从 C# 中的 async/await 到 Swift 中的闭包

Question

Coming from a C# background with async/await mindset I'm having some troubles to understand how to use closures in Swift properly.

I'm using the following method getMessagingId to get the firebase messaging ID. After I receive the ID via a closure I want to do something with the ID. However, I simply don't know how to get that value out of InstanceID.instanceID().instanceID(...) so that I can use it in the methods processMessagingId() and registerMessagingId() later. In C# I would use async/await to achieve that.

// Register this device
public func registerThisDevice() {
    // get the messaging ID
    var messagingId = getMessagingId()

    // process this messaging ID
    processMessagingId(messagingId)

    // register this messaging ID
    registerMessagingId(messagingId)
}

// get the messaging ID for firebase messaging
func getMessagingId() -> String {
    InstanceID.instanceID().instanceID(handler: { (result, error) in
        if let error = error {
            print("Error fetching messaging id: \(error)")
        } else if let result = result {
            print("Messaging id is \(result.token)")
        }
    })
    return ""
}

handler is a completion handler that also uses @escaping keyword.

I read a couple of articles about completion handlers and also @escaping. But I just can't wrap my head around this.

Answer 1

When you're doing async calls, you need to wait until async call ends and after that you can work with received data. For this we're using completion handlers in Swift.

Start with declaring completion parameter instead of having returning value

func getMessagingId(completion: @escaping (String) -> ())

What does that means?

Simplified, you declare what should happen when completion is called from this method. You want to pass parameter of type String which you can use in completion 's closure and then you want to call code inside completion 's closure

So now let's call our completion and as parameter let's pass our message

if let error = error {
    completion("Error fetching messaging id: \(error)")
} else if let result = result {
    completion("Messaging id is \(result.token)")
}

Ok, now let's call our method

getMessagingId { message in // here you give `String` parameter name
    print(message) // this code gets executed when you call `completion` in `getMessagingId`
}

---

Now once you're able to understand this, you can pass multiple parameters like for example success , error , messageId , whatever you need...

Answer 2

You can use this framework for Swift coroutines - https://github.com/belozierov/SwiftCoroutine

When you call await it doesn't block the thread but only suspends coroutine, so you can use it in the main thread as well.

func awaitGetMessagingId() throws -> String {
    let (result, error) = try Coroutine.await {
        InstanceID.instanceID().instanceID(handler: $0)
    }
    if let error = error { throw error }
    return result ?? ""
}

and then call this method inside coroutine:

DispatchQueue.main.startCoroutine {
    let id = try awaitGetMessagingId()
    print("Messaging id is \(id)")
}