时间：2022-10-11 12:10:35 | 栏目：Android代码 | 点击：次
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);