在Swift中的if语句中使用多个let-as [英] Using multiple let-as within a if-statement in Swift

问题描述

我从字典中解开两个值，在使用它们之前，我必须将它们转换为正确的类型。这就是我想出的：

I'm unwrapping two values from a dictionary and before using them I have to cast them and test for the right type. This is what I came up with:

var latitude : AnyObject! = imageDictionary["latitude"]
var longitude : AnyObject! = imageDictionary["longitude"]

if let latitudeDouble = latitude as? Double  {
   if let longitudeDouble = longitude as? Double {
       // do stuff here
   }
}

但是如果让查询合二为一，我想打包两个。所以它会是这样的：

But I would like to pack the two if let queries into one. So that it would something like that:

if let latitudeDouble = latitude as? Double, longitudeDouble = longitude as? Double {
    // do stuff here
}

这句法不起作用，所以我想知道是否有一种美妙的方式来做到这一点。

That syntax is not working, so I was wondering if there was a beautiful way to do that.

推荐答案

Swift 3更新：

Update for Swift 3:

以下内容适用于Swift 3：

The following will work in Swift 3:

if let latitudeDouble = latitude as? Double, let longitudeDouble = longitude as? Double {
    // latitudeDouble and longitudeDouble are non-optional in here
}

请务必记住，如果其中一个尝试的可选绑定失败，则不会执行 if-let 块内的代码。

Just be sure to remember that if one of the attempted optional bindings fail, the code inside the if-let block won't be executed.

注意：这些子句不一定都是'let'子句，你可以用逗号分隔任何一系列的布尔检查。

Note: the clauses don't all have to be 'let' clauses, you can have any series of boolean checks separated by commas.

例如：

if let latitudeDouble = latitude as? Double, importantThing == true {
    // latitudeDouble is non-optional in here and importantThing is true
}

Swift 1.2：

Swift 1.2:

Apple可能已经阅读了您的问题，因为您的希望 - 为了在Swift 1.2中正确编译代码（今天测试版）：

Apple may have read your question, because your hoped-for code compiles properly in Swift 1.2 (in beta today):

if let latitudeDouble = latitude as? Double, longitudeDouble = longitude as? Double {
    // do stuff here
}

---

Swift 1.1及更早版本：

---

Swift 1.1 and earlier:

这是个好消息 - 你完全可以做到这一点。关于两个值的元组的switch语句可以使用模式匹配将它们同时转换为 Double ：

Here's the good news - you can totally do this. A switch statement on a tuple of your two values can use pattern-matching to cast both of them to Double at the same time:

var latitude: Any! = imageDictionary["latitude"]
var longitude: Any! = imageDictionary["longitude"]

switch (latitude, longitude) {
case let (lat as Double, long as Double):
    println("lat: \(lat), long: \(long)")
default:
    println("Couldn't understand latitude or longitude as Double")
}

更新：此版本的代码现在可以正常运行。

Update: This version of the code now works properly.

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