For-in loop and type casting only for objects which match type

问题

I have seen answers here which explain how to tell the compiler that an array is of a certain type in a loop.

However, does Swift give a way so that the loop only loops over items of the specified type in the array rather than crashing or not executing the loop at all?

回答1:

You can use a for-loop with a case-pattern:

for case let item as YourType in array {
    // `item` has the type `YourType` here 
    // ...
}

This will execute the loop body only for those items in the array which are of the type (or can be cast to) YourType.

Example (from Loop through subview to check for empty UITextField - Swift):

for case let textField as UITextField in self.view.subviews {
    if textField.text == "" {
        // ...
    }
}

回答2:

Given an array like this

let things: [Any] = [1, "Hello", true, "World", 4, false]

you can also use a combination of flatMap and forEach fo iterate through the Int values

things
    .flatMap { $0 as? Int }
    .forEach { num in
        print(num)
}

or

for num in things.flatMap({ $0 as? Int }) {
    print(num)
}

In both cases you get the following output

// 1
// 4

来源：https://stackoverflow.com/questions/38813225/for-in-loop-and-type-casting-only-for-objects-which-match-type