Android: Skanowanie sieci Wi-Fi + lista do wyboru

Question

Próbuję utworzyć aplikację, która pozwoli ci skanować dostępne sieci Wi-Fi, a następnie wyświetlić je na liście do wybrania. Aby to zrobić Próbowałem z tym kodem:

package android.nacho.WifiScan; 

import java.util.List; 
import android.app.Activity; 
import android.content.BroadcastReceiver; 
import android.content.Context; 
import android.content.Intent; 
import android.content.IntentFilter; 
import android.net.wifi.ScanResult; 
import android.net.wifi.WifiManager; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.AdapterView; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 
import android.widget.TextView; 
import android.widget.AdapterView.OnItemClickListener; 

public class WifiScan extends Activity { 
    TextView mainText; 
    WifiManager mainWifi; 
    WifiReceiver receiverWifi; 
    List<ScanResult> wifiList; 
    StringBuilder sb = new StringBuilder(); 
    ListView listView = new ListView(this); 
    ArrayAdapter<String> adapter; 
    Context context = this; 

    static String[] NETWORKS; 

    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_wifi_scan); 
     mainText = (TextView) findViewById(R.id.text); 
     mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE); 
     receiverWifi = new WifiReceiver(); 
     registerReceiver(receiverWifi, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)); 
     mainWifi.startScan(); 
     mainText.setText("\\nStarting Scan...\\n"); 


    } 

    public boolean onCreateOptionsMenu(Menu menu) { 
     menu.add(0, 0, 0, "Refresh"); 
     return super.onCreateOptionsMenu(menu); 
    } 

    public boolean onMenuItemSelected(int featureId, MenuItem item) { 
     mainWifi.startScan(); 
     mainText.setText("Starting Scan"); 
     return super.onMenuItemSelected(featureId, item); 
    } 

    protected void onPause() { 
     unregisterReceiver(receiverWifi); 
     super.onPause(); 
    } 

    protected void onResume() { 
     registerReceiver(receiverWifi, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)); 
     super.onResume(); 
    } 

    class WifiReceiver extends BroadcastReceiver { 
     public void onReceive(Context c, Intent intent) { 


      wifiList = mainWifi.getScanResults(); 
      NETWORKS = new String[wifiList.size()]; 
      for(int i = 0; i < wifiList.size(); i++){ 

       NETWORKS[i]=i+"-"+(wifiList.get(i)).toString(); 


      } 


      System.out.println("debería imprimir: "+NETWORKS[2]); 


      adapter = new ArrayAdapter<String>(context, android.R.layout.simple_list_item_multiple_choice, NETWORKS); 
      // adapter.setDropDownViewResource(android.R.layout.select_dialog_multichoice); 
      listView.setAdapter(adapter); 
      listView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE); 
      listView.setOnItemClickListener(new OnItemClickListener() { 

       public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,long arg3) { 


       } 

      }); 

      setContentView(listView); 

     } 
    } 
} 

Ale jestem coraz to błędy:

06-18 12:36:32.670: W/dalvikvm(15499): threadid=1: thread exiting with 
uncaught exception (group=0x40dfb930) 

06-18 12:36:32.670: E/AndroidRuntime(15499): FATAL EXCEPTION: main 

06-18 12:36:32.670: E/AndroidRuntime(15499): 
java.lang.RuntimeException: Unable to instantiate activity 
ComponentInfo{android.nacho.WifiScan/android.nacho.WifiScan.WifiScan}: 
java.lang.NullPointerException 

06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106) 

06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 

06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
06-18 12:36:32.670: D/AndroidRuntime(15499): Shutting down VM 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
android.os.Looper.loop(Looper.java:137) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.app.ActivityThread.main(ActivityThread.java:5039) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
java.lang.reflect.Method.invokeNative(Native Method) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
java.lang.reflect.Method.invoke(Method.java:511) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
dalvik.system.NativeStart.main(Native Method) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): Caused by: java.lang.NullPointerException 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.content.ContextWrapper.getResources(ContextWrapper.java:89) 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78) 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.view.View.<init>(View.java:3224) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at android.view.View.<init>(View.java:3279) 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.view.ViewGroup.<init>(ViewGroup.java:431) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.widget.AdapterView.<init>(AdapterView.java:235) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
android.widget.AbsListView.<init>(AbsListView.java:766) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
android.widget.ListView.<init>(ListView.java:143) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.widget.ListView.<init>(ListView.java:139) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.widget.ListView.<init>(ListView.java:135) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.nacho.WifiScan.WifiScan.<init>(WifiScan.java:27) 06-18 
12:36:32.670: E/AndroidRuntime(15499):  at 
java.lang.Class.newInstanceImpl(Native Method) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
java.lang.Class.newInstance(Class.java:1319) 06-18 12:36:32.670: 
E/AndroidRuntime(15499): at 
android.app.Instrumentation.newActivity(Instrumentation.java:1054) 
06-18 12:36:32.670: E/AndroidRuntime(15499): at 
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 
06-18 12:36:32.670: E/AndroidRuntime(15499): ... 11 more 

Co robię źle ??

Answer 1

Spróbuj tego pracował dla mnie

public class MainActivity extends Activity { 


    WifiManager mainWifi; 
    WifiReceiver receiverWifi; 

    StringBuilder sb = new StringBuilder(); 

    private final Handler handler = new Handler(); 


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


     mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE); 

     receiverWifi = new WifiReceiver(); 
     registerReceiver(receiverWifi, new IntentFilter(
       WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)); 
     if(mainWifi.isWifiEnabled()==false) 
     { 
      mainWifi.setWifiEnabled(true); 
     } 


     doInback(); 
    } 

    public void doInback() 
    { 
     handler.postDelayed(new Runnable() { 

      @Override 
      public void run() 
      { 
       // TODO Auto-generated method stub 
       mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE); 

       receiverWifi = new WifiReceiver(); 
       registerReceiver(receiverWifi, new IntentFilter(
         WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)); 
       mainWifi.startScan(); 
       doInback(); 
      } 
     }, 1000); 

    } 

    public boolean onCreateOptionsMenu(Menu menu) { 
     menu.add(0, 0, 0, "Refresh"); 
     return super.onCreateOptionsMenu(menu);} 
    public boolean onMenuItemSelected(int featureId, MenuItem item) { 
     mainWifi.startScan(); 

     return super.onMenuItemSelected(featureId, item);} 


    @Override 
    protected void onPause() 
    { 
     unregisterReceiver(receiverWifi); 
     super.onPause(); 
    } 

    @Override 
    protected void onResume() 
    { 
     registerReceiver(receiverWifi, new IntentFilter(
       WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)); 
     super.onResume(); 
    } 

    class WifiReceiver extends BroadcastReceiver 
    { 
     public void onReceive(Context c, Intent intent) 
     { 

      ArrayList<String> connections=new ArrayList<String>(); 
      ArrayList<Float> Signal_Strenth= new ArrayList<Float>(); 

      sb = new StringBuilder(); 
      List<ScanResult> wifiList; 
      wifiList = mainWifi.getScanResults(); 
      for(int i = 0; i < wifiList.size(); i++) 
      { 

       connections.add(wifiList.get(i).SSID); 
      } 


     } 
    } 



} 

Answer 2

nie doInback() stwarza wiele WifiReceivers który następnie trafi do wyjątek, ponieważ nie może dostać wszystko niezarejestrowany ?!

Może s.th. w ten sposób można to naprawić:

if (receiveWifi==null) { 
    receiveWifi = new WifiReceiver(); 
}