GoogleMap尝试在外面的地图上绘制时给我null

Question

如何正确保护GoogleMap不为空？

它目前在DrawonMap()方法中表示mMap null，但在将其添加到onmapready()方法中时可以工作。

public class DetailActivity extends AppCompatActivity implements OnMapReadyCallback {

private GoogleMap mMap;
private String locationA;
private String locationB;
private ArrayList<LatLng> latLngList;

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

    // Check if Google Maps have been activated, else starting it again.
    if (mMap == null) {
        SupportMapFragment smf = (SupportMapFragment) this.getSupportFragmentManager().findFragmentById(R.id.map);
        smf.getMapAsync(this);
    }

    // Settings up the views
    TextView mDistance = (TextView) findViewById(R.id.distanceDetail);
    TextView mDuration = (TextView) findViewById(R.id.durationDetail);
    TextView mLocationA = (TextView) findViewById(R.id.locationA);
    TextView mLocationB = (TextView) findViewById(R.id.locationB);


    // Getting the values
    Intent i = getIntent();
    locationA = i.getStringExtra("locA");
    locationB = i.getStringExtra("locB");
    String distance = i.getStringExtra("distance");
    String duration = i.getStringExtra("duration");


    // initiate the values.
    mDistance.setText(distance);
    mDuration.setText(duration);
    mLocationA.setText(locationA);
    mLocationB.setText(locationB);


    drawOnMap();
}





public void drawOnMap(){

    latLngList = getIntent().getParcelableArrayListExtra("points");

    // Används för att rita ut marksen på kartan sedan, start och stop.
    // Get the first and last postion to write out the marks on map
    LatLng startLatLng = latLngList.get(0);
    LatLng endLatLng = latLngList.get(latLngList.size() - 1);

    // Zooms into the start position;
    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(startLatLng, 10));

    // Adding mark to start location
    mMap.addMarker(new MarkerOptions()
            .position(startLatLng)
            .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_GREEN))
            .title(locationA));

    // Adding mark to end location
    mMap.addMarker(new MarkerOptions()
            .position(endLatLng)
            .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_RED))
            .title(locationB));


    if (latLngList.size() > 0) {
        // Draw the route on map
        mMap.addPolyline(new PolylineOptions()
                .addAll(latLngList)
                .color(Color.argb(255, 55, 160, 255))
                .width(20));
    }
}



@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    UiSettings mapSettings = mMap.getUiSettings();
    mapSettings.setZoomControlsEnabled(true);
}

Answer 1

在map就绪后调用drawOnMap方法。

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    UiSettings mapSettings = mMap.getUiSettings();
    mapSettings.setZoomControlsEnabled(true);

    drawOnMap();
}