Question

如何使用＆＃39;警卫＆＃39;在swift中。我已经阅读了许多关于＆＃39;后卫的文章。但是我没有清楚地了解这个。请给我一个明确的想法。请给我示例输出以便跟随＆＃39;如果＆＃39;言。

if firstName != "" 
{
   if lastName != "" 
   {
      if address != "" 
      {
        // do great code
      }
   }
 }

Answer 1

guard语句用于将程序控制转移到范围之外如果不满足一个或多个条件。

func doSomething(data: String?) -> String {

    // If data is nil, then return
    guard let data = data else { return "Invalid data" }

    defer { print("This will always be printed if data isn't nil") }

    // data is now a non optional String
    if data.lowercaseString == "ok" { return "Data is \"ok\"" }

    return "I'm your father"
}

print(doSomething("ok"))

<强>输出：

This will always be returned if data isn't nil
Data is "ok"

更倾向于你的问题：

guard语句中任何条件的值必须具有该类型符合BooleanType协议。

func doSomething(data: String) -> String {

    guard !data.isEmpty else { return "Data is empty" }

    return data
}

print(doSomething("ok")) // ok
print(doSomething(""))   // Data is empty

Answer 2

你也可以使用后卫，你的代码将更具可读性

let firstName = "First"
let lastName = "Last"
let address = "" // empty

if firstName != ""
{
    if lastName != ""
    {
        if address != ""
        {
            print(1,firstName,lastName, address)
        } else {
            print(1,"address is empty")
        }
    } else {
        print(1,"lastName is empty")
    }
} else {
    print(1,"firstName is empty")
}

func foo(firstName: String, lastName: String, address: String) {
    guard !firstName.isEmpty else { print(2,"firstName is empty"); return }
    guard !lastName.isEmpty else { print(2,"lastName is empty"); return }
    guard !address.isEmpty else { print(2,"address is empty"); return }

    print(2,firstName,lastName, address)
}

foo(firstName, lastName: lastName, address: address)
/*
 1 address is empty
 2 address is empty
 */
foo(firstName, lastName: lastName, address: "Address")
/*
 2 First Last Address
 */

Answer 3

guard的目的是验证某些条件，如果不满足条件则强制停止执行但延迟定义在执行即将离开当前范围之前未执行的代码块。
在保护中，一旦成功解包了可选项，后续代码就可以使用展开的值，而Defer对于执行无条件清理非常有用，无论某些操作是成功还是失败。
< / LI>
Guards提供了深层嵌套if-let的替代方案，而defer提供了try / finally for right的替代方法。
两者都用于控制流和错误处理

使用警卫＆amp;推迟：

for imageName in imageNamesList {
    guard let image = UIImage(named: imageName)
else { continue }
    // do something with image
}

func deferExample() {
    defer {
        print(“This will be run last”)
    }
    defer {
         print(“This will be run second-last”)
    }
    print(“operation started”)
    // …
    print(“operation complete”)
}

请查看http://nstechstack.com/archives/question/223-1

Answer 4

让我们通过非常简单的if方法。

class MySampleClass {

    var sampleString:String?

    func printSampleString() -> Void {


        if let str = sampleString{ ///if sampleString is nil the if will escape

            print(str) ///sampleString is not nil
        }
    }
}

var obj : MySampleClass = MySampleClass()
obj.sampleString = "SampleString"///Assign value to sampleString
obj.printSampleString()

现在让我们来guard方法

class MySampleClass {

    var sampleString:String?

    func printSampleString() -> Void {


        guard let str = sampleString else { ///guard will ensure sampleString is not nil

            return ///if sampleString is nil,then return
        }

        print(str) //sampleString is not nil,so print str!

    }
}

var obj : MySampleClass = MySampleClass()
obj.sampleString = "SampleString"///Assign value to sampleString
obj.printSampleString()

如何在swift中使用guard而不是if

4 个答案: