What's the Swift equivalent of declaring `typedef SomeClass<SomeProtocol> MyType`?

Question

I’m currently writing some Swift code in a project that is predominately Objective-C. In our ObjC code, we have a header that declares typedef GPUImageOutput<GPUImageInput> MyFilter;. We can then declare e.g. a @property that can only be a GPUImageOutput subclass that implements GPUImageInput.

(NOTE: GPUImageOutput and GPUImageInput are not defined by me; they are part of the GPUImage library)

Our Swift code doesn't seem to recognize this, even though the header is #imported in our Bridging Header. I’ve tried to replicate the declaration in Swift, but neither of these are proper syntax:
typealias MyFilter = GPUImageOutput, GPUImageInput
typealias MyFilter = GPUImageOutput : GPUImageInput

Answer 1

You can't declare typealias like that.

The best we can do is something like this:

class MyClass {
    private var filter:GPUImageOutput

    init<FilterType:GPUImageOutput where FilterType:GPUImageInput>(filter:FilterType) {
        self.filter = filter
    }

    func setFilter<FilterType:GPUImageOutput where FilterType:GPUImageInput>(filter:FilterType) {
        self.filter = filter
    }

    func someMethod() {
        let output = self.filter
        let input = self.filter as GPUImageInput
        output.someOutputMethod()
        input.someInputMethod()
    }
}

Answer 2

In Swift 4 you can achieve this with the new & sign (Below an example of a parameter confirming to UIViewController and UITableViewDataSource:

func foo(vc: UIViewController & UITableViewDataSource) {
    // access UIViewController property
    let view = vc.view
    // call UITableViewDataSource method
    let sections = vc.numberOfSectionsInTableView?(tableView)
}