Access UI properties from background thread in project 9

[at1993]

Hi All,

I thought you are not supposed to access/change UI. However I've noticed in this sample, fetchJSON is executed in the background thread but is reading UI property navigationController?.tabBarItem.tag. Am I missing something?

@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)
}