java - 共享偏好 VS 上下文？

Question

我对 Android 的经验不是很多，所以到目前为止我编写的每一段代码都非常简单。现在我需要实现一个本地化和导航应用程序，因此我需要将代码分解为模块，以便我可以单独更改每个组件。我有一些变量需要在不同的类之间共享它们。我使用了静态变量，但我在一些帖子中读到静态变量不是首选。然后我发现了一些其他谈论上下文的帖子。因此，我创建了一个名为 Globals 的类，并在 list 文件中添加了以下几行:

 <application android:name="com.example.smartnav.Globals" 
  package="com.example.smartnav"
 android:icon="@drawable/ic_launcher"
 android:allowBackup="true"

   android:label="@string/app_name"/>

这是全局类:

    package com.example.smartnav;


import java.util.List;

import android.net.wifi.ScanResult;

import android.app.Application;

public class Globals extends Application {
      private Boolean Scanning=false;
      private String Logname;
      private int interval;
      private int numOfScans;
      private List<ScanResult> result;



      //getters
      public Boolean getScannig(){
        return Scanning;
      }
      public int getInterval()
      {
          return interval;
      }
      public int getScans()
      {
          return numOfScans;
      }
      public List<ScanResult> getRes()
      {
          return result;
      }
      public String getLog()
      {
          return Logname;
      }


    //setter

      public void setScanning(Boolean s){
        Scanning= s;
      }
      public void setRes(List<ScanResult> res)
      {
          result =res;
      }
      public void setInterval(int I)
      {
          interval = I;
      }
      public void setScans(int S)
      {
          numOfScans=S;
      }
      public void setLog(String s)
      {
          Logname= s;
      }

}

现在我有两个问题，第一个问题是每当我尝试使用 Globals 类时我的应用程序都会崩溃，这是代码:我是否错误地使用了上下文？

    public class MainActivity extends Activity {

    private Context context;
    public WifiManager Wifi;
    private  WifiReceiver receiverWifi;
    private IntentFilter filter;
    private List<ScanResult> result;
    private File AppDir;
    private static String filename;
    private File file;
    private FileWriter writer;
    private Globals AppState ;
    private int Interval;
    private int numOfScans;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Log.d("Main ","activity created");
        //
       AppState = ((Globals)getApplicationContext());
       context= this;
       Wifi=(WifiManager) getSystemService(Context.WIFI_SERVICE);
       receiverWifi = new WifiReceiver();
        filter= new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION);
        registerReceiver(receiverWifi, filter);

      Log.d("Main   ","wifi registered");
        // create the application directory

        AppDir = new File(Environment.getExternalStorageDirectory()+"/SmartNavi/Log");
        if(AppDir.isDirectory())
        {
            filename=Environment.getExternalStorageDirectory()+"/SmartNavi/Log/log.txt";
            file = new File(filename);
            if(!file.exists())
                try {
                    file.createNewFile();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            else 
            {
                Date d= new Date();
                filename=Environment.getExternalStorageDirectory()+"/SmartNavi/Log/log"+d.getTime()+".txt";
                file = new File(filename);
                try {
                    file.createNewFile();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            }
        else
        {
            AppDir.mkdirs();
            filename=Environment.getExternalStorageDirectory()+"/SmartNavi/Log/log.txt";
            file = new File(filename);
            if(!file.exists())
                try {
                    file.createNewFile();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            else 
            {
                Date d= new Date();
                filename=Environment.getExternalStorageDirectory()+"/SmartNavi/Log/log"+d.getTime()+".txt";
                file = new File(filename);
                try {
                    file.createNewFile();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        }

        //setting pars

        Interval=250;
        numOfScans=4;
        AppState.setInterval(Interval);
        AppState.setScans(numOfScans);
        AppState.setLog(filename);


        Wifi.startScan();

        try {
                writer = new FileWriter(file, true);
                writer.append("Smart Navigation. \n");
                writer.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
       // AsyncScanning.AsyncScan();
    }//on create



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }



    class WifiReceiver extends BroadcastReceiver {

        public void onReceive(Context c, Intent intent) {

            result=Wifi.getScanResults();
    //      AppState.setRes(result);
             try {
                writer = new FileWriter(file, true);
                writer.append(result.size()+" s \n");
                writer.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }//end of on receive

    }// end of class
} // end of smartNav

我的最后一个问题是:我在这里读到了一些答案，如果我的应用程序成为后台进程，那么上下文中的所有数据将被设置为空，我将失去我的上下文。有什么方法可以克服这一点吗？或者我应该切换到 SharedPreferences ？

编辑:这是 Logcat 的输出

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.smartnav/com.example.smartnav.MainActivity}: java.lang.ClassCastException: android.app.Application cannot be cast to com.example.smartnav.Globals

Answer 1

Now I have two questions, the first one is that my application keeps crashing whenever I try to use the Globals class, here is the code: Did I use context incorrectly?

您应该使用 getApplication() 方法，或者使您的应用程序类为单例，这样您就可以调用 Globals.getInstance().getMyVariable() 等。

My last question is this : I have read on some answers here that if my application becomes a background process then all the data in the context will be set to null, and I will lose my context. Is there is any method to overcome this point? or should I switch to SharedPreferences ?

如果您的应用程序成为后台，那么 Android 更有可能杀死您的应用程序，这样也会破坏您的所有静态对象。在您的 Globals 类中，您不应将数据存储在静态变量中，而应存储在某些持久存储中 - 如果数据较小，则使用 SharedPreferences，如果数据较大，则可以将其存储在json 并保存到应用程序内存，或使用 sqlite db。