iOS 對模型對象進行歸檔

歸檔是指一種形式的序列化，專門編寫用於保存數據的任何對象都應該支持歸檔。使用對模型對象進行歸檔的技術可以輕鬆將複雜的對象寫入文件，然後再從中讀取它們。

只要在類中實現的每個屬性都是標量或者都是遵循NSCoding協議的某個類的實例，你就可以對整個對象進行完全歸檔。大多數的Foundation和Cocoa Touch類 都遵NSCoding協議，所以對於有大多數類來說，歸檔不太難。

遵循NSCoding協議：

NSCoding聲明瞭兩個方法，

- (void)encodeWithCoder:(NSCoder *)aCoder;和- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder;

第一個是 將對象編碼到歸檔中，第二個則是對歸檔解碼，來創建一個對象。

在BIDFourLines中：

#import <Foundation/Foundation.h>

@interface BIDFourLines : NSObject <NSCoding, NSCopying>

@property (copy, nonatomic) NSArray *lines;

@end

#import "BIDFourLines.h"

static NSString * const kLinesKey = @"kLinesKey";

@implementation BIDFourLines 

#pragma mark - Coding

- (id)initWithCoder:(NSCoder *)aDecoder
{
    self = [super init];
    if (self) {
        self.lines = [aDecoder decodeObjectForKey:kLinesKey];
    }
    return self;
}

- (void)encodeWithCoder:(NSCoder *)aCoder;
{
    [aCoder encodeObject:self.lines forKey:kLinesKey];
}

#pragma mark - Copying

- (id)copyWithZone:(NSZone *)zone;
{
    BIDFourLines *copy = [[[self class] allocWithZone:zone] init];
    NSMutableArray *linesCopy = [NSMutableArray array];
    for (id line in self.lines) {
        [linesCopy addObject:[line copyWithZone:zone]];
    }
    copy.lines = linesCopy;
    return copy;
}

@end

註：上面遵循了NSCopying協議，遵循NSCopying協議對於任何模型對象來說都是非常好的事情，NSCopying協議中，有一個copyWithZone方法，可以用來複制對象。

在BIDViewController中：

#import "BIDViewController.h"
#import "BIDFourLines.h"

static NSString * const kRootKey = @"kRootKey";

@interface BIDViewController ()

@property (strong, nonatomic) IBOutletCollection(UITextField) NSArray *lineFields;

@end

@implementation BIDViewController

- (NSString *)dataFilePath
{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(
                                                         NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    return [documentsDirectory stringByAppendingPathComponent:@"data.archive"];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSString *filePath = [self dataFilePath];
    //如果存在歸檔
    if ([[NSFileManager defaultManager] fileExistsAtPath:filePath]) {
        NSData *data = [[NSMutableData alloc]
                        initWithContentsOfFile:filePath];
        NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc]
                                         initForReadingWithData:data];
        //解檔
        BIDFourLines *fourLines = [unarchiver decodeObjectForKey:kRootKey];
        
        [unarchiver finishDecoding];
        
        for (int i = 0; i < 4; i++) {
            UITextField *theField = self.lineFields[i];
            theField.text = fourLines.lines[i];
        }
    }
    
    //後臺通知
    UIApplication *app = [UIApplication sharedApplication];
    [[NSNotificationCenter defaultCenter]
     addObserver:self
     selector:@selector(applicationWillResignActive:)
     name:UIApplicationWillResignActiveNotification
     object:app];
}


//進入後臺時 歸檔
- (void)applicationWillResignActive:(NSNotification *)notification
{
    NSString *filePath = [self dataFilePath];
    BIDFourLines *fourLines = [[BIDFourLines alloc] init];
    fourLines.lines = [self.lineFields valueForKey:@"text"];
    NSMutableData *data = [[NSMutableData alloc] init];
    NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc]
                                 initForWritingWithMutableData:data];
    [archiver encodeObject:fourLines forKey:kRootKey];
    [archiver finishEncoding];
    [data writeToFile:filePath atomically:YES];
}