iOS 对模型对象进行归档

归档是指一种形式的序列化，专门编写用于保存数据的任何对象都应该支持归档。使用对模型对象进行归档的技术可以轻松将复杂的对象写入文件，然后再从中读取它们。

只要在类中实现的每个属性都是标量或者都是遵循NSCoding协议的某个类的实例，你就可以对整个对象进行完全归档。大多数的Foundation和Cocoa Touch类 都遵NSCoding协议，所以对于有大多数类来说，归档不太难。

遵循NSCoding协议：

NSCoding声明了两个方法，

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

第一个是 将对象编码到归档中，第二个则是对归档解码，来创建一个对象。

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

注：上面遵循了NSCopying协议，遵循NSCopying协议对于任何模型对象来说都是非常好的事情，NSCopying协议中，有一个copyWithZone方法，可以用来复制对象。

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];
    //如果存在归档
    if ([[NSFileManager defaultManager] fileExistsAtPath:filePath]) {
        NSData *data = [[NSMutableData alloc]
                        initWithContentsOfFile:filePath];
        NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc]
                                         initForReadingWithData:data];
        //解档
        BIDFourLines *fourLines = [unarchiver decodeObjectForKey:kRootKey];
        
        [unarchiver finishDecoding];
        
        for (int i = 0; i < 4; i++) {
            UITextField *theField = self.lineFields[i];
            theField.text = fourLines.lines[i];
        }
    }
    
    //后台通知
    UIApplication *app = [UIApplication sharedApplication];
    [[NSNotificationCenter defaultCenter]
     addObserver:self
     selector:@selector(applicationWillResignActive:)
     name:UIApplicationWillResignActiveNotification
     object:app];
}


//进入后台时 归档
- (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];
}