在所有的移动开发平台数据持久化都是很重要的部分：在j2me中是rms或保存在应用程序的目录中，在Symbian中可以保存在相应的磁盘目录中和数据库中。Symbian中因为权限认证的原因，在3rd上大多数只能访问应用程序的private目录或其它系统共享目录。在iPhone中，APPLE博采众长，提供了多种数据持久化的方法，下面笔者会逐个进行详细的讲解。
iPhone提供的数据持久化的方法，从数据保存的方式上讲可以分为三大部分：属性列表、对象归档、嵌入式数据库(SQLite3)、其他方法。
一、属性列表NSUserDefaults
NSUserDefaults类的使用和NSKeyedArchiver有很多类似之处，但是查看NSUserDefaults的定义可以看出，NSUserDefaults直接继承自NSObject而NSKeyedArchiver 继承自NSCoder。这意味着NSKeyedArchiver实际上是个归档持久化的类，也就可以使用NSCoder类的［encodeObject: (id)objv forKey:(NSString *)key］方法来对数据进行持久化存储。

- (void)applicationDidFinishLaunching:(UIApplication *)application {   

NSString *strOne = @"Persistent data1";  NSString *strTwo = @"Persistent data 2";    

NSMutableArray *persistentArray = [[NSMutableArray alloc] init];  

[persistentArray addObject:strOne];  [persistentArray addObject:strTwo];  

 //archive  NSUserDefaults *persistentDefaults = [NSUserDefaults standardUserDefaults];  

[persistentDefaults setObject:persistentArray forKey:@"myDefault"];  

NSString *descriptionDefault = [persistentDefaults description];  

NSLog(@"NSUserDefaults description is :%@",descriptionDefault);    

//unarchive  NSArray *UnpersistentArray =

[persistentDefaults objectForKey:@"myDefault"];

NSString *UnstrOne = [UnpersistentArray objectAtIndex:0];  

NSString *UnstrTwo = [UnpersistentArray objectAtIndex:1];    

NSLog(@"UnstrOne = %@,UnstrTwo = %@",UnstrOne,UnstrTwo);    

// Override point for customization after application launch  [window makeKeyAndVisible]; }

二、对象归档NSKeyedArchiver和NSKeyedUnarchiver
iPhone和Symbian 3rd一样，会为每一个应用程序生成一个私有目录，这个目录位于

/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications下，并随即生成一个数字字母串作为目录名，在每一次应用程序启动时，这个字母数字串都是不同于上一次的，上一次的应用程序目录信息被转换成名为.DS_Store隐藏文件，这个目录的文件结构如下图：

通常使用Documents目录进行数据持久化的保存，而这个Documents目录可以通过NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserdomainMask,YES)得到，代码如下：

- (void)applicationDidFinishLaunching:(UIApplication *)application {   

NSString *strOne = @"Persistent data1";  NSString *strTwo = @"Persistent data 2";    

NSArray *persistentArray = [NSArray arrayWithObjects:strOne,strTwo,nil];  

NSArray *pathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,    

NSAllDomainsMask, YES);    int pathLen = [pathArray count];    

NSLog(@"path number is :%d",pathLen);    

NSString *filePath;    for(int i = 0; i < pathLen; i++)  {   

filePath = [pathArray objectAtIndex:i];   NSLog(@"%d path is :%@",i,filePath);  }    

NSString *myFilename = [filePath stringByAppendingPathComponent:@"myFile.rtf"];    

NSLog(@"myfile‘s path is :%@",myFilename);    

// no files generated in correspond directory now    

[NSKeyedArchiver archiveRootObject:persistentArray toFile:myFilename];  

// now the myFile.rtf is generated    

// Override point for customization after application launch  [window makeKeyAndVisible]; }

NSSearchPathForDirectoriesInDomains()的第二个参数是个枚举值，在笔者的测试代码中，只有NSUserDomainMask和NSAllDomainsMask可以获取到目录数为1，其余的皆为0，打印出来的结果如下：

[Session started at 2009-11-10 21:30:08 +0800.]
2009-11-10 21:30:10.516 PersistentExample[763:207] path number is :1
2009-11-10 21:30:10.518 PersistentExample[763:207] 0 path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/C93DC783-F137-4660-AE5A-08C3E11C774B/Documents
2009-11-10 21:30:10.521 PersistentExample[763:207] myfile‘s path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/C93DC783-F137-4660-AE5A-08C3E11C774B/Documents/myFile.rtf
Terminating in response to SpringBoard‘s termination.

