NSUserDefault存储类

1. 类要实现NSCoding协议
   实现encodeWithCoder和initWithCoder方法
2. 先把类的instance archive成NSData，然后存起来

    TGTest *tt = [TGTest new];
    tt.num = 3;
    NSData *dd = [NSKeyedArchiver archivedDataWithRootObject:tt];
    [[NSUserDefaults standardUserDefaults] setObject:dd forKey:@"objtest"];

1. 读数据时，读出NSData，然后unarchive得到instance。

    NSData *dd = [[NSUserDefaults standardUserDefaults] objectForKey:@"objtest"];
    @try
    {
        TGTest *tt= [NSKeyedUnarchiver unarchiveObjectWithData:dd];
    }
    @catch (NSException *exception)
    {
        NSLog(@"%@",exception);
    }
    @finally
    {
        
    }