Bound value in a conditional binding must be of Op

问题:

I have a protocol defined:

protocol Usable {
    func use()
}

and a class that conforms to that protocol

class Thing: Usable {
    func use () {
        println ("you use the thing")
    }
}

I would like to programmatically test whether or not the Thing class conforms to the Usable protocol.

let thing = Thing()

// Check whether or not a class is useable
if let usableThing = thing as Usable { // error here
    usableThing.use()
}
else {
    println("can't use that")
}

But I get the error

Bound value in a conditional binding must be of Optional Type

If I try

let thing:Thing? = Thing()

I get the error

Cannot downcast from 'Thing?' to non-@objc protocol type 'Usable'

I then add @objc to the protocol and get the error

Forced downcast in conditional binding produces non-optional type 'Usable'

At which point I add ? after the as, which finally fixes the error.

How can I achieve this functionality with conditional binding with a non-@objc protocol, the same as in the "Advanced Swift" 2014 WWDC Video?

回答1:

You can get it to compile by making the cast as Usable? instead of as Usable, like this:

// Check whether or not a class is useable
if let usableThing = thing as Usable? { // error here
    usableThing.use()
}
else {
    println("can't use that")
}

回答2:

As metioned in the Swift doc, the is operator is the guy you need for the job:

The is operator checks at runtime to see whether the expression is of the specified type. If so, it returns true; otherwise, it returns false.

The check must not be known to be true or false at compile time.

Therefore, the following test would normally be what you need:

if thing is Usable { 
    usableThing.use()
} else {
    println("can't use that")
}

However, as the doc specifies, Swift can detect at compile time that the expression is always true and declares an error to help the developer.

回答3:

This works for me in the playground

protocol Usable {
    func use()
}

class Thing: Usable {
    func use () {
        println ("you use the thing")
    }
}

let thing = Thing()
let testThing : AnyObject = thing as AnyObject

if let otherThing = testThing as? Thing {
    otherThing.use()
} else {
    println("can't use that")
}

回答4:

You are getting

Bound value in a conditional binding must be of Optional Type

because thing as Usable must return an optional type so making it as? should solved the problem. Unfortunately, the error still persisted for some odd reason. Anyway, a workaround I found to get it to work is to extract out the variable assignment inside the if statement

let thing = Thing()

let usableThing = thing as? Usable

if useableThing { 
    usableThing!.use()
}
else {
    println("can't use that")
}

回答5:

swift protocols does not work in Playgrounds in the first beta, try to build a real project instead.