Swift using guard and fatal error specified in an function - ios

Question

I use enum for building my UITableView cells:
enum VAXSections: Int
{
case Segmented = 0
case Scrollable = 1
case ScheduledMode = 2
case ScheduledFooter = 3
case SilentHours = 4
case SilentFooter = 5
}
here how I use it:
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
{
guard let unwrappedSection = VAXSections(rawValue: indexPath.section) else {
showFatalError()
return nil
}
Few problem here I want to guard my section value if it's out of max case in my enum. For example if indexPath.section will be bigger then 5 then app should fall back. But if you see we can't return nil here, as cellForRowAtIndexPath has to return cell in any case.
I can solve problem by providing more readability with replacing showFatalError() fiction with:
guard let unwrappedSection = VAXSections(rawValue: indexPath.section) else {
fatalError("Check \(String(VAXUnitDashboardViewController.self)) UITableView datasource or check \(String(VAXSections.self)) enum.")
}
then I don't need to return any value. But then I turned in another problem. As I need to specify at least 3 datasource functions of my UITableView I need to duplicate fatal error which I wish to replace with one function that do the same all the time:
fatalError("Check \(String(VAXUnitDashboardViewController.self)) UITableView datasource or check \(String(VAXSections.self)) enum.")

Answer 1

enum VAXItems: String {
case Item1
case Item2
case Item3
}
enum VAXSections: String {
case Segmented
case Scrollable
case ScheduledMode
case ScheduledFooter
case SilentHours
case SilentFooter
}
struct VAXModel {
var type: VAXSections
var items: [VAXItems]
}
Then on your UIViewController you can have:
class ViewController: UIViewController {
private var model: [VAXModel] = []
override func viewDidLoad() {
super.viewDidLoad()
model = [
VAXModel(type: .ScheduledMode, items: [.Item1, .Item2, .Item3]),
VAXModel(type: .SilentFooter, items: [.Item1])
]
}
func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return model.count
}
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return model[section].items.count
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier(String(UITableViewCell), forIndexPath: indexPath)
let item = model[indexPath.section].items[indexPath.row]
switch item {
case .Item1: cell.textLabel?.text = item.rawValue
case .Item2: // Config
case .Item3: // Config
}
return cell
}
}

Answer 2

I don't think you need to have it in 3 places actually. Assuming that the 3 data source methods you are talking about are :
func numberOfSectionsInTableView(_ tableView: UITableView) -> Int
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int
func tableView(_ tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
You actually need to have your guard in only one of them.
numberOfSectionsInTableView is the first method that will be called, so if you fail here, the other two methods won't be called. If the number of sections is based on some calculations, you can also cut off the value, something like this : if calculatedNumberOfSections > 6 { return 6 } else { return calculatedNumberOfSections } (remember that section numbering is 0 based)
numberOfRowsInSection - if you guard here you have two options - either fail with fatalError or (better in my opinion) - return 0 if a section number higher than 5 gets passed. Returning 0 will result in cellForRowAtIndexPath not being called with that section.
cellForRowAtIndexPath - you already got this :)