Calling a REST service from Swift — a quick example

I figured this would be a recurring theme, so thought I’d write it down for posterity, not least so I didn’t lose it. The scenario is a common one — we have a mobile app for iOS written in Swift where in response to some kind of user trigger we need to go off to the server to do some work via a REST endpoint using JSON as our protocol format. Here’s a snippet I created to do exactly this, a variation on a couple of similar examples but with a couple of notes of things I found along the way.

Let’s assume we’re in a simple View Controller where we’re responding to stimulus from the user in the form of a button. I have an action handler called getData to respond to a button push.

@IBAction func getData(sender: AnyObject) {
   dispatch_async(dispatch_get_main_queue(), {
      self.getJsonData()
   })
}

You will notice a couple of things. Firstly we’re calling a another function on the controller that will actually do the heavy lifting. I’ve separated that out purely to make things easier to read and follow. The second thing is that I’m using the Grand Central Dispatch mechanism in Swift to spin off the action into a separate thread. This follows general good practice for user interface that you shouldn’t do work on the main thread that might block for some period of time, such as writing to the network or disk. Doing so could cause your app to hang and potentially be shut down by iOS as being unresponsive as a result (Apple themselves explain this nicely).

So now to the main guts of the code. In my simple case here I’m doing an HTTP GET of a JSON feed that returns an array of items that looks a bit like this:

[
  {
    "device": "TEST",
    "event": "Testing",
    "date": "2014-08-12T08:31:25.449Z",
    "_id": "53e9d0ddf5d6a92100b8254c"
  },
  {
    "device": "TEST",
    "event": "Testing2again",
    "date": "2014-08-12T09:25:25.062Z",
    "_id": "53e9dd85f5d6a92100b8254d"
  }
]

Here’s the function that does the work of going off to the server to get the feed.

func getJsonData() {
   var urlPath = "http://myserver/myaudits"
   var url = NSURL(string: urlPath)
        
   var session = NSURLSession.sharedSession()
   var task = session.dataTaskWithURL(url!, 
      completionHandler: {
         data, 
         response, 
         error -> Void in
         
            println("Task completed")
           
            if(error != nil) {
                println(error.localizedDescription)
            }
            var err: NSError?
            
            var results = NSJSONSerialization.JSONObjectWithData(data, 
               options: NSJSONReadingOptions.MutableContainers, 
               error: &err) as NSArray
            
            if(err != nil) {
                println("JSON Error \(err!.localizedDescription)")
            }

            println("\(results.count) JSON rows returned and parsed into an array")
            
            if (results.count != 0) {
                // For this example just spit out the first item "event" entry
                var rowData: NSDictionary = results[0] as NSDictionary
                var deviceValue = rowData["device"] as String;
                println("Got \(deviceValue) out")
            } else {
                println("No rows returned")
            }
        })
        
   task.resume()
}

Note that I’m explicitly casting the parsed JSON as an NSArray. One thing I discovered is that Swift isn’t massively tolerant if you get this wrong, for example if the JSON resolves to an Object, then you need to cast to an NSDictionary. Rather than return a useful message, it simply crashed the app. Now of course this was due to a genuine developer gaffe, but still it soaked up a fair amount of time trying to work out what was wrong.

With that, you should be all set. Hope this is useful.

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