/ parse Preference file 解析Preference文件
  SharedPreferences preferences = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);
  // get values from Map
  preferences.getBoolean("key", defaultValue)  
  preferences.get..("key", defaultValue)
  // you can get all Map but be careful you must not modify the collection returned by this
  // method, or alter any of its contents.
  //（Preference文件轉換成map）你可以獲取到一個map但是小心點最好不要修改map或它的內容
  Map<String, ?> all = preferences.getAll();
  // get Editor object
  SharedPreferences.Editor editor = preferences.edit();
  // add on Change Listener 添加監(jiān)聽器
  preferences.registerOnSharedPreferenceChangeListener(mListener);
  // remove on Change Listener 取消監(jiān)聽器
  preferences.unregisterOnSharedPreferenceChangeListener(mListener);
  // listener example 監(jiān)聽器例子
  SharedPreferences.OnSharedPreferenceChangeListener mOnSharedPreferenceChangeListener  
          = new SharedPreferences.OnSharedPreferenceChangeListener() {
      @Override
      public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
      }
  };

  // get Editor object
  SharedPreferences.Editor editor = preferences.edit();
  // put values in editor
  editor.putBoolean("key", value);  
  editor.put..("key", value);
  // remove single value by key
  editor.remove("key");
  // remove all values
  editor.clear();
  // commit your putted values to the SharedPreferences object synchronously
  // returns true if success 同步提交保存 成功返回true
  boolean result = editor.commit();
  // do the same as commit() but asynchronously (faster but not safely)
  // returns nothing 異步保存 不返回結果
  editor.apply();

  // There are 1000 String values in preferences
  SharedPreferences first = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);  
  // call time = 4 milliseconds第一次讀取文件花了4毫秒
  SharedPreferences second = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);  
  // call time = 0 milliseconds第二次0
  SharedPreferences third = context.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);  
  // call time = 0 milliseconds第三次0

  first.edit().putInt("key",15).commit();
  int firstValue = first.getInt("key",0)); // firstValue is 15  
  int secondValue = second.getInt("key",0)); // secondValue is also 15

  first.getString("key", null)  
  // call time = 147 milliseconds 第一次拿需要解析比較慢，然后會放到map中
  first.getString("key", null)  
  // call time = 0 milliseconds 第二次直接從map中拿 ，不用解析 很快
  second.getString("key", null)  
  // call time = 0 milliseconds 和第二次一樣
  third.getString("key", null)  
  // call time = 0 milliseconds

  public class MigrationManager {  
      private final static String KEY_PREFERENCES_VERSION = "key_preferences_version";
      private final static int PREFERENCES_VERSION = 2;
      public static void migrate(Context context) {
          SharedPreferences preferences = context.getSharedPreferences("pref", Context.MODE_PRIVATE);
          checkPreferences(preferences);
      }
      private static void checkPreferences(SharedPreferences thePreferences) {
          final double oldVersion = thePreferences.getInt(KEY_PREFERENCES_VERSION, 1);
          if (oldVersion < PREFERENCES_VERSION) {
              final SharedPreferences.Editor edit = thePreferences.edit();
              edit.clear();
              edit.putInt(KEY_PREFERENCES_VERSION, currentVersion);
              edit.commit();
          }
      }
  }

    public class PreferencesManager {
        private static final String PREF_NAME = "com.example.app.PREF_NAME";
        private static final String KEY_VALUE = "com.example.app.KEY_VALUE";
        private static PreferencesManager sInstance;
        private final SharedPreferences mPref;
        private PreferencesManager(Context context) {
            mPref = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
        }
        public static synchronized void initializeInstance(Context context) {
            if (sInstance == null) {
                sInstance = new PreferencesManager(context);
            }
        }
        public static synchronized PreferencesManager getInstance() {
            if (sInstance == null) {
                throw new IllegalStateException(PreferencesManager.class.getSimpleName() +
                        " is not initialized, call initializeInstance(..) method first.");
            }
            return sInstance;
        }
        public void setValue(long value) {
            mPref.edit()
                    .putLong(KEY_VALUE, value)
                    .commit();
        }
        public long getValue() {
            return mPref.getLong(KEY_VALUE, 0);
        }
        public void remove(String key) {
            mPref.edit()
                    .remove(key)
                    .commit();
        }
        public boolean clear() {
            return mPref.edit()
                    .clear()
                    .commit();
        }
    }