我试图从侦听器类中调用一个方法，但是即使我为该类声明一个对象，它也会抛出nullpointexception。我真的不明白为什么。
我尝试向该方法发送参数，但这都不起作用。
我收到的错误是由于方法checkGpsStat（）中的调用引起的：
LocationServices.getSettingsClient(MainActivity.this).checkLocationSettings(builder.build())
错误：
试图调用虚拟方法'android.content.Context 空对象上的android.content.Context.getApplicationContext（）' 参考
CLASS：-LocationReceiver
public class LocationReceiver extends BroadcastReceiver {
private static final String TAG = "LocationReceiver";
private Context mContext;
private Snackbar snackbar;
private MainActivity mActicity = new MainActivity();
public LocationReceiver(Context context, Snackbar snackbar){
this.mContext = context;
this.snackbar = snackbar;
}
@Override
public void onReceive(Context context, Intent intent) {
LocationManager locationManager = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
if (intent.getAction().matches("android.location.PROVIDERS_CHANGED")) {
boolean gpsEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);
boolean networkEnabled = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
if(gpsEnabled && networkEnabled) {
if (snackbar != null) {
snackbar.dismiss();
}
Log.d(TAG, "GPS enabled");
} else {
View view = snackbar.getView();
FrameLayout.LayoutParams params = (FrameLayout.LayoutParams)view.getLayoutParams();
params.gravity = Gravity.TOP;
view.setLayoutParams(params);
view.setBackgroundColor(Color.parseColor("#123123"));
snackbar.show();
mActicity.checkGpsState(); // This call causes crash!
Log.d(TAG, "GPS disabled");
}
}
}
方法：-checkGpsState（）
public void checkGpsState(){
Log.d(TAG,"checkGpsState started...");
LocationRequest locationRequest = LocationRequest.create();
locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder()
.addLocationRequest(locationRequest);
// Below code is where the error is firing from! "mainactivity.this".
Task<LocationSettingsResponse> result =
LocationServices.getSettingsClient(MainActivity.this).checkLocationSettings(builder.build());
result.addOnCompleteListener(new OnCompleteListener<LocationSettingsResponse>() {
@Override
public void onComplete(@NonNull Task<LocationSettingsResponse> task) {
try {
LocationSettingsResponse response = task.getResult(ApiException.class);
// All location settings are satisfied. The client can initialize location
// requests here.
createLocationRequest(); // Start updating location!
} catch (ApiException exception) {
switch (exception.getStatusCode()) {
case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
// Location settings are not satisfied. But could be fixed by showing the
// user a dialog.
try {
// Cast to a resolvable exception.
ResolvableApiException resolvable = (ResolvableApiException) exception;
// Show the dialog by calling startResolutionForResult(),
// and check the result in onActivityResult().
resolvable.startResolutionForResult(
MainActivity.this,
LocationRequest.PRIORITY_HIGH_ACCURACY);
} catch (IntentSender.SendIntentException e) {
// Ignore the error.
} catch (ClassCastException e) {
// Ignore, should be an impossible error.
}
break;
case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
// Location settings are not satisfied. However, we have no way to fix the
// settings so we won't show the dialog.
break;
}
}
}
});
}
答案 0 :(得分：1)
哇，终于！我能够解决这个问题。对此感到非常高兴！
我不确定这是否是最佳解决方案，或者它是否安全。但这对我有用，我希望对其他遇到此问题的人也有用。
因此，解决方案来了！ 代码左侧的数字是我实现此步骤所要执行的步骤。希望对您有帮助！
CLASS：-LocationReceiver
public class LocationReceiver extends BroadcastReceiver {
private static final String TAG = "LocationReceiver";
private Context mContext;
private Snackbar snackbar;
1 private MainActivity mainActivity; // Declare activity object
//NOTE: In the constructor, we pass the activity that fires for this listener, in this case MainActivity.
2 public LocationReceiver(Context context, MainActivity mActivity, Snackbar snackbar){ // Add activity to constructor parameter
this.mContext = context;
this.snackbar = snackbar;
3 mainActivity = mActivity; // set activity with the passed activity.
}
@Override
public void onReceive(Context context, Intent intent) {
LocationManager locationManager = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
if (intent.getAction().matches("android.location.PROVIDERS_CHANGED")) {
boolean gpsEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);
boolean networkEnabled = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
if(gpsEnabled && networkEnabled) {
if (snackbar != null) {
snackbar.dismiss();
}
Log.d(TAG, "GPS is enabled");
} else {
View view = snackbar.getView();
FrameLayout.LayoutParams params = (FrameLayout.LayoutParams)view.getLayoutParams();
params.gravity = Gravity.TOP;
view.setLayoutParams(params);
view.setBackgroundColor(Color.parseColor("#123123"));
snackbar.show();
// If user turned off location, pass both context, AND activity to the method that handles the gps-dialog.
//MARK: - pass both context and activity that we receieved from constructor!
4 mainActivity.checkGpsState(mContext, mainActivity); // Call method in another activity, pass both context and activity (both is for certain calls in the method in the other activity)
Log.d(TAG, "GPS is disabled");
}
}
}
}
方法：-checkGpsState（）
//checkGps
5 public void checkGpsState(Context mContext, Activity mActivity){ // Here we need to receive the parameter.
Log.d(TAG,"checkGpsState started...");
LocationRequest locationRequest = LocationRequest.create();
locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder()
.addLocationRequest(locationRequest);
//MARK: - pass the context from parameter!
6 Task<LocationSettingsResponse> result =
LocationServices.getSettingsClient(mContext).checkLocationSettings(builder.build()); // Here comes the usage of the context we passed as parameter!
result.addOnCompleteListener(new OnCompleteListener<LocationSettingsResponse>() {
@Override
public void onComplete(@NonNull Task<LocationSettingsResponse> task) {
try {
LocationSettingsResponse response = task.getResult(ApiException.class);
// All location settings are satisfied. The client can initialize location
// requests here.
createLocationRequest(); // Start updating location!
} catch (ApiException exception) {
switch (exception.getStatusCode()) {
case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
// Location settings are not satisfied. But could be fixed by showing the
// user a dialog.
try {
// Cast to a resolvable exception.
ResolvableApiException resolvable = (ResolvableApiException) exception;
// Show the dialog by calling startResolutionForResult(),
// and check the result in onActivityResult().
// MARK: - Here we pass the activity from parameter!
7 resolvable.startResolutionForResult(
mActivity,
LocationRequest.PRIORITY_HIGH_ACCURACY); // And here comes the usage for the activity we passed as parameter
} catch (IntentSender.SendIntentException e) {
// Ignore the error.
} catch (ClassCastException e) {
// Ignore, should be an impossible error.
}
break;
case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
// Location settings are not satisfied. However, we have no way to fix the
// settings so we won't show the dialog.
break;
}
}
}
});
}