Extending Dictionary with key and value constraints

问题

I want to extend dictionary and constrain it to specific key type of NSDate with values of type Array<MyClass>

MyClass is a swift class with no subclass.

extension Dictionary where Key: NSDate, Value: Array<MyClass>{
    func myFunction() -> Int{
        // Some thing useful
    }
}

The above code returns a build error:

Type 'Value' constrained to non-protocol type 'Array'

Ok so it seems that it needs to be constrained to a specific protocol, but strangely enough when I constrain Value to:

extension Dictionary where Key: NSDate, Value: MyClass

The compiler does not complain and the build succeeds. But MyClass is not a protocol. So what gives?

I've managed to constrain it like this:

extension Dictionary where Key: NSDate, Value: CollectionType, Value.Generator.Element: MyClass{
    func myFunction() -> Int{
        var count = 0
        for (_, graphicItemArray) in self{
            count+= (graphicItemArray as! [MyClass]).count
        }

        return count
    }
}

But this requires a forced cast because I need to get the count property in a Integer form.

So why can't I constrain it explicitly to value type of [MyClass] ?

回答1:

Not being able to use a where clause with a struct may be an error. Checking the language reference for where clauses and following into

conformance-requirement → type-identifier­ | ...

Seems like type-identifier should accept a struct, even the example they use is Int.

This maybe be something to file a bug for.

---

Related to your specific problem, it seems to work if you use _ArrayType which is a protocol that Array implements and has the methods you need.

extension Dictionary where Key: NSDate, Value: _ArrayType ...

I am not sure tho if using that kind of internal protocols would flag your app and make it fail Apple review.

来源：https://stackoverflow.com/questions/32815718/extending-dictionary-with-key-and-value-constraints