Android Pie and WiFiManger.getScanResults() returns empty list or Android Oreo WiFiManger.startScan() returns false

Question

Consider the original example from https://developer.android.com/guide/topics/connectivity/wifi-scan#java:

public class MainActivity extends AppCompatActivity {
    private WifiManager wifiManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        wifiManager = (WifiManager) getApplicationContext().getSystemService(Context.WIFI_SERVICE);

        BroadcastReceiver wifiScanReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context c, Intent intent) {
                boolean success = intent.getBooleanExtra(WifiManager.EXTRA_RESULTS_UPDATED, false);
                if (success) {
                    scanSuccess();
                } else {
                    // scan failure handling
                    scanFailure();
                }
            }
        };

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

        boolean success = wifiManager.startScan();
        if (!success) {
            scanFailure();
        }
    }

    private void scanSuccess() {
        List<ScanResult> results = wifiManager.getScanResults();
    }

    private void scanFailure() {
        // handle failure: new scan did NOT succeed
        // consider using old scan results: these are the OLD results!
        List<ScanResult> results = wifiManager.getScanResults();
    }

I created a simple project in Android studio, here the gradle file:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "it.xxx.wifiscan"
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

and the manifest.xml file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:tools="http://schemas.android.com/tools"
          package="it.resis.wifiscan"
          xmlns:android="http://schemas.android.com/apk/res/android">

    <uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE"/>
    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION "/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="GoogleAppIndexingWarning">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>

                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
    </application>

</manifest>

if I run it on Xiaomi Mi A2 - Android Pie 9 Api 28

wifiManager.startScan()

return flase!

If I run it on Nokia 6 - Android Oreo 8.1.0 Api 27

wifiManager.startScan()

returns true, but

wifiManager.getScanResults();

returns and empty list.

I don't understand, I'm going mad.

Answer 1

Starting with Android 6, accessing scan results for either Wi-Fi or Bluetooth requires you to hold either ACCESS_COARSE_LOCATION or ACCESS_FINE_LOCATION. I see that you have declared ACCESS_COARSE_LOCATION in your manifest, but you will also need to modify your Activity to request that permission at runtime for Android 6.0 and above. For example:

private static final String RC_LOCATION = 1;

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

    String location = android.Manifest.permission.ACCESS_COARSE_LOCATION;
    if (ActivityCompat.checkSelfPermission(this, location) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(new String[] { locationPermission }, RC_LOCATION);
    } else {
        startWifiScan();
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] results) {
    if (requestCode == RC_LOCATION) {
        if (results[0] == PackageManager.PERMISSION_GRANTED) {
            startWifiScan();
        } else {
            // user rejected permission request
        }
    } else {
        super.onRequestPermissionsResult(requestCode, permissions, results);
    }
}

private void startWifiScan() {
    // do wi-fi scanning
}