ios - Try catch whole block in Swift 2 -

i know how make try-catch single function. problem want try-catch whole block of code - simple if goes wrong ignore whole block.

let me show simple example. simple parser facebook graph api /me/taggable_friends:

let params:[nsobject : anyobject] = ["fields": "first_name, last_name, id, picture"] let request = fbsdkgraphrequest(graphpath: "/me/taggable_friends", parameters: params, httpmethod: "get")  request.startwithcompletionhandler { (connection, result, error) -> void in              if let error = error {                 print(error)             }             else{                  let resultdictionary:nsdictionary! = result as! [string: anyobject]                 let array = resultdictionary["data"] as! nsarray                 object in array {                     let firstname = object["first_name"] as! string                     let lastname = object["last_name"] as! string                     let tagid = object["id"] as! string                     let picture = object["picture"] as! nsdictionary                     let picturedata = picture["data"] as! nsdictionary                     let pictureurl = picturedata["url"] as! string                      let friend = facebookfriend(firstname: firstname, lastname: lastname, tagid: tagid, picture: pictureurl)                      print(pictureurl)                      self.friends.append(friend)                 }               }         } 

that code working , quietly fail error not nil. let's facebook has changed or somehow values in json aren't available. app crash. have make if-let statements every line of code. it'll make long , hard read. in java i'd make try-catch whole block. in swift don't know how that. what's more compiler saying "as!" (so unwrapping optional value) can't raise exception false :)

this guard statement comes in handy, basic syntax is:

guard let foo = expression else {     handle error, , return (or throw) } 

in case, you're wanting protect bunch of optional parsing vagaries of facebook changing api @ point in future. using guard , continue in case allows safely skip rest of block:

    object in array {         guard             let firstname = object["first_name"] as? string,             let lastname = object["last_name"] as? string,             let tagid = object["id"] as? string,             let picture = object["picture"] as? nsdictionary,             let picturedata = picture["data"] as? nsdictionary,             let pictureurl = picturedata["url"] as? string else {                 print("json format has changed")                 continue         }          let friend = facebookfriend(firstname: firstname, lastname: lastname, tagid: tagid, picture: pictureurl)          print(pictureurl)          self.friends.append(friend)     } 

generally speaking, swift try/catch blocks aren't analogous try/catch in other languages, particularly java. in java exceptions (to extent) handleable, such null references, array bounds issues, etc. in swift, try/catch errors definitively not meant handle exceptions, meant handle errors. there's no way use try/catch protect as! string failing. have expect , protect against possibility, precisely guard comes play, potentially in combination throw.