Working with C APIs from Swift

Question

I'm trying to keep track of the network status. I went through the code of the FXReachability. Specifically the following method.

- (void)setHost:(NSString *)host
{
    if (host != _host)
    {
        if (_reachability)
        {
            SCNetworkReachabilityUnscheduleFromRunLoop(_reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);
            CFRelease(_reachability);
        }
        _host = [host copy];
        _status = FXReachabilityStatusUnknown;
        _reachability = SCNetworkReachabilityCreateWithName(kCFAllocatorDefault, [_host UTF8String]);
        SCNetworkReachabilityContext context = { 0, ( __bridge void *)self, NULL, NULL, NULL };
        SCNetworkReachabilitySetCallback(_reachability, ONEReachabilityCallback, &context);
        SCNetworkReachabilityScheduleWithRunLoop(_reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);
    }
}

What it does is it keeps checking the connection to the specified host. I'm trying to convert this method to Swift and I'm having a couple of problems.

1. Converting the host string to UTF8.

I have to convert the host string to UTF8 and pass it into the SCNetworkReachabilityCreateWithName method. I can't find the exact Swift equivalent of UTF8String property in Objective-C. But there is a property called utf8 for the String type. According to the documentation,

You can access a UTF-8 representation of a String by iterating over its utf8 property. This property is of type String.UTF8View, which is a collection of unsigned 8-bit (UInt8) values, one for each byte in the string’s UTF-8 representation:

How can I get a complete UTF8 representation of a string instead of a collection of unsigned 8-bit (UInt8) values?

---

2. Callback function for SCNetworkReachabilitySetCallback.

Th second parameter of this function expects something of type SCNetworkReachabilityCallBack. I dived in to the source file and found out that it's actually a typealias.

typealias SCNetworkReachabilityCallBack = CFunctionPointer<((SCNetworkReachability!, SCNetworkReachabilityFlags, UnsafeMutablePointer<Void>) -> Void)>

I defined it like this.

let reachabilityCallBack: SCNetworkReachabilityCallBack = {

}()

But I get the error Missing return in a closure expected to return 'SCNetworkReachabilityCallBack' when in that typealias you can clearly see that it returns Void.

Is this the wrong way to do this?

---

These are the problems I'm facing. I've been at this for hours now with no luck so I'd really appreciate any help.

Thank you.

Answer 1

Your first problem can be solved with

let reachability = host.withCString {
    SCNetworkReachabilityCreateWithName(nil, $0).takeRetainedValue()
}

Inside the closure, $0 is a pointer to the NUL-terminated UTF-8 representation of the String.

Update: As Nate Cook said in a now deleted answer and also here, you can actually pass a Swift string to a function taking a UnsafePointer<UInt8> directly:

let reachability = SCNetworkReachabilityCreateWithName(nil, host).takeRetainedValue()

---

Unfortunately there is (as far as I know) currently no solution to your second problem. SCNetworkReachabilitySetCallback expects a pointer to a C function as second parameter, and there is currently no method to pass a Swift function or closure. Note that the documentation for SCNetworkReachabilityCallBack shows only Objective-C but no Swift.

See also Does Swift not work with function pointers?.

---

Update for Swift 2: It is now possible to pass a Swift closure to a C function taking a function pointer parameter. Also SCNetworkReachabilityCreateWithName() does not return an unmanaged object anymore:

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
}, &context) 

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes)

Update for Swift 3 (Xcode 8):

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
    }, &context)

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(),
                                         CFRunLoopMode.commonModes.rawValue)