swift's name mangling is preventing simple symbol lookup. — Nikolai Ruhe, Jul 13 '16 at 15:23

Sulthan · Answer 2 · 2016-07-13T14:43:38.760

0

No, it's not possible in Swift.

Even respondsToSelector uses the Obj-C runtime and can be used only for functions available in Obj-C.

edited Jul 13 '16 at 14:43

answered Jul 13 '16 at 14:00

Sulthan

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How will you get past the compiler if the function does not exist? – Feldur Jul 13 '16 at 15:31
@Feldur That's why we have `#available`. – Sulthan Jul 13 '16 at 15:58
@Feldur You can always declare a function (as `extern`, but that's the default) and then leave it up to the dynamic linker to resolve the symbol. That's what happens all of the time when linking to system frameworks. – Nikolai Ruhe Jul 13 '16 at 15:59
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Wa's the question not relative to doing this in Swift? Extern does not seem to be a swift keyword – Feldur Jul 13 '16 at 16:29

Check existence of global function in Swift

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