Question

我试图将核心数据中的多个对象保存到for循环中的IPodSongs实体，即当前位于for song in result{}循环中的歌曲的标题。但是我的代码只保存循环中的最后一首歌，并且只是保持覆盖同一个对象。我不需要覆盖同一个对象，而是每次都需要创建一个新对象。我做错了什么？

func fetchiPodSongsOnSignup() {

        var appDel: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate

        var context: NSManagedObjectContext = appDel.managedObjectContext!

        var newSong = NSEntityDescription.insertNewObjectForEntityForName("IPodSongs", inManagedObjectContext: context) as! NSManagedObject

        var request = NSFetchRequest(entityName: "IPodSongs")

        request.returnsObjectsAsFaults = false

        var results = context.executeFetchRequest(request, error: nil)


        let query = MPMediaQuery.songsQuery()

        let result = query.collections as! [MPMediaItemCollection]

        for song in result {

            for song in song.items as! [MPMediaItem] {

                newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

                println(newSong)

                context.save(nil)

            }
        }

Answer 1

您无需创建将新对象保存到Core Data的请求。你做错了是创建一个托管对象，插入它然后通过循环改变它，而不是为每首歌创建一个新对象。

这应该有效：

func fetchiPodSongsOnSignup() {

    var appDel: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate

    var context: NSManagedObjectContext = appDel.managedObjectContext!


    let query = MPMediaQuery.songsQuery()

    let result = query.collections as! [MPMediaItemCollection]

    for song in result {

        for song in song.items as! [MPMediaItem] {

            var newSong = NSEntityDescription.insertNewObjectForEntityForName("IPodSongs", inManagedObjectContext: context) as NSManagedObject

            newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

            println(newSong)


        }
    }
    if context.save(&errorPointer == false {
        printlin("Error received while saving")
    }

}

同时将if context.save(&errorPointer == false {printlin("Error received while saving")}放在最后。

Answer 2

您在所有迭代中使用相同的插入对象：

for song in song.items as! [MPMediaItem] {
      newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

您确实正在更改歌曲的对象值，但您使用的是同一个实例，而后者又是核心数据库中的同一个实体。

您需要为每首新歌使用新实例：

 var newSong = NSEntityDescription.insertNewObjectForEntityForName("IPodSongs", inManagedObjectContext: context) as! NSManagedObject
 newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

Answer 3

试试这个，这会有用......

插入for循环并保存在外部循环...

func fetchiPodSongsOnSignup() {

var appDel: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate

var context: NSManagedObjectContext = appDel.managedObjectContext!


let query = MPMediaQuery.songsQuery()

let result = query.collections as! [MPMediaItemCollection]

for song in result {

    for song in song.items as! [MPMediaItem] {

        var newSong = NSEntityDescription.insertNewObjectForEntityForName("IPodSongs", inManagedObjectContext: context) as NSManagedObject

        newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

        context.insert(newSong)

    }
}
context.save(nil)
}

Answer 4

这最有可能对你有用！试试吧！

for song in result {

        for song in song.items as! [MPMediaItem] {
            var newSong = NSEntityDescription.insertNewObjectForEntityForName("IPodSongs", inManagedObjectContext: context) as! NSManagedObject
            newSong.setValue("\(song.valueForProperty(MPMediaItemPropertyTitle))", forKey: "title")

            println(newSong)

            context.save(nil)

        }
    }

Swift中的核心数据：仅保存for循环中的最后一个对象

4 个答案: