使用SharedPreferences在Android存储对象详细代码

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

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);

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