ios - 如何使用 Swift 处理空对象？

Question

我有一个跟踪每月开支的应用程序。我有一个 Expense 实体，它有一个 month 属性，用于跟踪创建当前月份的费用。然后我会在表格 View 中显示每个月的费用，如此处所示。用户只有在下个月或上个月有费用时才可以左右切换

@IBAction func backMonthButtonPressed(sender: AnyObject) {
    print("Back Button Pressed")
    currentMonth = NSCalendar.currentCalendar().dateByAddingUnit(.Month, value: -1, toDate: currentMonth, options: [])!
    if checkMonth(currentMonth) {
        updateFetch()
        setMonth()
    } else {
        currentMonth = NSCalendar.currentCalendar().dateByAddingUnit(.Month, value: 1, toDate: currentMonth, options: [])!
    }
    tableView.reloadData()

}

@IBAction func nextMonthButtonPressed(sender: AnyObject) {

    print("Next Button Pressed")

    currentMonth = NSCalendar.currentCalendar().dateByAddingUnit(.Month, value: 1, toDate: currentMonth, options: [])!
    if checkMonth(currentMonth) {
        updateFetch()
        setMonth()
    } else {
        currentMonth = NSCalendar.currentCalendar().dateByAddingUnit(.Month, value: -1, toDate: currentMonth, options: [])!
    }
    tableView.reloadData()
}

func checkMonth(month : NSDate) -> Bool {
    let app = UIApplication.sharedApplication().delegate as! AppDelegate
    let context = app.managedObjectContext //scratch pad

    let fetchRequest = NSFetchRequest(entityName: "Expense")
    fetchRequest.predicate = NSPredicate(format: "month == %@", month.MonthYearDateFormatter())
    let count = context.countForFetchRequest(fetchRequest, error: nil)
    if count > 0 {
        print("There are expenses for this month \(month.MonthYearDateFormatter()). Show expenses")
        return true

    } else {
        print("There are no expenses for this month \(month.MonthYearDateFormatter()). Do Nothing")
        return false
    }
}

我的问题是，在不太可能发生的情况下，用户在 6 月份创建了费用，但在 8 月份没有创建费用。我怎样才能让用户在不跳过它的情况下仍然看到他/她在八月份的费用。有任何想法吗？

Answer 1

我在阐述之前做了一些优化:

@IBAction func backMonthButtonPressed(sender: AnyObject) {
    self.processMonth(step: -1)
}

@IBAction func nextMonthButtonPressed(sender: AnyObject) {
    self.processMonth(step: 1)
}

// a method uses almost the same code for both cases, so it was merged
func processMonth(step: Int) {
    let direction = (step < 1 ? "Back" : "Next")
    print("\(direction) Button Pressed")

    currentMonth = NSCalendar.currentCalendar().dateByAddingUnit(.Month, value: step, toDate: currentMonth, options: [])!

    //if checkMonth(currentMonth) {
    // I wouldn't test this because it locks you out from seeing empty month.
        updateFetch()
        setMonth()
    //}

    tableView.reloadData()
}

你所问的问题的答案:

如果您的 UITableView 的数据源设置正确，您应该能够度过空白的月份

// changed return type from `Bool` to `void` as I suggested in the method not to test empty month, as it could be useless

func checkMonth(month : NSDate) {
    let app = UIApplication.sharedApplication().delegate as! AppDelegate
    let context = app.managedObjectContext //scratch pad

    let fetchRequest = NSFetchRequest(entityName: "Expense")
    fetchRequest.predicate = NSPredicate(format: "month == %@", month.MonthYearDateFormatter())
    let count = context.countForFetchRequest(fetchRequest, error: nil)
    if count > 0 {
        print("There are expenses for this month \(month.MonthYearDateFormatter()). Show expenses")

    } else {
        print("There are no expenses for this month \(month.MonthYearDateFormatter()). Do Nothing")
        // here you can make some additional actions, like seeting the empty table with "no expenses this month"
    }
}

无论如何，正如 @Paulw11 指出的那样，如果您的数据源不会耗尽大小，您宁愿从 viewDidLoad/viewDidAppear 的数据模型中获取数据，然后每月呈现根据 currentMonth 变量(关于用户当前看到的月份)。

因此，您只能在 Controller 加载时以及每次用户更改当前月份 View 时调用 setMonth() 方法。