问题

Is there a way to check strings for nil and "" in Swift? In Rails, I can use blank() to check.

I currently have this, but it seems overkill:

    if stringA? != nil {
        if !stringA!.isEmpty {
            ...blah blah
        }
    }

回答1:

If you're dealing with optional Strings, this works:

(string ?? "").isEmpty

The ?? nil coalescing operator returns the left side if it's non-nil, otherwise it returns the right side.

You can also use it like this to return a default value:

(string ?? "").isEmpty ? "Default" : string!

回答2:

You could perhaps use the if-let-where clause:

Swift 3:

if let string = string, !string.isEmpty {
    /* string is not blank */
}

Swift 2:

if let string = string where !string.isEmpty {
    /* string is not blank */
}

回答3:

Using the guard statement

I was using Swift for a while before I learned about the guard statement. Now I am a big fan. It is used similarly to the if statement, but it allows for early return and just makes for much cleaner code in general.

To use guard when checking to make sure that a string is neither nil nor empty, you can do the following:

let myOptionalString: String? = nil

guard let myString = myOptionalString, !myString.isEmpty else {
    print("String is nil or empty.")
    return // or break, continue, throw
}

/// myString is neither nil nor empty (if this point is reached)
print(myString)

This unwraps the optional string and checks that it isn't empty all at once. If it is nil (or empty), then you return from your function (or loop) immediately and everything after it is ignored. But if the guard statement passes, then you can safely use your unwrapped string.

However, String implements isEmpty property by conforming to protocol Collection. Therefore we can replace the previous code's generic constraint (Wrapped == String) with a broader one (Wrapped: Collection) so that Array, Dictionary and Set also benefit our new isNilOrEmpty property:

extension Optional where Wrapped: Collection {

    var isNilOrEmpty: Bool {
        return self?.isEmpty ?? true
    }

}

Usage with Strings:

let optionalString: String? = nil
print(optionalString.isNilOrEmpty) // prints: true

let optionalString: String? = ""
print(optionalString.isNilOrEmpty) // prints: true

let optionalString: String? = "Hello"
print(optionalString.isNilOrEmpty) // prints: false

Usage with Arrays:

let optionalArray: Array<Int>? = nil
print(optionalArray.isNilOrEmpty) // prints: true

let optionalArray: Array<Int>? = []
print(optionalArray.isNilOrEmpty) // prints: true

let optionalArray: Array<Int>? = [10, 22, 3]
print(optionalArray.isNilOrEmpty) // prints: false

Sources:

Extending optionals in Swift
Swift Tip: Non-Empty Collections

回答7:

I know there are a lot of answers to this question, but none of them seems to be as convenient as this (in my opinion) to validate UITextField data, which is one of the most common cases for using it:

extension Optional where Wrapped == String {
    var isNilOrEmpty: Bool {
        return self?.trimmingCharacters(in: .whitespaces).isEmpty ?? true
    }
}

You can just use

textField.text.isNilOrEmpty

You can also skip the .trimmingCharacters(in:.whitespaces) if you don't consider whitespaces as an empty string or use it for more complex input tests like

var isValidInput: Bool {
    return !isNilOrEmpty && self!.trimmingCharacters(in: .whitespaces).characters.count >= MIN_CHARS
}

回答8:

I would recommend.

if stringA.map(isEmpty) == false {
    println("blah blah")
}

map applies the function argument if the optional is .Some.
The playground capture also shows another possibility with the new Swift 1.2 if let optional binding.

回答9:

If you want to access the string as a non-optional, you should use Ryan's Answer, but if you only care about the non-emptiness of the string, my preferred shorthand for this is

if stringA?.isEmpty == false {
    ...blah blah
}

Since == works fine with optional booleans, I think this leaves the code readable without obscuring the original intention.

If you want to check the opposite: if the string is nil or "", I prefer to check both cases explicitly to show the correct intention:

if stringA == nil || stringA?.isEmpty == true {
    ...blah blah
}

回答10:

SWIFT 3

extension Optional where Wrapped == String {

    /// Checks to see whether the optional string is nil or empty ("")
    public var isNilOrEmpty: Bool {
        if let text = self, !text.isEmpty { return false }
        return true
    }
}

Use like this on optional string:

if myString.isNilOrEmpty { print("Crap, how'd this happen?") }

回答11:

Swift 3 For check Empty String best way

if !string.isEmpty{

// do stuff

}

回答12:

You can create your own custom function, if that is something you expect to do a lot.

func isBlank (optionalString :String?) -> Bool {
    if let string = optionalString {
        return string.isEmpty
    } else {
        return true
    }
}



var optionalString :String? = nil

if isBlank(optionalString) {
    println("here")
}
else {
    println("there")
}

回答13:

Swift 3 solution Use the optional unwrapped value and check against the boolean.

if (string?.isempty == true) {
    // Perform action
}

回答14:

Create a String class extension:

extension String
{   //  returns false if passed string is nil or empty
    static func isNilOrEmpty(_ string:String?) -> Bool
    {   if  string == nil                   { return true }
        return string!.isEmpty
    }
}// extension: String

Notice this will return TRUE if the string contains one or more blanks. To treat blank string as "empty", use...

return string!.trimmingCharacters(in: CharacterSet.whitespaces).isEmpty

... instead. This requires Foundation.

