blocks|key|105328|text|现在内置了使用.isEmpty检测空字符串的功能|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|105329|if+emptyString.isEmpty+{
++++print("Nothing+to+see+here")
}|code-block|syntax|javascript|105330|苹果预发行文档："Strings+and+Characters"。|105331|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html#//apple_ref/doc/uid/TP40014097-CH7-XID_368^0|7|8|0|0|8|O|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

There is now the built in ability to detect empty string with <code>.isEmpty</code>:

<pre><code>if emptyString.isEmpty {
 print("Nothing to see here")
}
</code></pre>

Apple Pre-release documentation: <a href="https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html#//apple_ref/doc/uid/TP40014097-CH7-XID_368" rel="noreferrer">"Strings and Characters"</a>.

blocks|key|4611998|text|检查字符串是否为nil或空的一种简明方法是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4611999|var+myString:+String?+=+nil

if+(myString+??+"").isEmpty+{
++++print("String+is+nil+or+empty")
}|code-block|syntax|javascript|4612000|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

A concise way to check if the string is nil or empty would be:

<pre><code>var myString: String? = nil

if (myString ?? "").isEmpty {
 print("String is nil or empty")
}
</code></pre>

blocks|key|4611974|text|我正在完全重写我的答案(again)。这一次是因为我成了guard+statement和early+return的粉丝。这使得代码更加整洁。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4611975|非可选字符串|4611976|检查长度是否为零。|4611977|let+myString:+String+=+""

if+myString.isEmpty+{
++++print("String+is+empty.")
++++return+//+or+break,+continue,+throw
}

//+myString+is+not+empty+(if+this+point+is+reached)
print(myString)|code-block|syntax|javascript|4611978|如果if语句通过，那么您就可以安全地使用该字符串，因为它不是空的。如果它是空的，那么函数将提前返回，之后什么都不会发生。|4611979|可选字符串|4611980|检查是否为nil或零长度。|4611981|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)|4611982|这将展开可选的，并同时检查它是否为空。在传递guard语句之后，您可以安全地使用未包装的非空字符串。|4611983|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/posts/31327259/revisions|1|https://thatthinginswift.com/guard-statement-swift/|2|https://www.hackingwithswift.com/new-syntax-swift-2-guard^0|S|5|C|5|0|S|F|1|18|C|2|0|0|0|0|2|2|0|0|0|0|M|5|0^^$0|@$1|2|3|4|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@$9|1D|A|1E|1|1F]|$9|1G|A|1H|1|1I]|$9|1J|A|1K|1|1L]]|E|$]]|$1|F|3|G|5|6|7|1M|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1N|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1O|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1P|8|@$9|1Q|A|1R|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1S|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1T|8|@]|D|@]|E|$]]|$1|U|3|V|5|L|7|1U|8|@]|D|@]|E|$M|N]]|$1|W|3|X|5|6|7|1V|8|@$9|1W|A|1X|B|C]]|D|@]|E|$]]|$1|Y|3|-4|5|6|7|1Y|8|@]|D|@]|E|$]]]|Z|$10|$5|11|12|13|E|$14|15]]|16|$5|11|12|13|E|$14|17]]|18|$5|11|12|13|E|$14|19]]]]

I am completely rewriting my answer (<a href="https://stackoverflow.com/posts/31327259/revisions">again</a>). This time it is because I have become a fan of the <a href="https://thatthinginswift.com/guard-statement-swift/" rel="noreferrer"><code>guard</code> statement</a> and <a href="https://www.hackingwithswift.com/new-syntax-swift-2-guard" rel="noreferrer">early return</a>. It makes for much cleaner code.
<h1>Non-Optional String</h1>
Check for zero length.
<pre><code>let myString: String = &quot;&quot;

if myString.isEmpty {
 print(&quot;String is empty.&quot;)
 return // or break, continue, throw
}

// myString is not empty (if this point is reached)
print(myString)
</code></pre>
If the <code>if</code> statement passes, then you can safely use the string knowing that it isn't empty. If it is empty then the function will return early and nothing after it matters.
<h1>Optional String</h1>
Check for nil or zero length.
<pre><code>let myOptionalString: String? = nil

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

// myString is neither nil nor empty (if this point is reached)
print(myString)
</code></pre>
This unwraps the optional and checks that it isn't empty at the same time. After passing the <code>guard</code> statement, you can safely use your unwrapped nonempty string.

blocks|key|105365|text|下面是我检查字符串是否为空的方法。“空白”指的是一个空的或只包含空格/换行符的字符串。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|105366|struct+MyString+{
++static+func+blank(text:+String)+->+Bool+{
++++let+trimmed+=+text.trimmingCharacters(in:+CharacterSet.whitespacesAndNewlines)
++++return+trimmed.isEmpty
++}
}|code-block|syntax|javascript|105367|使用方法：|105368|MyString.blank("+")+//+true|105369|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Here is how I check if string is blank. By 'blank' I mean a string that is either empty or contains only space/newline characters.

<pre><code>struct MyString {
 static func blank(text: String) -&gt; Bool {
 let trimmed = text.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)
 return trimmed.isEmpty
 }
}
</code></pre>

How to use:

<pre><code>MyString.blank(" ") // true
</code></pre>

blocks|key|105655|text|您还可以使用可选扩展，这样就不必担心解包或使用==+true|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|105656|extension+String+{
++++var+isBlank:+Bool+{
++++++++return+self.trimmingCharacters(in:+.whitespacesAndNewlines).isEmpty
++++}
}
extension+Optional+where+Wrapped+==+String+{
++++var+isBlank:+Bool+{
++++++++if+let+unwrapped+=+self+{
++++++++++++return+unwrapped.isBlank
++++++++}+else+{
++++++++++++return+true
++++++++}
++++}
}|code-block|syntax|javascript|105657|注意:在optional上调用此函数时，请确保不要使用?，否则它仍将需要解包。|105658|entityMap^0|N|7|0|0|R|1|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

You can also use an optional extension so you don't have to worry about unwrapping or using <code>== true</code>:

<pre><code>extension String {
 var isBlank: Bool {
 return self.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
 }
}
extension Optional where Wrapped == String {
 var isBlank: Bool {
 if let unwrapped = self {
 return unwrapped.isBlank
 } else {
 return true
 }
 }
}
</code></pre>

Note: when calling this on an optional, make sure not to use <code>?</code> or else it will still require unwrapping.

blocks|key|4612043|text|要同时执行nil检查和长度，Swift2.0和iOS+9以后的版本可以使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4612044|if(yourString?.characters.count+>+0){}|code-block|syntax|javascript|4612045|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

To do the nil check and length simultaneously 
Swift 2.0 and iOS 9 onwards you could use

<pre><code>if(yourString?.characters.count &gt; 0){}
</code></pre>

blocks|key|4612136|text|如果字符串为==+""，isEmpty将按您所想的那样执行，它将返回true。其他一些答案指出了一种情况，即您有一个可选字符串。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4612137|请使用Optional+Chaining！|offset|length|4612138|如果字符串不是nil，则使用isEmpty，否则不使用。|4612139|下面，将不会设置optionalString，因为字符串为空|4612140|let+optionalString:+String?+=+nil
if+optionalString?.isEmpty+==+true+{
+++++optionalString+=+"Lorem+ipsum+dolor+sit+amet"
}|code-block|syntax|javascript|4612141|显然，您不会使用上面的代码。收益来自JSON解析或其他有或没有值的情况。这保证了如果有一个值，代码就会运行。|4612142|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/OptionalChaining.html^0|0|3|H|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@$D|10|E|11|1|12]]|A|$]]|$1|F|3|G|5|6|7|13|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|14|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|15|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

isEmpty will do as you think it will, if string == "", it'll return true.
Some of the other answers point to a situation where you have an optional string.

PLEASE use <a href="https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/OptionalChaining.html" rel="nofollow noreferrer">Optional Chaining</a>!!!!

If the string is not nil, isEmpty will be used, otherwise it will not.