[Session started at 2009-11-10 21:32:27 +0800.]
2009-11-10 21:32:30.091 PersistentExample[803:207] path number is :1
2009-11-10 21:32:30.092 PersistentExample[803:207] 0 path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/763E6772-E754-452F-8532-80C2CE4466B5/Documents
2009-11-10 21:32:30.100 PersistentExample[803:207] myfile‘s path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/763E6772-E754-452F-8532-80C2CE4466B5/Documents/myFile.rtf
Terminating in response to SpringBoard‘s termination.

从打印的结果如下，每次应用程序启动时生成的数字字母串目录名字并不一样。在调用[NSKeyedArchiver archiveRootObject:persistentArray toFile:myFilename]方法前，文件myFile.rtf并每生成，只有在调用此方法后才产生相应的文件。
下面需要把数据从属性列表中读取出来，在上面的代码中，笔者使用NSArray保存数据。但在大多数应用程序中，数据的尺寸并不是固定的，这个时候就需要使用NSMutalbeArray动态的保存数据，代码优化如下：

 -   (void)applicationDidFinishLaunching:(UIApplication *)application { 
 NSString *myFilename;
 // archive
 {
  NSString *strOne = @"Persistent data1";
  NSString *strTwo = @"Persistent data 2";
  
  NSMutableArray *persistentArray = [[NSMutableArray alloc] init];
  [persistentArray addObject:strOne];
  [persistentArray addObject:strTwo];

  NSArray *pathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,    NSAllDomainsMask, YES);
  
  int pathLen = [pathArray count];
  NSLog(@"path number is :%d",pathLen);
  
  NSString *filePath;
  
  for(int i = 0; i < pathLen; i++)
  {
   filePath = [pathArray objectAtIndex:i];
   
   NSLog(@"%d path is :%@",i,filePath);
  }
  
  myFilename = [filePath stringByAppendingPathComponent:@"myFile.rtf"];
  
  NSLog(@"myfile‘s path is :%@",myFilename);
  
  [NSKeyedArchiver archiveRootObject:persistentArray toFile:myFilename];
 }
 
 // unarchive
 {
  NSArray *unarchiveArray = [NSKeyedUnarchiver unarchiveObjectWithFile:myFilename];
  NSString *UnstrOne = [unarchiveArray objectAtIndex:0];
  NSString *UnstrTwo = [unarchiveArray objectAtIndex:1];
  
  NSLog(@"UnstrOne = %@,UnstrTwo = %@",UnstrOne,UnstrTwo);
 }
 
 
 // Override point for customization after application launch
 [window makeKeyAndVisible];

输出结果如下：

[Session started at 2009-11-10 22:41:57 +0800.]
2009-11-10 22:41:59.344 PersistentExample[1082:207] path number is :1
2009-11-10 22:41:59.346 PersistentExample[1082:207] 0 path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/055CD17C-864E-4A83-ABF0-5F01EE85BD5A/Documents
2009-11-10 22:41:59.355 PersistentExample[1082:207] myfile‘s path is :/Users/sundfsun2009/Library/Application Support/iPhone Simulator/User/Applications/055CD17C-864E-4A83-ABF0-5F01EE85BD5A/Documents/myFile.rtf
2009-11-10 22:41:59.357 PersistentExample[1082:207] UnstrOne = Persistent data1,UnstrTwo = Persistent data 2
Terminating in response to SpringBoard‘s termination.

从上面的图中可以看到，目录中还有个tmp目录，读者也可以把数据保存在tmp目录中，获取这个目录使用NSTemporaryDirectory()方法。

三、嵌入式数据库(SQLite3)
嵌入式数据库持久化数据就是把数据保存在iphone的嵌入式数据库系统SQLite3中，本质上来说，数据库持久化操作是基于文件持久化基础之上的。 要使用嵌入式数据库SQLite3，首先需要加载其动态库libsqlite3.dylib，这个文件位于/Xcode3.1.4/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.1.SDK/usr/lib目录下。在Framework文件夹上右击，选择“Adding->Existing Files...”，定位到上述目录并加载到文件夹。
首先在头文件中做如下修改：

#import <UIKit/UIKit.h>

#include "sqlite3.h"
#define kFileName @"mydb.sql"

@interface PersistentExampleAppDelegate : NSObject <UIApplicationDelegate> {
 sqlite3 *database;
 UIWindow *window;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

- (void)applicationDidFinishLaunching:(UIApplication *)application {   

 NSArray *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);

 NSString *paths = [[path objectAtIndex:0] stringByAppendingPathComponent:kFileName];  

 NSFileManager *fileManager = [NSFileManager defaultManager];