Use it thus...

if String.isNilOrEmpty("hello world") == true 
{   print("it's a string!")
}

回答15:

Swift 3 This works well to check if the string is really empty. Because isEmpty returns true when there's a whitespace.

extension String {
    func isEmptyAndContainsNoWhitespace() -> Bool {
        guard self.isEmpty, self.trimmingCharacters(in: .whitespaces).isEmpty
            else {
               return false
        }
        return true
    }
}

Examples:

let myString = "My String"
myString.isEmptyAndContainsNoWhitespace() // returns false

let myString = ""
myString.isEmptyAndContainsNoWhitespace() // returns true

let myString = " "
myString.isEmptyAndContainsNoWhitespace() // returns false

回答16:

You should do something like this:
if !(string?.isEmpty ?? true) { //Not nil nor empty }

Nil coalescing operator checks if the optional is not nil, in case it is not nil it then checks its property, in this case isEmpty. Because this optional can be nil you provide a default value which will be used when your optional is nil.

回答17:

Using isEmpty

"Hello".isEmpty  // false
"".isEmpty       // true

Using allSatisfy

extension String {
  var isBlank: Bool {
    return allSatisfy({ $0.isWhitespace })
  }
}

"Hello".isBlank        // false
"".isBlank             // true

Using optional String

extension Optional where Wrapped == String {
  var isBlank: Bool {
    return self?.isBlank ?? true
  }
}

var title: String? = nil
title.isBlank            // true
title = ""               
title.isBlank            // true

Reference : https://useyourloaf.com/blog/empty-strings-in-swift/

回答18:

This is a general solution for all types that conform to the Collection protocol, which includes String:

extension Optional where Wrapped: Collection {
    var isNilOrEmpty: Bool {
        return self?.isEmpty ?? true
    }
}

回答19:

When dealing with passing values from local db to server and vice versa, I was having too much trouble with ?'s and !'s and what not.

So I made a Swift3.0 utility to handle null cases and i can almost totally avoid ?'s and !'s in the code.

func str(_ string: String?) -> String {
    return (string != nil ? string! : "")
}

Ex:-

Before :

    let myDictionary: [String: String] = 
                      ["title": (dbObject?.title != nil ? dbObject?.title! : "")]

After :

    let myDictionary: [String: String] = 
                        ["title": str(dbObject.title)]

and when its required to check for a valid string,

    if !str(dbObject.title).isEmpty {
        //do stuff
    }

This saved me having to go through the trouble of adding and removing numerous ?'s and !'s after writing code that reasonably make sense.

回答20:

Use the ternary operator (also known as the conditional operator, C++ forever!):

if stringA != nil ? stringA!.isEmpty == false : false { /* ... */ }

The stringA! force-unwrapping happens only when stringA != nil, so it is safe. The == false verbosity is somewhat more readable than yet another exclamation mark in !(stringA!.isEmpty).

I personally prefer a slightly different form:

if stringA == nil ? false : stringA!.isEmpty == false { /* ... */ }

In the statement above, it is immediately very clear that the entire if block does not execute when a variable is nil.

回答21:

helpful when getting value from UITextField and checking for nil & empty string

@IBOutlet weak var myTextField: UITextField!

Heres your function (when you tap on a button) that gets string from UITextField and does some other stuff

@IBAction func getStringFrom_myTextField(_ sender: Any) {

guard let string = myTextField.text, !(myTextField.text?.isEmpty)!  else { return }
    //use "string" to do your stuff.
}

This will take care of nil value as well as empty string.

It worked perfectly well for me.

回答22:

In my opinion, the best way to check the nil and empty string is to take the string count.

var nilString : String?
print(nilString.count) // count is nil

var emptyString = ""
print(emptyString.count) // count is 0

// combine both conditions for optional string variable
if string?.count == nil || string?.count == 0 {
   print("Your string is either empty or nil")
}

回答23:

you can use this func

 class func stringIsNilOrEmpty(aString: String) -> Bool { return (aString).isEmpty }

来源：https://stackoverflow.com/questions/29381994/check-string-for-nil-empty

标签

swift

string

optional

is-empty

Check string for nil & empty

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