Swift 2 - Check Type of empty Array (Introspection)

问题

I'm currently working on introspection in Swift 2 and have Problems getting the specific type for an Array (in this example an Array<String>).

var prop = obj.valueForKey("strings")!

if prop is Array<String> {
println("true")
}
if prop is Array<Int> {
println("true")
}

Output is:

true
true

while it should be

true
false

Is there a way to find out the type for the members of the Array? For example, if I daclared the Array as Array<String> I want to get String or at least be able to check if it is. MirrorType also did not lead to any success on that by now.

回答1:

There are 2 ways to achieve what you want:

if prop.dynamicType == Array<Int>.self (or [Int].self) which is better than if prop.dynamicType == [Int]().dynamicType { because [Int]() creates an unused instance of "array of integers".
Typically, when you check if an array is specific-typed, you plan to use it in a certain way (as a result, you will likely cast your array to [Int]). Having that said, I recommend using if let arrayOfInts = prop as? Array<Int> {. Using this construct, you will check for type compatibility and prepare your array to be treated in a special way (using the casted arrayOfInts reference).

Anyway, it's up to you to decide what to do.

回答2:

Perhaps what you want is the type of each individual item inside the Array rather than the type of the Array itself? If you are using collection types in Swift, all the items stored in the Array (or Dictionary) are of the same type (except if you declare the Array as Array for example to break the rules... which is not usually necessary, or wanted).

By declaring an Array with its initially values you are automatically telling the compiler what type they are. If you do something like this:

let obj = [1,2,3]
var property = obj[0]

if property is String {
    print("true")
}
if property is Int {
    print("true")
}

The compiler will already tell you that property is String always fails, and there is actually no need to do that test (because we already know that it will always fail).

If you are working with Objective-C APIs and types on the other hand, there may be occasions where you will need to test for type, this is a good example of testing for type in an Objective-C collection that has items of different types:

let userDefaults = NSUserDefaults.standardUserDefaults()
let lastRefreshDate: AnyObject? = userDefaults.objectForKey("LastRefreshDate")
if let date = lastRefreshDate as? NSDate {
print("\(date.timeIntervalSinceReferenceDate)")
}

Hope this helps.

来源：https://stackoverflow.com/questions/33119752/swift-2-check-type-of-empty-array-introspection

标签

arrays

swift2

introspection