android - How can I draw a static target circle on Google Maps?

Question

I've been looking into drawing a circle for a static radius on top of Google Maps and all the answers I came across describe drawing markers and circles which are tied to a lat long coordinate.

What I require is this:

This circle and the marker floats above Google Maps fragment, i.e.: when you pan and zoom, it stays static. And here's the tricky part: I want to be able to get the covered area in the map for processing (eg: the lat, long of the center marker and the radius of the circle depending on the level of zoom on the Map).

How can I achieve this? Thanks in advance.

Answer 1

You can create a custom View to draw the circle. I have based my example on Draw transparent circle filled outside

Here you can find a tutorial about how to create custom views.

In my example I am creating a custom RadarOverlayView with a radius parameter that I use to compute the area.

My custom view code:

public class RadarOverlayView extends LinearLayout {
    private Bitmap windowFrame;
    private float radius = 0f;
    private int centerX = 0;
    private int centerY = 0;

    public RadarOverlayView(Context context) {
        super(context);
    }

    public RadarOverlayView(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray a = context.getTheme().obtainStyledAttributes(
                attrs, R.styleable.RadarOverlayView, 0, 0);

        try {
            radius = a.getDimension(R.styleable.RadarOverlayView_radius, 0f);
        } finally {
            a.recycle();
        }
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);

        if (windowFrame == null) {
            createWindowFrame();
        }
        canvas.drawBitmap(windowFrame, 0, 0, null);
    }

    @Override
    public boolean isEnabled() {
        return false;
    }

    @Override
    public boolean isClickable() {
        return false;
    }

    protected void createWindowFrame() {
        windowFrame = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
        Canvas osCanvas = new Canvas(windowFrame);

        centerX = getWidth() / 2;
        centerY = getHeight() / 2;

        if (radius > 0) {
            Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

            // Draw the circunference
            paint.setStyle(Paint.Style.STROKE);
            paint.setColor(Color.RED);
            paint.setAlpha(200);
            paint.setStrokeWidth(5);
            osCanvas.drawCircle(centerX, centerY, radius, paint);

            // Draw the circle
            paint.setStyle(Paint.Style.FILL);
            paint.setColor(Color.RED);
            paint.setAlpha(100);
            osCanvas.drawCircle(centerX, centerY, radius, paint);

            // Draw the center icon
            paint.setAlpha(255);
            Bitmap centerBitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);
            osCanvas.drawBitmap(centerBitmap, centerX - centerBitmap.getWidth() / 2,
                    centerY - centerBitmap.getHeight() / 2,
                    paint);
        }
    }

    @Override
    public boolean isInEditMode() {
        return true;
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        windowFrame = null;
    }

    public float getRadius() {
        return radius;
    }

    public int getCenterX() {
        return centerX;
    }

    public int getCenterY() {
        return centerY;
    }
}

My attrs.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="RadarOverlayView">
        <attr name="radius" format="dimension" />
    </declare-styleable>
</resources>

My activity_maps.xml layout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:orientation="vertical">
    <fragment
        android:id="@+id/map"
        android:name="myPackage.MySupportMapFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MapsActivity"/>
    <myPackage.RadarOverlayView
        android:id="@+id/radar"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_centerInParent="true"
        app:radius="150dp" />
</RelativeLayout>

My Activity:

public class MapsActivity extends FragmentActivity implements GoogleMap.OnCameraChangeListener {
    private GoogleMap mMap;
    private RadarOverlayView radarView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_maps);
        radarView = (RadarOverlayView) findViewById(R.id.radar);
        setUpMapIfNeeded();
    }

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

    private void setUpMapIfNeeded() {
        if (mMap == null) {
            mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map))
                    .getMap();
            if (mMap != null) {
                setUpMap();
            }
        }
    }

    private void setUpMap() {
        mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
        mMap.getUiSettings().setAllGesturesEnabled(true);
        mMap.getUiSettings().setZoomControlsEnabled(true);

        mMap.setOnCameraChangeListener(this);
    }

    @Override
    public void onCameraChange(final CameraPosition cameraPosition) {
        // Compute the area of the circle each time the camera changes

        LatLng center = mMap.getProjection().fromScreenLocation(
                new Point(radarView.getCenterX(), radarView.getCenterY()));
        LatLng right = mMap.getProjection().fromScreenLocation(
                new Point(radarView.getCenterX() + Math.round(radarView.getRadius()),
                        radarView.getCenterY()));

        Location locationCenter = new Location("center");
        locationCenter.setLatitude(center.latitude);
        locationCenter.setLongitude(center.longitude);

        Location locationRight = new Location("right");
        locationRight.setLatitude(right.latitude);
        locationRight.setLongitude(right.longitude);

        double geoRadius = locationCenter.distanceTo(locationRight);
        double geoArea = Math.PI * Math.pow(geoRadius, 2);

        // Uncomment to inspect the difference between
        // RadarOverlayView circle and geographic circle:
        // mMap.clear();
        // Circle circle = mMap.addCircle(new CircleOptions()
        //        .center(cameraPosition.target)
        //        .radius(geoRadius)
        //        .strokeColor(Color.GREEN)
        //        .fillColor(Color.BLUE));

        Toast.makeText(this, "Area: " + geoArea, Toast.LENGTH_SHORT).show();
    }
}

The result looks like this and shows a Toast with the area covered by the circle each time the camera changes:

Limitations:

The example is drawing a perfect circle in a View, but this circle is not guaranteed to be geographically accurate depending on the zoom level.

You can see that, at high zoom levels, there is a huge difference between the circle drawn by the custom view and a geographically accurate circle (based on the camera target and radius) if you uncomment the mMap.addCircle code on the onCameraChange method:

This difference that is caused by the projection of the map (WGS84), is huge at high zoom levels, and decreases at lower zoom levels: