blocks|key|4493649|text|与if不同，guard创建可从其块外部访问的变量。展开大量的Optional是很有用的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4493650|entityMap^0|1|2|6|5|U|8|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

Unlike <code>if</code>, <code>guard</code> creates the variable that can be accessed from outside its block. It is useful to unwrap a lot of <code>Optional</code>s.

blocks|key|4493616|text|一个好处是消除了大量嵌套的if+let语句。15:30左右观看WWDC“Swift的新特性”视频，其中标题为“末日金字塔”。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4493617|entityMap^0|D|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

One benefit is elimination a lot of nested <code>if let</code> statements. See the WWDC "What's New in Swift" video around 15:30, the section titled "Pyramid of Doom".

blocks|key|4494842|text|与if语句类似，guard根据表达式的布尔值执行语句。与if语句不同，guard语句仅在不满足条件时运行。您可以认为guard更像是一个断言，但您可以优雅地退出，而不是崩溃。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4494843|请参阅：http://ericcerney.com/swift-guard-statement/|offset|length|4494844|entityMap|0|LINK|mutability|MUTABLE|url|http://ericcerney.com/swift-guard-statement/^0|0|4|18|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

Like an if statement, guard executes statements based on a Boolean value of an expression. Unlike an if statement, guard statements only run if the conditions are not met. You can think of guard more like an Assert, but rather than crashing, you can gracefully exit.

refer to:
<a href="http://ericcerney.com/swift-guard-statement/" rel="nofollow">http://ericcerney.com/swift-guard-statement/</a>

blocks|key|4493704|text|它真的真的使带有多个查找和可选参数的序列流更加简洁和清晰，并减少了大量的if嵌套。请参阅Erica+Sadun发布的on+replacing+Ifs。……可能会得意忘形，下面是一个例子：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4493705|++++let+filteredLinks+=+locationsLinkedToList.filter({$0.actionVerb+==+movementCommand})
++++guard+let+foundLink+=+filteredLinks.first+else+{return+("<Person>+cannot+go+in+that+direction.",+nil,+nil)}
++++guard+filteredLinks.count+==+1+else+{return+("<Person>+cannot+decide+which+route+to+take.",+nil,+nil)}
++++guard+let+nextLocation+=+foundLink.toLocation+else+{return+("<Person>+cannot+go+in+that+direction.",+nil,+nil)}|code-block|syntax|javascript|4493706|看看能不能坚持下去。|4493707|entityMap|0|LINK|mutability|MUTABLE|url|http://ericasadun.com/2015/12/29/migrating-ifs-to-guards-in-swift/^0|1M|G|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

It really really does make the flow of a sequence with several lookups and optionals much more concise and clear and reduces lots of if nesting. See Erica Sadun post <a href="http://ericasadun.com/2015/12/29/migrating-ifs-to-guards-in-swift/" rel="nofollow">on replacing Ifs</a>. .... Could get carried away, an example below:

<pre><code> let filteredLinks = locationsLinkedToList.filter({$0.actionVerb == movementCommand})
 guard let foundLink = filteredLinks.first else {return ("&lt;Person&gt; cannot go in that direction.", nil, nil)}
 guard filteredLinks.count == 1 else {return ("&lt;Person&gt; cannot decide which route to take.", nil, nil)}
 guard let nextLocation = foundLink.toLocation else {return ("&lt;Person&gt; cannot go in that direction.", nil, nil)}
</code></pre>

See if that sticks.

blocks|key|4494918|text|来源：Guard+in+Swift|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4494919|让我们看一下这个例子，以便更清楚地理解它|4494920|示例1：|4494921|func+validate()+{+++++++++
++++guard+3>2+else+{+++++++++++++
++++print+("False")+++++++++++++
++++return+++++++++
++++}+++++++++
++++print+("True")+//True+++++
}+
validate()|code-block|syntax|javascript|4494922|在上面的示例中，我们看到3大于2，并且跳过guard+else子句中的语句并打印True。|4494923|示例2：|4494924|func+validate()+{+++++++++
++++guard+1>2+else+{+++++++++++++
++++print+("False")++++++++++++//False+
++++return+++++++++
++++}+++++++++
++++print+("True")++++++
}+
validate()|4494925|在上面的示例中，我们看到1小于2，并执行了guard+else子句中的语句，打印False，然后返回。|4494926|Example+3:+gaurd+let,+unwrapping+optionals+through+guard+let

func+getName(args+myName:+String?)+{
+++++guard+let+name+=+myName,+!name.isEmpty+else+{
+++++print+("Condition+is+false")++++++++++//+Condition+is+false++++++++++++return+++++++++
+++++}+++++++++
+++++print("Condition+is+met\(name)")+++++
}+
getName(args:+"")|4494927|在上面的例子中，我们使用guard+let来展开optionals。在函数getName中，我们定义了一个string+myName类型的变量，它是可选的。然后我们使用guard+let来检查变量myName是否为空，如果没有赋值给name并再次检查，则name不为空。如果两个条件都符合，即为真，则将跳过else块，并打印“conditions+is+Conditions+with+name”。|4494928|基本上，我们在这里检查用逗号分隔的两件事，首先是展开和可选，然后检查它是否满足条件。|4494929|在这里，我们没有向函数传递任何东西，即空字符串，因此打印条件为false。|4494930|func+getName(args+myName:+String?)+{
+++++guard+let+name+=+myName,+!name.isEmpty+else+{
+++++print+("Condition+is+false")++++++++++
+++++return+++++++++
+++++}++++++++
+++++print("Condition+is+met+\(name)")+//+Condition+is+met+Hello++++
}+getName(args:+"Hello")|4494931|在这里，我们将“Hello”传递给函数，您可以看到输出被打印为“Condition+is+met+Hello”。|4494932|entityMap|0|LINK|mutability|MUTABLE|url|https://thenucleargeeks.com/2019/05/09/guard-in-swift/^0|3|E|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1E|8|@]|9|@$A|1F|B|1G|1|1H]]|C|$]]|$1|D|3|E|5|6|7|1I|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|1J|8|@]|9|@]|C|$]]|$1|H|3|I|5|J|7|1K|8|@]|9|@]|C|$K|L]]|$1|M|3|N|5|6|7|1L|8|@]|9|@]|C|$]]|$1|O|3|P|5|6|7|1M|8|@]|9|@]|C|$]]|$1|Q|3|R|5|J|7|1N|8|@]|9|@]|C|$K|L]]|$1|S|3|T|5|6|7|1O|8|@]|9|@]|C|$]]|$1|U|3|V|5|J|7|1P|8|@]|9|@]|C|$K|L]]|$1|W|3|X|5|6|7|1Q|8|@]|9|@]|C|$]]|$1|Y|3|Z|5|6|7|1R|8|@]|9|@]|C|$]]|$1|10|3|11|5|6|7|1S|8|@]|9|@]|C|$]]|$1|12|3|13|5|J|7|1T|8|@]|9|@]|C|$K|L]]|$1|14|3|15|5|6|7|1U|8|@]|9|@]|C|$]]|$1|16|3|-4|5|6|7|1V|8|@]|9|@]|C|$]]]|17|$18|$5|19|1A|1B|C|$1C|1D]]]]

Source: <a href="https://thenucleargeeks.com/2019/05/09/guard-in-swift/" rel="nofollow noreferrer">Guard in Swift</a>

Let's see the example to understand it clearly 

Example 1:

<pre><code>func validate() { 
 guard 3&gt;2 else { 
 print ("False") 
 return 
 } 
 print ("True") //True 
} 
validate()
</code></pre>

In the above example we see that 3 is greater than 2 and the statement inside the guard else clause are skipped and True is printed.

Example 2:

<pre><code>func validate() { 
 guard 1&gt;2 else { 
 print ("False") //False 
 return 
 } 
 print ("True") 
} 
validate()
</code></pre>

In the above example we see that 1 is smaller than 2 and the statement inside the guard else clause are executed and False is printed followed by return.

<pre><code>Example 3: gaurd let, unwrapping optionals through guard let

func getName(args myName: String?) {
 guard let name = myName, !name.isEmpty else {
 print ("Condition is false") // Condition is false return 
 } 
 print("Condition is met\(name)") 
} 
getName(args: "")
</code></pre>

In the above example we are using guard let to unwrap the optionals. In the function getName we’ve defined a variable of type string myName which is optional. We then use guard let to check whether the variable myName is nil or not, if not assign to name and check again, name is not empty. If both the conditions qualified i.e. true the else block will be skipped and print “Conditions is met with name”.

Basically we are checking two things here separated by comma, first unwrapping and optional and checking whether that satisfies condition or not.

Here we are passing nothing to the function i.e. empty string and hence Condition is false is print.

<pre><code>func getName(args myName: String?) {
 guard let name = myName, !name.isEmpty else {
 print ("Condition is false") 
 return 
 } 
 print("Condition is met \(name)") // Condition is met Hello 
} getName(args: "Hello")
</code></pre>

Here we are passing “Hello” to the function and you can see the output is printed “Condition is met Hello”.

Swift 2 introduced the <code>guard</code> keyword, which could be used to ensure that various data is configured ready to go. An example I saw on <a href="http://www.hackingwithswift.com/swift2">this website</a> demonstrates an submitTapped function:

<pre><code>func submitTapped() {
 guard username.text.characters.count &gt; 0 else {
 return
 }

 print("All good")
}
</code></pre>

I am wondering if using <code>guard</code> is any different than doing it the old fashioned way, using an <code>if</code> condition. Does it give benefits, which you could not get by using a simple check?

Swift's guard keyword

Swift

Swift 2引入了guard关键字，该关键字可用于确保各种数据配置就绪。我在上看到的一个示例演示了一个submitTapped函数：func submitTapped() {    guard username.text.characters.count > 0 else {        return    }    print("All good")}我想知道使用guard是否与使用if条件

问斯威夫特的卫士关键字
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