 BOOL findFile = [fileManager fileExistsAtPath:paths];  

 NSLog(@"Database file path = %@",paths);    

// 如果找到了数据库文件  if(findFile)  {   

NSLog(@"Database file have already existed.");  

  if(sqlite3_open([paths UTF8String], &database) != SQLITE_OK)//打开数据库失败  

 {    

sqlite3_close(database);   

 NSAssert(0,@"Failed to open database");

  }  }else  {  

 NSLog(@"Database file does not exsit!");

  if(sqlite3_open([paths UTF8String], &database) != SQLITE_OK)//打开数据库失败

  {  

  sqlite3_close(database);  

  NSAssert(0,@"Failed to open database");

  }  }    

char *errorMsg;

//创建表

 NSString *createSQL = @"create table if not exists fields (row integer primary key, field_data text);"; 

 if(sqlite3_exec(database, [createSQL UTF8String],NULL,NULL,&errorMsg)!=SQLITE_OK)

 {   

sqlite3_close(database);   NSAssert1(0,@"Error creating table: %s",errorMsg);  

}

   NSString *strOne = @"Persistent data1";  

NSString *strTwo = @"Persistent data 2";    

NSMutableArray *persistentArray = [[NSMutableArray alloc] init];

 [persistentArray addObject:strOne];

 [persistentArray addObject:strTwo];  

 for (int i = 0; i < [persistentArray count]; i++) {

  NSString *upDataSQL = [[NSString alloc] initWithFormat:@"insert or replace into

fields (row,field_data) values (%d,‘%@‘);",i,[persistentArray objectAtIndex:i]];

  char* errorMsg;  

 if(sqlite3_exec(database,[upDataSQL UTF8String],NULL,NULL,&errorMsg)!= SQLITE_OK)

  {

   sqlite3_close(database);    NSAssert(0,@"Failed to open database");

  }

 }  

 //unarchive  NSString *query = @"select row, field_data from fields order by row";//查找表中的数据   

sqlite3_stmt *statement;  if(sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, nil)

== SQLITE_OK)  {   

while(sqlite3_step(statement) == SQLITE_ROW)   

{  

  int row = sqlite3_column_int(statement, 0);   

 char *rowData = (char *)sqlite3_column_text(statement, 1);      

  NSString *fieldName = [[NSString alloc] initWithFormat:@"show%d",row];  

  NSString *fieldValue = [[NSString alloc] initWithUTF8String:rowData];
   NSLog(@"fieldName is :%@,fieldValue is :%@",fieldName,fieldValue);    

   [fieldName release];    [fieldValue release];   

}

  sqlite3_finalize(statement);

 }  

 // Override point for customization after application launch  [window makeKeyAndVisible];

}

在上面的代码中，我们使用  NSFileManager *fileManager = [NSFileManager defaultManager]; BOOL findFile = [fileManager fileExistsAtPath:paths]; 来判断数据库文件是否已经存在，其实在大多数情况下是没有必要的，sqlite3_open()方法会自动帮我们判断数据库文件是否存在，如果不存在则创建心的数据库文件。

四、其它方法
除了上面的三种方法来保存持久化数据以外，我们还可以用写文件到磁盘的方式来保存持久化数据。

-  (void)applicationDidFinishLaunching:(UIApplication *)application {   

NSString *strOne = @"Persistent data1";  NSString *strTwo = @"Persistent data 2";    

NSMutableArray *persistentArray = [[NSMutableArray alloc] init];  [persistentArray addObject:strOne];  

[persistentArray addObject:strTwo];        

NSArray *filePathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);  

NSString *filePath = [[filePathArray objectAtIndex:0] stringByAppendingPathComponent:@"mydatas.plist"];  

 [[NSArray arrayWithObjects:persistentArray,nil] writeToFile:filePath atomically:NO];    

//load  NSMutableArray *saveDataArray = [[NSMutableArray alloc] init];

 if([[NSFileManager defaultManager] fileExistsAtPath:filePath])  

 saveDataArray = [NSMutableArray arrayWithContentsOfFile:filePath];  

 else   

saveDataArray = [NSMutableArray arrayWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"Savedatas" ofType:@"plist"]];

NSArray *strArray = [saveDataArray objectAtIndex:0];  

 NSString *UnstrOne = [strArray objectAtIndex:0];  NSString *UnstrTwo = [strArray objectAtIndex:1];
 // Override point for customization after application launch  [window makeKeyAndVisible];

}

ios数据持久化-属性列表NSUserDefaults、对象归档NSKeyedArchiver和NSKeyedUnarchive、嵌入式数据库(SQLite3)