使用SharedPreferences在Android存储对象

为什么不用SQLite? 原因多种：除了面向对象和关系数据库之间的阻抗不匹配时，SQLite可能是矫枉过正(带来了更多的开销)对于一些简单的用例，或者其用法和语句可能会完全不喜欢。其他Android ORM 框架(ORMLite, greenDAO, Sugar ORM, ActiveAndroid, 等)或NOSQL移动数据库，如Couchbase精简版(Beta版在此时间)。 Couchbase精简版基本上是一个JSON数据库，它们旨在降低复杂性，但是同时又违背了抽象漏洞定律(所有不证自明的抽象都是有漏洞的)

代码如下：

/** stores the user object in SharedPreferences */

public class UserPrefs{

/** This application's preferences label */

private static final String PREFS_NAME = "com.our.package.UserPrefs";

/** This application's preferences */

private static SharedPreferences settings;

/** This application's settings editor*/

private static SharedPreferences.Editor editor;

/** Constructor takes an android.content.Context argument*/

public UserPrefs(Context ctx){

if(settings == null){

settings = ctx.getSharedPreferences(PREFS_NAME,

Context.MODE_PRIVATE );

}

/*

* Get a SharedPreferences editor instance.

* SharedPreferences ensures that updates are atomic

* and non-concurrent

*/

editor = settings.edit();

}

//...

}

其中User代码如下：

/** User object to be saved in db */

public class User{

private int id; // used for object storage

private String userName;

private boolean registered;

private double score;

/** Constructor */

public User(int id, String userName, boolean registered, double score){

this.id = id;

this.userName = userName;

this.registered = registered;

this.score = score;

}

// getters and setters here...

}

将SharedPreferences看成一个Map存储，CRUD操作如下：

/** generic field keys */

private static final String KEY_USERNAME = "com.our.package.KEY_USERNAME";

private static final String KEY_REGISTERED = "com.our.package.KEY_REGISTERED";

private static final String KEY_SCORE = "com.our.package.KEY_SCORE";

/** Store or Update */

public void setUser(User user){

if(user == null)

return; // don't bother

int id = user.getId();

editor.putString(

getFieldKey(id, KEY_USERNAME),

user.getUsername() );

editor.putBoolean(

getFieldKey(id, KEY_REGISTERED),

user.isRegistered() );

editor.putFloat(

getFieldKey(id, KEY_SCORE),

user.getScore() );

editor.commit();

}

/** Retrieve */

public User getUser(int id){

String name = settings.getString(

getFieldKey(id, KEY_USERNAME),

"" ); // default value

boolean registered =  settings.getBoolean(

getFieldKey(id, KEY_REGISTERED),

false); // default value

double score =  settings.getFloat(

getFieldKey(id, KEY_SCORE),

0); // default value

return new User(id, name, registered, score);

}

/** Delete */

public void deleteUser(User user){

if(user == null)

return; // don't bother

int id = user.getId();

editor.remove( getFieldKey(id, KEY_USERNAME) );

editor.remove( getFieldKey(id, KEY_REGISTERED) );

editor.remove( getFieldKey(id, KEY_SCORE) );

editor.commit();

}

主键是通过getFieldKey方法， getFieldKey() 给我们每个用户每个字段的唯一标识。：

/** The prefix for flattened user keys */

public static final String KEY_PREFIX =

"com.our.package.KEY";

/** Method to return a unique key for any field belonging to a given object

* @param id of the object

* @param fieldKey of a particular field belonging to that object

* @return key String uniquely identifying the object's field

*/

private String getFieldKey(int id, String fieldKey) {

return  KEY_PREFIX + id + "_" + fieldKey;

}

客户端调用如下：

// get a SharedPreferences instance

UserPrefs prefs = new UserPrefs( this.getApplicationContext() );

// get id from server or local storage

// then find User with that id

User user = prefs.getUser(id);

// operations on User, e.g.

user.setRegistered(true);

user.setScore(new_score);

// save

prefs.setUser(user);

// ...or delete

prefs.deleteUser(user),

Gson 是一个Java库，它提供了简单的toJSON()和fromJson()方法来转换Java对象到JSON格式，反之亦然。我们可以简单地存储JSON格式整个字符串到SharedPreferences：

// convert User object user to JSON format

Gson gson = new Gson();

String user_json = gson.toJson(user);

// store in SharedPreferences

String id = "" + user.getId(); // get storage key

editor.putString(id, user_json);

editor.commit();

// time flies...

// do the reverse operation

user_json = settings.getString(id, "");

user = gson.fromJson(user_json, User.class);