Easy GCD using performSelector(inBackground:)

There’s another way of using GCD, and it’s worth covering because it’s a great deal easier in some specific circumstances. It’s called performSelector(), and it has two interesting variants: performSelector(inBackground:) and performSelector(onMainThread:).

Both of them work the same way: you pass it the name of a method to run, and inBackground will run it on a background thread, and onMainThread will run it on a foreground thread. You don’t have to care about how it’s organized; GCD takes care of the whole thing for you. If you intend to run a whole method on either a background thread or the main thread, these two are easiest.

For project 7, we can use this method to clear up the confusion with our showError() method. For example, we could refactor the fetching code into a fetchJSON() method that can then run in the background like this:

override func viewDidLoad() {
    super.viewDidLoad()

    performSelector(inBackground: #selector(fetchJSON), with: nil)
}

@objc func fetchJSON() {
    let urlString: String

    if navigationController?.tabBarItem.tag == 0 {
        urlString = "https://api.whitehouse.gov/v1/petitions.json?limit=100"
    } else {
        urlString = "https://api.whitehouse.gov/v1/petitions.json?signatureCountFloor=10000&limit=100"
    }

    if let url = URL(string: urlString) {
        if let data = try? Data(contentsOf: url) {
            parse(json: data)
            return
        }
    }

    performSelector(onMainThread: #selector(showError), with: nil, waitUntilDone: false)
}

func parse(json: Data) {
    let decoder = JSONDecoder()

    if let jsonPetitions = try? decoder.decode(Petitions.self, from: json) {
        petitions = jsonPetitions.results
        tableView.performSelector(onMainThread: #selector(UITableView.reloadData), with: nil, waitUntilDone: false)
    }
}

@objc func showError() {
    let ac = UIAlertController(title: "Loading error", message: "There was a problem loading the feed; please check your connection and try again.", preferredStyle: .alert)
    ac.addAction(UIAlertAction(title: "OK", style: .default))
    present(ac, animated: true)
}

As you can see, it makes your code easier because you don’t need to worry about closure capturing, so we’ll come back to this again in the future. Note: because performSelector() uses #selector, we need to mark both fetchJSON() and showError() with the @objc attribute.

Because the code is now much simpler, we can add an else block to our JSON decoding, like this:

if let jsonPetitions = try? decoder.decode(Petitions.self, from: json) {
    petitions = jsonPetitions.results
    tableView.performSelector(onMainThread: #selector(UITableView.reloadData), with: nil, waitUntilDone: false)
} else {
    performSelector(onMainThread: #selector(showError), with: nil, waitUntilDone: false)
}

This refactored code also makes the return call inside fetchJSON() work as intended: the showError() method is never called when things go well, because the whole method is exited. What you choose depends on your project’s needs, but I think it’s much easier to understand the program flow using this final approach.

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