浅谈 NSUserDefaults

NSUserDefaults 适用于快速读取小规模的数据

NSUserDefaults *standardDefaults = [NSUserDefaults standardUserDefaults];

写入数据

NSString *string = @"hahaha";
[standardDefaults setObject:string forKey:@"myKey"];
[standardDefaults synchronize];// 写完别忘了同步

读取数据

NSString *value = [standardDefaults objectForKey:@"myKey"];

NSUserDefaults 可以理解成键值对

有时在写数据之前，想判断下这个健是否已经设置过默认值

NSUserDefaults *standardDefaults = [NSUserDefaults standardUserDefaults];
if ([standardDefaults stringForKey:@"favoriteColor"] == nil) {
    [standardDefaults setObject:@"Green" forKey:@"favoriteColor"];
    [standardDefaults synchronize];
}

其实可以使用 registerDefaults:

NSUserDefaults *standardDefaults = [NSUserDefaults standardUserDefaults];
[standardDefaults registerDefaults:@{@"favoriteColor": @"Green"}];
[standardDefaults synchronize];

每次程序启动的时候调用 registerDefaults: 方法都是安全的，完全可以将这个方法的调用放到 applicationDidFinishLaunching: 方法中，这个方法永远都不会覆盖用户设置的值。

但是并不是所有类型的对象都能够直接放入 NSUserDefaults，NSUserDefaults 只支持 Foundation 类型的对象，如果自定义 NSObject 对象存入 NSUserDefaults 就要实现 NSCoding 协议了，比如：

@interface Person : NSObject

@property (nonatomic, copy) NSString *age;
@property (nonatomic, copy) NSString *name;

- (id)initWithCoder:(NSCoder *)coder;
- (void)encodeWithCoder:(NSCoder *)coder;

@implementation Person
- (id)initWithCoder:(NSCoder *)coder {
    if (self = [super init]) {
        self.age  = [coder decodeObjectForKey:@"age"];
        self.name = [coder decodeObjectForKey:@"name"];
    }
    return self;
}

- (void)encodeWithCoder: (NSCoder *)coder {
    [coder encodeObject:self.age forKey:@"age"];
    [coder encodeObject:self.name forKey:@"name"];
}

然后再存取时通过 NSData 做载体：

存入

NSUserDefaults *standardDefaults = [NSUserDefaults standardUserDefaults];
Person *person = [[Person alloc] init];
NSData *personData = [NSKeyedArchiver archivedDataWithRootObject:person];
[standardDefaults setObject:personData forKey:@"personKey"];
[standardDefaults synchronize];

读取

NSData *newPersonData = [standardDefaults objectForKey:"personKey"];
Person *Person = [NSKeyedUnarchiver unarchiveObjectWithData:newPersonData];