Is this string object creation in swift? - ios

Question

I'm trying to parse csv file from my ios project(swift 2.3) and found this website. In the tutorial code, it has the following section of code :
if let content = String(contentsOfURL: contentsOfURL,
encoding: encoding, error: error) {
...........
}
And I'm not sure what it does. Does it create a String object?

Answer 1

Does it create a String object?
Yes, it creates a string from the contents of the URL given by contentsOfURL and using the character encoding given by encoding. It's analogous to the following Objective-C code:
NSString *content = [NSString stringWithContentsOfURL:contentsOfURL
encoding:encoding
error:&error];
The if let part is a form of conditional statement. let is used to assign a value to an immutable variable. Using it in a conditional as in your example only allows the body of the conditional statement to execute if that assignment succeeds. So, if some error occurs while the data at the given URL is being fetched or if the string cannot be created for some reason, the condition fails and the body isn't executed. The whole snippet might be written like this in Objective-C:
NSString *content = [NSString stringWithContentsOfURL:contentsOfURL
encoding:encoding
error:&error];
if (content != nil) {
// do something with content
}

Answer 2

That code creates a string, but it does it by fetching the contents of a URL. Usually that URL points to a resource on the Internet. In that case it's a very bad way to fetch a string, since it is a synchronous network call that can hang or fail. It's a very bad idea to do synchronous networking calls on the main thread.
You could wrap that code in a GCD call to a background queue, but instead I'd suggest using NSURLSession and submitting a data task. Your search terms would be NSURLSession (or just URLSession in Swift 3) and the function func dataTask(with url: URL). (It might be easier search on it's Objective-C name, dataTaskWithURL since Google searches don't work very well with special characters.)
Take a look at a GitHub project I created called Async_demo. It has a singleton class called DownloadManager that downloads a blob of data from a specified URL. It's written to return the data as a Data object, but it would be a simple matter to convert that result from Data to a String.
The key bit of code is this:
typealias DataClosure = (Data?, Error?) -> Void
func downloadFileAtURL(_ url: URL, completion: #escaping DataClosure) {
//We create a URLRequest that does not allow caching so you can see the download take place
let request = URLRequest(url: url,
cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
timeoutInterval: 30.0)
let dataTask = URLSession.shared.dataTask(with: request) {
//------------------------------------------
//This is the completion handler, which runs LATER,
//after downloadFileAtURL has returned.
data, response, error in
//Perform the completion handler on the main thread
DispatchQueue.main.async() {
//Call the copmletion handler that was passed to us
completion(data, error)
}
//------------------------------------------
}
dataTask.resume()
//When we get here the data task will NOT have completed yet!
}