wifiManager.getScanResult()返回空值

Question

我正在搜索可用wifi网络，但getScanResults()方法返回空列表。

我包含了所需的所有权限：

android.permission.ACCESS_COARSE_LOCATION
android.permission.CHANGE_WIFI_STATE
android.permission.ACCESS_FINE_LOCATION
android.permission.ACCESS_WIFI_STATE

主要的activity类是：

public class Home extends Activity {`

    Context context;
    WifiManager wifiManager = null;
    WiFiReceiver wifiReceiver = null;

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

        context = this;
        wifiManager = (WifiManager) 
        context.getSystemService(Context.WIFI_SERVICE);
        wifiReceiver = new WiFiReceiver(wifiManager);

        IntentFilter intentFilter = new IntentFilter();
        intentFilter.addAction(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION);
        registerReceiver(wifiReceiver, intentFilter);

        wifiManager.startScan();
        List<ScanResult> results = wifiReceiver.results;
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        unregisterReceiver(wifiReceiver);
    }
}

广播接收器是：

public class WiFiReceiver extends BroadcastReceiver {`

    public List<ScanResult> results;
    private WifiManager wifiManager;

    public WiFiReceiver(WifiManager wifiManager) {
        this.wifiManager = wifiManager;
    }

    @Override
    public void onReceive(Context context, Intent intent) {
        boolean success = intent.getBooleanExtra(WifiManager.EXTRA_RESULTS_UPDATED, false);
        if (success) {
            results = wifiManager.getScanResults();
            Log.e("wiFi Manager", "Done");

        } else {
            Log.e("wiFi Manager", "Scan failure");
        }
    }
}

Answer 1

问题是，您假设startScan()将立即产生结果，但它实际上只是执行了所说的操作，即开始扫描。在WiFiReceiver中的onReceive被触发之前，您正在访问then results变量，这就是它始终为空的原因(您的日志记录应该会确认这一点)。

您需要做的就是像here代码一样，在结果准备就绪时使用回调函数来获取结果。注意onReceive方法是如何调用scanSuccess()的，并且结果只能在scanSuccess()中访问，而不是在调用startScan()之后立即访问。还要注意它们是如何通过检查布尔startScan()返回来检查启动扫描是否实际成功的

Answer 2

从API level 23 (Android 6.0 Marshmallow)，我们需要从用户端请求运行时权限。特别是ACCESS_FINE_LOCATION。您需要在wifiManager.startScan()之前检查是否授予权限，如果没有授予权限，则需要调用requestPermissions()。

下面是一个示例：

public void startScanningWifi(){if ( Build.VERSION.SDK_INT >= 23){

    if (ActivityCompat.checkSelfPermission(context, Manifest.
            permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED
            && ActivityCompat.checkSelfPermission(context,
            Manifest.permission.ACCESS_COARSE_LOCATION) !=
            PackageManager.PERMISSION_GRANTED ){
        
        requestPermissions(new String[]{
                        Manifest.permission.ACCESS_FINE_LOCATION},
                REQUEST_CODE_ASK_PERMISSIONS);
        Log.i(TAG, "User location NOT ENABLED, waiting for permission");

    }else{
        //Start scanning for wifi
    }}

您还需要在活动中包含此方法

    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           String[] permissions, int[] grantResults) {
        switch (requestCode) {
            case REQUEST_CODE_ASK_PERMISSIONS:
                if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                //start scanning
                    wifiManager.startScan();
                } else {
                    // Permission for location Denied
                    Toast.makeText( this,"Well cant help you then!" ,
                            Toast.LENGTH_SHORT)
                            .show();
                }
                break;
            default:
                super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }

别忘了在你的活动中声明这一点

private final int REQUEST_CODE_ASK_PERMISSIONS = 1;