Objective C 总结（十三）：持久化

Objective-C提供了几种常用的数据保存方案

1. 属性列表
2. 对象序列化和反序列化
3. SQLite
4. Core Data

属性列表

plist中可存储的类型：NSArray, NSDictionary, NSString, NSData, NSDate, NSNumber,最常用的根类型是NSArray,NSDictionary

这些类提供了读和写的方法， 同样提供的工厂方法

- (id) initWithContentsOfFile : (NSString *) filePath;
- (void) writeToFile: (NSString *) filePath
           atomically: (BOOL) flag;

NSArray *plist = [NSArray arrayWithContentsOfFile: @"/tmp/test.plist"];
[plist writeToFile: @"/tmp/testSaved.plist" atomically: YES];

对象序列化

实现NSCoding协议

@implementation Person

- (id) initWithCoder: (NSCoder *) decoder
{
    self = [super initWithCoder: decoder];
    if(self)
    {
        self.firstName = [decoder decodeObjectForKey: @"firstName"];
    }
    return self;
}
- (void) encodeWithCoder: (NSCoder *) coder
{
    [super encodeWithCoder: coder];
    [coder encodeObject: self.firstName
                      forKey: @"firstName"];
}

@end

NSCoder是抽象类，需要使用它的子类的实现， 如NSKeyedArchiver, NSKeyedUnarchiver

NSData *personData = [NSKeyedArchiver archiverDataWithRootObject: somePerson];

[personData writeToFile:　@"/tmp/somePerson.data"];

somePerson = [NSKeyedUnarchiver unarchiveObjectWithData: personData];

上例使用了工厂方法，同样可以使用实例方法

NSMutableData *data = [[NSMutableData alloc] init];
NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData: data];
[archiver encodeObject: somePerson forKey: "bobData"];
[archiver finishEncoding];
BOOL isSuccess = [data writeToFile: @"/tmp/bob.data" atomically: YES];
[archiver release];
[data release];


NSData *bobData = [[NSData alloc] initWithContentsOfFile: @"/tmp/bob.data"];
NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData: bobData];
somePerson = [unarchiver decodeObjectForKey: @"bobData"];
[unarchiver release];
[bobData release];

SQLite 

SQLite是基于文件是数据库，提供了C API，使用字符串为C字符串，不能用NSString

sqlite3 *database;
int result = sqlite3_open("/documents/mydata.db", &database);
if(result== SQLITE_OK)
{}
char *errorMsg;
const char *createSql = "CREATE TABLE IF NOT EXISTS Person (ID INTEGER PRIMARY KEY AUTOINCREMENT, firstName TEXT");
result = sqlite3_exec(database, createSql, NULL, NULL, &errorMsg);


const char *sql = "SELECT * FROM Person WHERE firstName='bob'";
sqlite3_stmt *sqlStatement;
result = sqlite3_prepare_v2(database, sql, -1, &sqlStatement, NULL);
if(result==SQLITE_OK)
{
    while(sqlite3_step(sqlStatement) == SQLITE_ROW)
    {
        // 第二个参数为索引
        int id = sqlite3_column_int(sqlStatement, 0);
        char *firstName = (char *)sqlite3_column_text(sqlStatement, 1);
    }
    sqlite3_finalize(sqlStatement);
}

sqlite3_close(database);

绑定变量语法

char *sql = "INSERT INTO Person VALUES(?)";
sqlite3_stmt *statement;
result = sqlite3_prepare_v2(database,sql, -1, &statement, NULL);
if(result == SQLITE_OK)
{
    sqlite3_bind_text(statement,1, "john", -1, NULL);
}
if(sqlite3_step(statement) != SQLITE_DONE)
{
    NSLog("check errors");
}
sqlite3_finalize(statement);

第四个参数表示数据长度，-1表示整个字符串

最后一个可选的回调函数，用于执行内存清理工作。