RxSwift+fromAsync example

RxSwift+fromAsync.swift

// Consider this function:
func fetchPost(by postId: Int, completion: @escaping (Post?, Error?) -> Void) {
  ...
}
// Using our extension to use an observable
// fetchPostObservable looks like: fetchPostObservable(postId) -> Observable<Post>
let fetchPostObservable = Observable.fromAsync(postService.getPost(postId:completion:))

// Now to fetch a post you just have to call it just like you would call any function which returns an Observable:
fecthPostObservable(10).subscribe(onNext: { (post) in
    print("Fetching post with id 10 succeeded")
}, onError: { (error) in
    print("Fetching post failed: \(error)"
})