Extending structs with protocols in Swift - ios

Question

I have the following piece of code. The protocols and extensions I've added don't seem to be doing quite what I expect. What am I missing?
struct foo {
var getFoo: String
var getBar: bar
struct bar {
var getBar: String
}
}
protocol FooProtocol {
var getFoo: String { get }
var getBar: BarProtocol { get }
}
extension foo: FooProtocol {} // Type 'foo' does not conform to protocol 'FooProtocol'
protocol BarProtocol {
var getBar: String { get }
}
extension foo.bar: BarProtocol {}
The problem in this code is the line extension foo: FooProtocol {}, where we're trying to say that, in effect foo.getBar.getBar should be a valid property. However, Xcode throws up a compilation error saying "Type 'foo' does not conform to protocol 'FooProtocol'.
I can fix this by changing the following:
protocol FooProtocol {
var getFoo: String { get }
var getBar: foo.bar { get } // instead of var getBar: BarProtocol { get }
}
However, it seems the line extension foo.bar: BarProtocol {} should mean FooProtocol doesn't care if we give it foo.bar or BarProtocol. What am I missing here?
#Drobs answer was correct. Here's a bonus question.
BONUS
Now suppose I need to implement another struct that conforms to FooProtocol. Neither of these approaches work. What's the fix?
struct FooImplementation: FooProtocol { // Type 'FooImplementation' does not conform to protocol 'FooProtocol'
var getFoo: String
var getBar: BarProtocol
}
Do I need to use some sort of typealias to achieve this?

Answer 1

You want to use an associatedtype in the protocol. In that case it essentially says give me a type that implements the protocol BarProtocol.
protocol FooProtocol {
associatedtype BarType: BarProtocol
var getFoo: String { get }
var getBar: BarType { get }
}
protocol BarProtocol {
var getBar: String { get }
}
struct Foo: FooProtocol {
var getFoo: String
var getBar: Bar
struct Bar: BarProtocol {
var getBar: String
}
}