// Define success type
struct MyOperationSuccess {
    // …
}

// Make subclass of AsyncOperation whose success type is `MyOperationSuccess`
class MyOperation: AsyncOperation<MyOperationSuccess> {
    // …
    
    override func didStartExecuting() {
        // Do something…
        // When completed, call `self.complete()` with `MyOperationSuccess` instance
        // When failed, call `self.fail()` with `Error`
    }
}

// Make `MyOperation` instance
let op = MyOperation()
// Start the operation with `completionBlock`. Alternatively, you can add the operation into `OperationQueue`.
op.start {
    do {
        let result = try op.getResult() // Type of 'result' is `MyOperationSuccess`
        // Do something with result…
    } catch AsyncOperationError.cancelled(let canceller) {
        // Handle cancellation…
    } catch {
        // Handle other errors…
    }
}