Below, the optionalString will NOT be set because the string is nil

<pre><code>let optionalString: String? = nil
if optionalString?.isEmpty == true {
 optionalString = "Lorem ipsum dolor sit amet"
}
</code></pre>

Obviously you wouldn't use the above code. The gains come from JSON parsing or other such situations where you either have a value or not. This guarantees code will be run if there is a value.

blocks|key|4612101|text|检查仅检查文本中的空格和换行符|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4612102|extension+String
{
++++var++isBlank:Bool+{
++++++++return+self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).isEmpty
++++}
}|code-block|syntax|javascript|4612103|使用|4612104|if+text.isBlank
{
++//text+is+blank+do+smth
}|4612105|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Check check for only spaces and newlines characters in text

<pre><code>extension String
{
 var isBlank:Bool {
 return self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).isEmpty
 }
}
</code></pre>

using 

<pre><code>if text.isBlank
{
 //text is blank do smth
}
</code></pre>

blocks|key|4612070|text|对于可选字符串，如何：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4612071|if+let+string+=+string+where+!string.isEmpty
{
++++print(string)
}|code-block|syntax|javascript|4612072|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

For optional Strings how about:

<pre><code>if let string = string where !string.isEmpty
{
 print(string)
}
</code></pre>

blocks|key|105680|text|if+myString?.startIndex+!=+myString?.endIndex+{}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|105681|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>if myString?.startIndex != myString?.endIndex {}
</code></pre>

blocks|key|105720|text|public+extension+Swift.Optional+{
++++
++++func+nonEmptyValue<T>(fallback:+T)+->+T+{
++++++++
++++++++if+let+stringValue+=+self+as?+String,+stringValue.isEmpty+{
++++++++++++return+fallback
++++++++}
++++++++
++++++++if+let+value+=+self+as?+T+{
++++++++++++return+value
++++++++}+else+{
++++++++++++return+fallback
++++++++}
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|105721|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre class="lang-swift prettyprint-override"><code>public extension Swift.Optional {
 
 func nonEmptyValue&lt;T&gt;(fallback: T) -&gt; T {
 
 if let stringValue = self as? String, stringValue.isEmpty {
 return fallback
 }
 
 if let value = self as? T {
 return value
 } else {
 return fallback
 }
 }
}
</code></pre>

blocks|key|4612084|text|关于|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4612085|if+let+notEmptyString+=+optionalString+where+!notEmptyString.isEmpty+{
++++//+do+something+with+emptyString+
++++NSLog("Non-empty+string+is+%25@",+notEmptyString)
}+else+{
++++//+empty+or+nil+string
++++NSLog("Empty+or+nil+string")
}|code-block|syntax|javascript|4612086|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

What about

<pre><code>if let notEmptyString = optionalString where !notEmptyString.isEmpty {
 // do something with emptyString 
 NSLog("Non-empty string is %@", notEmptyString)
} else {
 // empty or nil string
 NSLog("Empty or nil string")
}
</code></pre>

blocks|key|105709|text|您可以使用此扩展：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|105710|extension+String+{

++++static+func+isNilOrEmpty(string:+String?)+->+Bool+{
++++++++guard+let+value+=+string+else+{+return+true+}

++++++++return+value.trimmingCharacters(in:+.whitespaces).isEmpty
++++}

}|code-block|syntax|javascript|105711|然后像这样使用它：|105712|let+isMyStringEmptyOrNil+=+String.isNilOrEmpty(string:+myString)|105713|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You can use this extension:

<pre><code>extension String {

 static func isNilOrEmpty(string: String?) -&gt; Bool {
 guard let value = string else { return true }

 return value.trimmingCharacters(in: .whitespaces).isEmpty
 }

}
</code></pre>

and then use it like this:

<pre><code>let isMyStringEmptyOrNil = String.isNilOrEmpty(string: myString)
</code></pre>

In Objective C, one could do the following to check for strings:

<pre class="lang-c prettyprint-override"><code>if ([myString isEqualToString:@""]) {
 NSLog(@"myString IS empty!");
} else {
 NSLog(@"myString IS NOT empty, it is: %@", myString);
}
</code></pre>

How does one detect empty strings in Swift?

Check empty string in Swift?

在目标C中，可以执行以下操作来检查字符串：if ([myString isEqualToString:@""]) {    NSLog(@"myString IS empty!");} else {    NSLog(@"myString IS NOT empty, it is: %@", myString);}如何检测Swift中的空字符串？

问是否检查Swift中的空字符串？
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问是否检查Swift中的空字符串？
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