Swift Testing optionals for nil

Question

I m using Xcode 6 Beta 4  I have this weird situation where I cannot figure out how to appropriately test for optionals   If I have an optional xyz  is the correct way to test   if  xyz     Do something   or  if  xyz    nil     Do something   The documents say to do it the first way  but I ve found that sometimes  the second way is required  and doesn t generate a compiler error  but other times  the second way generates a compiler error   My specific example is using the GData XML parser bridged to swift   let xml   GDataXMLDocument      XMLString  responseBody      options  0      error   amp xmlError    if  xmlError    nil    Here  if I just did   if xmlError   it would always return true  However  if I do   if  xmlError    nil    then it works  as how it works in Objective-C    Is there something with the GData XML and the way it treats optionals that I am missing

User · Accepted Answer

In Xcode Beta 5  they no longer let you do    var xyz   NSString   if xyz        Do something using  xyz       This produces an error      does not conform to protocol  BooleanType Protocol    You have to use one of these forms   if xyz    nil         Do something using  xyz      if let xy   xyz         Do something using  xy

User · Answer

Instead of if  ternary operator might come handy when you want to get a value based on whether something is nil   func f x  String   - gt  String       return x    nil    empty     non-empty

User · Answer

Another approach besides using if or guard statements to do the optional binding is to extend Optional with    extension Optional        func ifValue   valueHandler   Wrapped  - gt  Void            switch self           case  some let wrapped   valueHandler wrapped          default  break                      ifValue receives a closure and calls it with the value as an argument when the optional is not nil  It is used this way   var helloString  String     Hello  World    helloString ifValue       print  0     prints  Hello  World      helloString   nil  helloString ifValue       print  0     This code never runs     You should probably use an if or guard however as those are the most conventional  thus familiar  approaches used by Swift programmers

User · Answer

Although you must still either explicitly compare an optional with nil or use optional binding to additionally extract its value  i e  optionals are not implicitly converted into Boolean values   it s worth noting that Swift 2 has added the guard statement to help avoid the pyramid of doom when working with multiple optional values   In other words  your options now include explicitly checking for nil   if xyz    nil          Do something with xyz     Optional binding   if let xyz   xyz          Do something with xyz         Note that we can reuse the same variable name      And guard statements   guard let xyz   xyz else          Handle failure and then exit this code block        e g  by calling return  break  continue  or throw     return       Do something with xyz  which is now guaranteed to be non-nil   Note how ordinary optional binding can lead to greater indentation when there is more than one optional value   if let abc   abc       if let xyz   xyz              Do something with abc and xyz                   You can avoid this nesting with guard statements   guard let abc   abc else          Handle failure and then exit this code block     return    guard let xyz   xyz else          Handle failure and then exit this code block     return       Do something with abc and xyz

User · Answer

Swift 3 0  4 0  There are mainly two ways of checking optional for nil  Here are examples with comparison between them  1  if let  if let is the most basic way to check optional for nil  Other conditions can be appended to this nil check  separated by comma  The variable must not be nil to move for the next condition  If only nil check is required  remove extra conditions in the following code   Other than that  if x is not nil  the if closure will be executed and x val will be available inside  Otherwise the else closure is triggered   if let x val   x  x val  gt  5         x val available on this scope   else        2  guard let  guard let can do similar things  It s main purpose is to make it logically more reasonable  It s like saying Make sure the variable is not nil  otherwise stop the function  guard let can also do extra condition checking as if let   The differences are that the unwrapped value will be available on same scope as guard let  as shown in the comment below  This also leads to the point that in else closure  the program has to exit the current scope  by return  break  etc   guard let x val   x  x val  gt  5 else       return     x val available on this scope

User · Answer

One of the most direct ways to use optionals is the following   Assuming xyz is of optional type  like Int  for example   if let possXYZ   xyz          do something with possXYZ  the unwrapped value of xyz    else          do something now that we know xyz is  None     This way you can both test if xyz contains a value and if so  immediately work with that value   With regards to your compiler error  the type UInt8 is not optional  note no      and therefore cannot be converted to nil  Make sure the variable you re working with is an optional before you treat it like one

User · Answer

Also you can use Nil-Coalescing Operator  The nil-coalescing operator  a    b  unwraps an optional a if it contains a value  or returns a default value b if a is nil  The expression a is always of an optional type  The expression b must match the type that is stored inside a   let value   optionalValue    defaultValue  If optionalValue is nil  it automatically assigns value to defaultValue

User · Answer

If you have conditional and would like to unwrap and compare  how about taking advantage of the short-circuit evaluation of compound boolean expression as in  if xyz    nil  amp  amp  xyz      some non-nil value         Granted  this is not as readable as some of the other suggested posts  but gets the job done and somewhat succinct than the other suggested solutions

User · Answer

One option that hasn t specifically been covered is using Swift s ignored value syntax   if let     xyz          something that should only happen if xyz is not nil     I like this since checking for nil feels out of place in a modern language like Swift  I think the reason it feels out of place is that nil is basically a sentinel value  We ve done away with sentinels pretty much everywhere else in modern programming so nil feels like it should go too

User · Answer

To add to the other answers  instead of assigning to a differently named variable inside of an if condition   var a  Int    5  if let b   a         do something     you can reuse the same variable name like this   var a  Int    5  if let a   a          do something     This might help you avoid running out of creative variable names     This takes advantage of variable shadowing that is supported in Swift

User · Answer

From swift programming guide      If Statements and Forced Unwrapping      You can use an if statement to find out whether an optional contains a   value  If an optional does have a value  it evaluates to true  if it   has no value at all  it evaluates to false    So the best way to do this is      swift  gt  3 if xyz    nil      and if you are using the xyz in if statement Than you can unwrap xyz in if statement in constant variable  So you do not need to unwrap every place in if statement where xyz is used   if let yourConstant   xyz          use youtConstant you do not need to unwrap  xyz      This convention is suggested by apple and it will be followed by devlopers

User · Answer

var xyz   NSDictionary      case 1  xyz     1   one      case 2   empty dictionary  xyz   NSDictionary       case 3  do nothing  if xyz   NSLog  xyz is not nil      else     NSLog  xyz is nil            This test worked as expected in all cases   BTW  you do not need the brackets

User · Answer

Now you can do in swift the following thing which allows you to regain a little bit of the objective-c if nil else  if textfieldDate text  isEmpty    true

User · Answer

Swift 5 Protocol Extension  Here is an approach using protocol extension so that you can easily inline an optional nil check   import Foundation  public extension Optional        var isNil  Bool            guard case Optional none   self else               return false                    return true             var isSome  Bool            return  self isNil             Usage  var myValue  String   if myValue isNil          do something    if myValue isSome          do something

[ios] Swift: Testing optionals for nil

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