iphone开发中的数据存储：archiving model Objects

让Model类遵循NSCoding 和 NSCopying 协议

并实现三个方法：

- (void)encodeWithCoder:(NSCoder *)encoder {

/*
[super encodeWithCoder:encoder];
//If you are subclassing a class that also conforms to NSCoding, you need to make sure you call encodeWithCoder: on your superclass,
*/
[encoder encodeObject:foo forKey:kFooKey];
[encoder encodeObject:bar forKey:kBarKey];
[encoder encodeInt:someInt forKey:kSomeIntKey]; 
[encoder encodeFloat:someFloat forKey:kSomeFloatKey]
}

- (id)initWithCoder:(NSCoder *)decoder 
{ 
    if (self = [super init]) {
//if(self = [super initWithCoder:decoder])
    foo = [decoder decodeObjectForKey:kFooKey];
    bar = [decoder decodeObjectForKey:kBarKey]; 
    someInt = [decoder decodeIntForKey:kSomeIntKey]; 
    someFloat = [decoder decodeFloatForKey:kAgeKey];

}
    return self;
}

- (id)copyWithZone:(NSZone *)zone {
MyClass *copy = [[[self class] allocWithZone:zone] init];
 copy.foo = [self.foo copyWithZone:zone];
copy.bar = [self.bar copyWithZone:zone];
copy.someInt = self.someInt;
copy.someFloat = self.someFloat;
return copy;
}

在存储四个textField的字符的例子中，Model代码如下

#pragma mark NSCoding
- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:field1 forKey:kField1Key];
    [encoder encodeObject:field2 forKey:kField2Key];
    [encoder encodeObject:field3 forKey:kField3Key];
    [encoder encodeObject:field4 forKey:kField4Key];
}

- (id)initWithCoder:(NSCoder *)decoder {
    if (self = [super init]) {
        field1 = [decoder decodeObjectForKey:kField1Key];
        field2 = [decoder decodeObjectForKey:kField2Key];
        field3 = [decoder decodeObjectForKey:kField3Key];
        field4 = [decoder decodeObjectForKey:kField4Key];
    }
    return self;
}

#pragma mark -
#pragma mark NSCopying
- (id)copyWithZone:(NSZone *)zone {
    BIDFourLines *copy = [[[self class] allocWithZone: zone] init];
    copy.field1 = [self.field1 copyWithZone:zone];
    copy.field2 = [self.field2 copyWithZone:zone];
    copy.field3 = [self.field3 copyWithZone:zone];
    copy.field4 = [self.field4 copyWithZone:zone];
    return copy;
}

依然在ViewController中定义两个方法：

- (NSString *)dataFilePath;

- (void)applicationWillResignActive:(NSNotification *)notification;

#define kFilename         @"archive"
#define kDataKey          @"Data"

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


- (void)applicationWillResignActive:(NSNotification *)notification {
    BIDFourLines *fourLines = [[BIDFourLines alloc] init];
    fourLines.field1 = field1.text;
    fourLines.field2 = field2.text;
    fourLines.field3 = field3.text;
    fourLines.field4 = field4.text;
    
    NSMutableData *data = [[NSMutableData alloc] init];
    NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc]
                                 initForWritingWithMutableData:data];
    [archiver encodeObject:fourLines forKey:kDataKey];
    [archiver finishEncoding];
    [data writeToFile:[self dataFilePath] atomically:YES];
}

之后在viewDidLoad中实现：

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSString *filePath = [self dataFilePath];
    if ([[NSFileManager defaultManager] fileExistsAtPath:filePath]) {
        NSData *data = [[NSMutableData alloc]
            initWithContentsOfFile:[self dataFilePath]];
        NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc]
            initForReadingWithData:data];
        BIDFourLines *fourLines = [unarchiver decodeObjectForKey:kDataKey];
        [unarchiver finishDecoding];
        
        field1.text = fourLines.field1;
        field2.text = fourLines.field2;
        field3.text = fourLines.field3;
        field4.text = fourLines.field4;
    }
    
    UIApplication *app = [UIApplication sharedApplication];
    [[NSNotificationCenter defaultCenter] addObserver:self
        selector:@selector(applicationWillResignActive:)
        name:UIApplicationWillResignActiveNotification
        object:app];
}