Swift集合对象的协议与关联类型

Question

I have a question (maybe regarding type erasure). Imagine the following scenario:

public protocol DataItem {

    associatedtype T
    var action: ((_ item: T) -> Void)? {get}
}

struct UserItem: DataItem {

    typealias T = UserItem

    // Custom Properties
    let name: String

    // Protocol: DataItem
    let action: ((T) -> Void)?
}

struct DriverItem: DataItem {

    typealias T = DriverItem

    // Custom Properties
    let licenseNumber: String

    // Protocol: DataItem
    let action: ((T) -> Void)?
}

let items = [
    UserItem(name: "Dexter", action: { (item) in print(item.name)}),
    DriverItem(licenseNumber: "1234567890", action: { (item) in print(item.licenseNumber)})
]

items.forEach {
    $0.action?($0)
}

I have a DataItem which is an abstract data item for a UITableViewCell and which has an action property that should be called when the cell is selected. My question is how can I create an array of DataItem objects, select an item from this list (or iterate over it) and call the respective action, that will print the name of the UserItem and the license number of the DriverItem. But with that implementation above the compiler complains with the following message that the items list can only be of type [Any]...

Heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional

That way I cannot call the action declared in the protocol DataItem. I tried to wrap my head around type erasures but couldn't understand it yet...

Would be happy if anyone comes up with a solution...

Answer 1

It's not that hard with no additional requirements:

struct UserItem {
    let name: String

    func action() {
        print(name)
    }
}

struct DriverItem {
    let licenseNumber: String

    func action() {
        print(licenseNumber)
    }
}

let data = UserItem(name: "Test")
let closure = { print("Some object data \(data.name)") }
let items = [
    UserItem(name: "Dexter").action,
    DriverItem(licenseNumber: "1234567890").action,
    closure
]

items.forEach {
    $0()
}

Answer 2

The error just asks you to add a type to items:

let items: Array<Any> = [
    UserItem(name: "Dexter", action: { (item) in print(item.name)}),
    DriverItem(licenseNumber: "1234567890", action: { (item) in print(item.licenseNumber)})
]

Not sure if this won't lead to more errors because I am not sure if you can call a completion block while creating the array.

---

Edit

I think what you are trying to do is get a call when the variable is called. The way to do this would be to use the get you already typed but didnt complete, like so:

var licenseNumber: String {
    get {
        print("doSomething!")
        return self.licenseNumber
    }
}

The array would become something like:

let items: Array<Any> = [
    UserItem(name: "Dexter"),
    DriverItem(licenseNumber: "1234567890")
]