iOS Persistence and Core Data L2- Udacity的筆記

筆記目錄：
1.保存數據的基礎知識
2.如何使用NSCoder/NSKeyedArchiver保存數據(所在章節)
3.Core Data的使用
4.Core Data的構建
5.Core Data的框架
以上文章的簡介：我的女朋友CoreData和她閨蜜們

NSCoder/NSKeyedArchiver

前言

這是Lesson 2，這一課將主要介紹如何利用NSCoder/NSKeyedArchiver將一個包含對象(object)的數組或者字典，保存到Document/目錄下。

保存一個包含對象的數組

假設有一個數組(NSArray)objects包含著NSData對象。對象圖(object graph)如圖

object graph

創建路徑

可以通過定義如下這樣的compute propertityactorsFilePath來返回指向Document/目錄的路徑(path)

var actorsFilePath : String {
    let manager = NSFileManager.defaultManager()
    let url = manager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first as! NSURL
    return url.URLByAppendingPathComponent("objectsArray").path!
}

NSKeyedArchiver

可以用這個類將整個對象圖(object graph)保存到Document/目錄下。
永這個類很簡單，只需如下一行代碼就能將數組objects保存到相應的文件中。

NSKeyedArchiver.archiveRootObject(objects, toFile: actorsFilePath)

然后取回數值

if let array = NSKeyedUnarchiver.unarchiveObjectWithFile(actorsFilePath) as? [NSData] {
    objects = array
}

KeyedArchiver能夠保存這些類型的對象：NSArray, NSDictionary, NSDate, NSNumber, NSString, and NSData.

NSCoding

如果上面的那些類型不能滿足要求。則可以通過使用Protocol NSCoding來自定義自己喜歡類型的對象，例如下面的例子Person

NSCoding是protocol。
提供下兩個有用的method：

• encodeWithCoder(:)
• init(coder:)

將要保存的類擴展 為NSObject(Nobody really gave this much thought when they were archiving Objective-C classes. But now it seems a little strange. It is possible that this will go away at some point.)并使用NSCoding protocol。

例如：定義一個類Person

class Person : NSObject, NSCoding {
 
    struct Keys {
        static let Name = "name"
        static let ProfilePath = "profile_path"
        static let Movies = "movies"
        static let ID = "id"
    }
    
    var name = ""
    var id = 0
    var imagePath = ""
    var movies = [Movie]()
    
    init(dictionary: [String : AnyObject]) {
        name = dictionary[Keys.Name] as! String
        id = dictionary[TheMovieDB.Keys.ID] as! Int
        
        if let pathForImgage = dictionary[Keys.ProfilePath] as? String {
            imagePath = pathForImgage
        }
    }
    
    var image: UIImage? {
        get {
            return TheMovieDB.Caches.imageCache.imageWithIdentifier(imagePath)
        }
        
        set {
            TheMovieDB.Caches.imageCache.storeImage(image, withIdentifier: imagePath)
        }
    }
    
    
    // MARK: - NSCoding
    
    func encodeWithCoder(archiver: NSCoder) {
        
        // archive the information inside the Person, one property at a time
        archiver.encodeObject(name, forKey: Keys.Name)
        archiver.encodeObject(id, forKey: Keys.ID)
        archiver.encodeObject(imagePath, forKey: Keys.ProfilePath)
        archiver.encodeObject(movies, forKey: Keys.Movies)
    }

    required init(coder unarchiver: NSCoder) {
        super.init()
        
        // Unarchive the data, one property at a time
        name = unarchiver.decodeObjectForKey(Keys.Name) as! String
        id = unarchiver.decodeObjectForKey(Keys.ID) as! Int
        imagePath = unarchiver.decodeObjectForKey(Keys.ProfilePath) as! String
        movies = unarchiver.decodeObjectForKey(Keys.Movies) as! [Movie]
    }
}

下一章節將開始，教保存Object的另一種常用方法Core Data。Core Data比NSCoding多了不少特性，但是弄起來就相對蛋疼一點。下一章節Core Data的使用