iOS---数据离线缓存

static FMDatabase *_db;
+ (void)initialize
{
    // 1.打开数据库
    NSString *path = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:@"statuses.sqlite"];
    _db = [FMDatabase databaseWithPath:path];
    [_db open];
    
    // 2.创表
    // 表中有三列，一个是主键，另一个是数据对象的id，一个是数据对象
    [_db executeUpdate:@"CREATE TABLE IF NOT EXISTS t_status (id integer PRIMARY KEY, status blob NOT NULL, idstr text NOT NULL);"];
}

+ (void)saveStatuses:(NSArray *)statuses
{
    // statuses是字典数组
    // 要将一个对象存进数据库的blob字段,最好先转为NSData
    // 一个对象要遵守NSCoding协议,实现协议中相应的方法,才能转成NSData
    for (NSDictionary *status in statuses) {
        // NSDictionary --> NSData
        NSData *statusData = [NSKeyedArchiver archivedDataWithRootObject:status];
        [_db executeUpdateWithFormat:@"INSERT INTO t_status(status, idstr) VALUES (%@, %@);", statusData, status[@"idstr"]];
    }
}

+ (NSArray *)statusesWithParams:(NSDictionary *)params
{
    // 根据请求参数生成对应的查询SQL语句
    NSString *sql = nil;
    // params[@"since_id"] 上拉刷新的标识
    if (params[@"since_id"]) {
        sql = [NSString stringWithFormat:@"SELECT * FROM t_status WHERE idstr > %@ ORDER BY idstr DESC LIMIT 20;", params[@"since_id"]];
    } else if (params[@"max_id"]) { // 下拉刷新
        sql = [NSString stringWithFormat:@"SELECT * FROM t_status WHERE idstr <= %@ ORDER BY idstr DESC LIMIT 20;", params[@"max_id"]];
    } else { // 第一次加载的时候
        sql = @"SELECT * FROM t_status ORDER BY idstr DESC LIMIT 20;";
    }
    
    // 执行SQL
    FMResultSet *set = [_db executeQuery:sql];
    NSMutableArray *statuses = [NSMutableArray array];
    while (set.next) {
        NSData *statusData = [set objectForColumnName:@"status"];
        NSDictionary *status = [NSKeyedUnarchiver unarchiveObjectWithData:statusData];
        [statuses addObject:status];
    }
    return statuses;
}