如果UserDefaults为空，如何检查一次 [英] How to check a single time if UserDefaults is empty

问题描述

我的应用计算从日期到NSDate（）之间的天数。当我发布它时，用户只能保存一个日期，标题和背景图片。

My app counts the days between a date and NSDate(). When I released it, a user could only save one date, a title and a background image.

现在，我有一个UICollectionView，可以选择保存多个日期，它将通过在其各自的数组中附加日期，标题和图像字符串来创建一个单元格。

Now I have a UICollectionView with the option to save more than one date, and it will create a cell by appending a date, title and image string to their respective arrays.

该应用程序已完全更改，因此我在努力检查用户是否已保存日期，如果有的话，则将该信息添加到数组中

The app has been completely changed, so I'm struggling with how to check whether a user has saved a date, and if they have, add that info to the arrays to create a cell.

但是，我希望只检查一次-第一次从更新或全新安装打开应用程序。

But, I want this to only been checked once - the first time the app is opened from update or fresh install.

这是我的想法，它不能正常工作。

Here is my thinking about it, it doesn't work by the way.

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as! MyCollectionViewCell

    if userDefault.objectForKey("day") == nil {
    } else {
        // Add the first date created from previous version
        let day = userDefault.objectForKey("day") as? String
        let dateFormatter = NSDateFormatter()
        dateFormatter.dateFormat = "dd MMMM yyyy hh:mm a"
        let date = dateFormatter.dateFromString(day!)!
        let date1 = dateFormatter.stringFromDate(date)
        addDateToArray(date1)

        // Add the since text
        let text = userDefault.objectForKey("sinceText") as? String
        addSinceLabelToArray(text!)

         //Add the image background
        let image = userDefault.objectForKey("khoury") as! String
        addThemeToImagesArray(image)
    }

上面的代码会发生什么它返回nil。我希望它可以创建第一个单元格，以便用户可以看到他们保存的日期。

What happens with the code above is it returns nil. I am expecting it to create the first cell, so that the user can see the date they have saved.

推荐答案

您可以使用 NSUserDefaults 中的另一个布尔值可检测首次运行：

You can use another boolean value in NSUserDefaults to detect the first run:

let defaults = NSUserDefaults.standardUserDefaults()

if (!defaults.boolForKey("migrated")) {
    defaults.setBool(true, forKey: "migrated")
    // ... anything else
}

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