Swift 2 if case 用法

发布时间：2020-12-14 02:05:06 所属栏目：百科来源：网络整理

导读：Swift 2: Pattern Matching with “if case” Posted onJune 20th,2015 While looking through slides forWhat’s New in Swiftagain,I’ve decided to instead play around with and write down the new Swift 2 idioms here for easy reference for myself

Swift 2: Pattern Matching with “if case”

Posted onJune 20th,2015

While looking through slides forWhat’s New in Swiftagain,I’ve decided to instead play around with and write down the new Swift 2 idioms here for easy reference for myself (and hopefully others). I’m going to start with Pattern Matching with if case.

The Setup

Let’s say you’re making a Sign In Form in your app. You’ll have the following fields:

 
        enum 
        SignUpFormField 
         { 
       
        case 
        FirstName 
        ( 
        String 
        ) 
       
        case 
        LastName 
        ( 
        String 
        ) 
       
        case 
        EmailAddress 
        ( 
        String 
        ) 
       
        case 
        DOB 
        ( 
        NSDate 
        ) 
       
        }

And of course,as the user enters his or her Date of Birth,you want toshow them that they were born before Taylor Swiftif that is actually the case. Taylor Swift’s birthday is just a constant in your app:

 
        let 
        taylorSwiftsBday:  
        NSDate 
        = { 
       
        let 
        gregorianCalendar =  
        NSCalendar 
        (calendarIdentifier:  
        NSCalendarIdentifierGregorian 
        ) 
       
        let 
        dateComponents =  
        NSDateComponents 
        () 
       
        dateComponents.day = 13 
       
        dateComponents.month = 12 
       
        dateComponents.year = 1989 
       
        return 
        gregorianCalendar?.dateFromComponents(dateComponents) ??  
        NSDate 
        () 
       
        }()

BS2 (Before Swift 2)

So now,it’s time to write the function to compare the user’s birthday to Taylor Swift’s birthday. BS2,you might have done it as follows:

 
        func 
        bornBeforeTaylorSwift(signUpFormField:  
        SignUpFormField 
        ) { 
       
        switch 
        signUpFormField { 
       
        case 
        . 
        DOB 
        ( 
        let 
        otherBday) 
       
        where 
        taylorSwiftsBday.compare(otherBday) == . 
        OrderedDescending 
        : 
       
        print 
        ( 
        "Fun fact: You were born before Taylor Swift!" 
        ) 
       
        default 
        : 
       
        break 
       
        } 
       
        }

Notice the need for a full onswitch statement / syntax,and thedefault case used to do nothing.

Swift 2

These issues mentioned above been solved in Swift 2 with a much simplerpattern matching “if case”:

 
        func 
        bornBeforeTaylorSwift(signUpFormField:  
        SignUpFormField 
        ) { 
       
        if 
        case 
         . 
        DOB 
        ( 
        let 
        otherBday) = signUpFormField 
       
        where 
        taylorSwiftsBday.compare(otherBday) == . 
        OrderedDescending 
       
        { 
       
        print 
        ( 
        "Fun fact: You were born before Taylor Swift!" 
        ) 
       
        } 
       
        }

Advanced Pattern Matching

Note that you can use the usual advanced pattern matching techniques in the “if case” statement that you’ve probably used in switch statements:

 
        let 
        numberOfTaylorSwiftSongsFavorited = 93 
       
        if 
        case 
         0...225 = numberOfTaylorSwiftSongsFavorited { 
       
        print 
        ( 
        "this is a valid number of favorited Taylor Swift Songs" 
        ) 
       
        }

I personally still have to get my brain to think of this way of writing my code naturally,but I’m sure it’ll come with practice and lots of refactoring!

（编辑：李大同）

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