使用distanceTo无法显示手机移动距离，请求排查问题

Let's Fix Your Distance Tracking Issue #

Hey there! No need to apologize for your English—we're all here to troubleshoot together. Let's break down the problems in your code and address the permission concerns first:

1. Permission Handling Problem #

Your current code only checks if permissions are granted, but it doesn't request permissions if they're missing. On Android 6.0 (API 23+) and above, you need to ask the user explicitly for location permissions. Skipping this step means your app can't access location data at all, which would definitely break your distance tracking.

2. Critical Logic Bugs in Your Code #

There are a few issues that would prevent the distance from calculating correctly (or even crash your app):

• You're calling onLocationChanged(null) directly in onCreate—this sets mCurrentLocation to null, leading to null pointer exceptions when you try to call distanceTo().
• You don't check if location is null in onLocationChanged—GPS can take time to get a fix, so early callbacks might pass null.
• Your distance calculation logic doesn't handle cases where location data isn't yet available.

Fixed Code Example #

Location mCurrentLocation, lStart, lEnd;
float dist = 0;
private static final int LOCATION_PERMISSION_REQUEST_CODE = 100;

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

    // Check and request permissions first
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED
            && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this,
                new String[]{Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION},
                LOCATION_PERMISSION_REQUEST_CODE);
        return;
    }

    // Start location updates only if permissions are granted
    startLocationTracking();
}

private void startLocationTracking() {
    LocationManager lm = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED
            || ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) == PackageManager.PERMISSION_GRANTED) {
        lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);
    }
}

@Override
public void onLocationChanged(Location location) {
    // Skip if location data is invalid
    if (location == null) {
        return;
    }

    mCurrentLocation = location;
    if (lStart == null) {
        // Initialize with first valid location
        lStart = location;
        lEnd = location;
    } else {
        lEnd = location;
    }
    updateDistanceUI();
}

private void updateDistanceUI() {
    TextView txt2 = findViewById(R.id.textView2);
    // Calculate distance only if both locations are valid
    if (lStart != null && lEnd != null) {
        dist += lStart.distanceTo(lEnd);
        txt2.setText(String.format("%.2f m", dist)); // Format for cleaner display
        lStart = lEnd; // Update start point for next calculation
    } else {
        txt2.setText("Waiting for location...");
    }
}

// Handle permission request result
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if (requestCode == LOCATION_PERMISSION_REQUEST_CODE) {
        boolean hasFinePermission = grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED;
        boolean hasCoarsePermission = grantResults.length > 1 && grantResults[1] == PackageManager.PERMISSION_GRANTED;
        
        if (hasFinePermission || hasCoarsePermission) {
            startLocationTracking();
        } else {
            Toast.makeText(this, "Location permissions are required to track distance", Toast.LENGTH_LONG).show();
        }
    }
}

Key Changes Explained #

• Proper Permission Flow: Now we request permissions if they're missing, and start location updates only after the user grants them.
• Null Safety Checks: We avoid crashes by validating location before using it.
• Removed Invalid onLocationChanged(null) Call: This was causing initial null values that broke the calculation.
• Better UI Feedback: We show a message while waiting for the first GPS fix.
• Formatted Distance Output: Using String.format makes the distance value easier to read.

Test this code outdoors (GPS needs clear sky access) and make sure to grant location permissions when prompted—it should start tracking your distance correctly!

内容的提问来源于stack exchange，提问作者Raúl