How to test closures in swift

Im new in writing testcases, so please do not refrain from any suggestions.

Problem - How do i write testcase for (closures/ higher order functions) not the api calling closures excluding them, inorder to get 100% coverage, im stuck only in this part.

CODE -

init () {
    
   

    var items:[ReturnPaymentsOptionsModel] = []
    items.append(ReturnPaymentsOptionsModel(text: "Unexpected payment or \nunknown sender", isSelected: false))
    items.append(ReturnPaymentsOptionsModel(text: "Incorrect currency", isSelected: false))
    items.append(ReturnPaymentsOptionsModel(text: "Incorrect amount", isSelected: false))
    items.append(ReturnPaymentsOptionsModel(text: "Duplicate", isSelected: false))
    items.append(ReturnPaymentsOptionsModel(text: "Auction/Order cancelled", isSelected: false))
    items.append(ReturnPaymentsOptionsModel(text: "Charges exceeds the principle amount", isSelected: false))

    let observableToggleItems = Observable.just(items)
    
    let tappedItemAction = Observable.combineLatest(observableToggleItems, itemTapped)
        .map { argument -> [ReturnPaymentsOptionsModel] in // for this  closure im stuck
            
            let (items, indexPath) = argument
            var modifiedItems = items
            
            for index in modifiedItems.indices {
                
                if index == indexPath.row {
                    modifiedItems[index].isSelected.toggle()
                } else {
                    modifiedItems[index].isSelected = false
                }
            }
            
            return modifiedItems
        }.startWith(items)
    
    didFetchToggleItems = Observable.merge(getToggleItems.withLatestFrom(observableToggleItems), tappedItemAction)
}

Solution

I'll assume you don't have issues with testing the Rx chain here, but with the closure itself.

Then it would be better if you extracted that entire closure to a function you can unit test directly, and then just call the function in the closure in the .map, something like:

function modifyItems(argument: ([Item], IndexPath)) -> [ITTReturnPaymentsOptionsModel] {
    // Your closure here
}

And then something like:

let tappedItemAction = Observable.combineLatest(observableToggleItems, itemTapped)
        .map { modifyItems($0) }
        .startWith(items)

This will allow you to unit test the function separately from the Rx chain.