Refer to variable from do block in catch - swift

Question

I need to access a variable inside a do statement. Will it behave like the if-else statement in the sense that you con't use variables outside of the if statement?
import UIKit
import PlaygroundSupport
class MyViewController : UIViewController {
override func loadView() {
do {
//let url = URL?("https://www.hackingwithswift.com")
let TECIONEXContent = try String(contentsOf: URL("https://www.hackingwithswift.com"))
} catch { print("error")}
//I need to access TECIONEXContent variable outside the do statement
// Error: Use of unresolved identifier 'TECIONEXContent'
var TECGrid = TECIONEXContent.components(separatedBy: "\n")
}
}
The error is on the last line, 'unresolved identifier'.

Answer 1

Will it behave like the if-else statement in the sense that you con't use variables outside of the if statement?
Yes. But just like an if-else statement, you can define the variable before the do-catch:
E.g. in an if-else statement:
let foo: String
if bar > 1 {
foo = "bigger than one"
} else {
foo = "one or smaller"
}
Or, in your case:
let url = URL(string: "https://www.hackingwithswift.com")!
let contents: String
do {
contents = try String(contentsOf: url)
} catch {
print(error)
return
}
let grid = contents.components(separatedBy: "\n")
Or, you aren’t really doing anything with the error message, you can eliminate the do-catch altogether:
guard let contents = try? String(contentsOf: url) else {
print("error")
return
}
let grid = contents.components(separatedBy: "\n")
Frankly, all of that having been said, using String(contentsOf:) is probably not the best pattern, anyway, because that performs a synchronous network request, which risks having the OS “watchdog” process kill your app unceremoniously if the main thread is blocked; and even if that doesn’t happen, it’s not a good user experience to freeze the app while the network request is in progress. Usually we’d use URLSession:
let url = URL(string: "https://www.hackingwithswift.com")!
URLSession.shared.dataTask(with: url) { data, response, error in
guard
let data = data,
let httpResponse = response as? HTTPURLResponse,
let string = String(data: data, encoding: .utf8) else {
print(error ?? "Unknown error")
return
}
guard 200 ..< 300 ~= httpResponse.statusCode else {
print("Expected 2xx response, but got \(httpResponse.statusCode)")
return
}
let grid = string.components(separatedBy: "\n")
DispatchQueue.main.async {
// use `grid` here
}
}.resume()
Unrelated, but:
The convention is to start variable names with lowercase letters.
You implemented loadView. It’s rare that we do that, and instead we implement viewDidLoad, making sure to call super.viewDidLoad(), too.
If you’re doing this in a playground, you’d obviously also set needsIndefiniteExecution, if you haven’t already.
Thus:
import UIKit
import PlaygroundSupport
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
performRequest()
}
func performRequest() {
let url = URL(string: "https://www.hackingwithswift.com")!
URLSession.shared.dataTask(with: url) { data, response, error in
guard
let data = data,
let httpResponse = response as? HTTPURLResponse,
let string = String(data: data, encoding: .utf8) else {
print(error ?? "Unknown error")
return
}
guard 200 ..< 300 ~= httpResponse.statusCode else {
print("Expected 2xx response, but got \(httpResponse.statusCode)")
return
}
let grid = string.components(separatedBy: "\n")
DispatchQueue.main.async {
print(grid)
// use `grid` here
}
}.resume()
}
}
PlaygroundPage.current.liveView = ViewController()
PlaygroundPage.current.needsIndefiniteExecution = true

Answer 2

In your code are four(!) very bad practices.
Never load data synchronously with API like String(contentsOf from a remote URL. Use asynchronous API like URLSession.
Never print a meaningless literal string in a catch block. Print the error instance.
According to the naming convention variable names should be lowerCamelCased.
In a do - catch block put always all good code in the do scope. This solves your issue.
do {
let tecionexContent = try String(contentsOf: URL("https://www.hackingwithswift.com")!)
let tecGrid = tecionexContent.components(separatedBy: "\n")
} catch { print(error) }
recommended
URLSession.shared.dataTask(with: URL("https://www.hackingwithswift.com")!) { data, _ , error in
if let error = error { print(error); return }
let tecionexContent = String(data: data!, encoding: .utf8)!
let tecGrid = tecionexContent.components(separatedBy: "\n")
}.resume()

Answer 3

The Issue
The problem with your code is that you are defining the variable inside a block/closure, which is a local scope. Your variable should be at a scope where both the blocks are able to see it. This means that in the catch block, the variable does not exist. Specifically you had attempted to reference the string variable you called TECIONEXContent from outside of the closure in which it was instantiated.
On some general style points: please stick with the swift naming convention for variables (i.e. camel case) whereas your classes etc should be capitalized. (For the purposes of running the below code in the playground I've used an arbitrary function name, but you could employ it from your lifecycle methods).
Basic Playground Demo Code
import UIKit
import PlaygroundSupport
class MyViewController : UIViewController {
private var tecGrid: [String]? {
didSet {
// Update some UI Here (insuring your on the main thread)
print(self.tecGrid)
}
}
func test() {
do {
var texionicContent = try String(contentsOf:URL(string: "https://www.hackingwithswift.com")!)
tecGrid = texionicContent.components(separatedBy: "\n")
}
catch let error {
print("Catch the error")
}
}
}
let play = MyViewController(nibName: nil, bundle: nil)
play.test()