java - 谷歌地图从异步任务 onPostExecute 在 Canvas 上叠加绘图

Question

我有一个谷歌地图 mapView，上面有一个自定义叠加层。该叠加层获取用户正在查看的当前坐标，然后转到一个网站，在该网站上获取要叠加到 map 上的图像。这是一种不好的做法，因为 Web 请求可能需要几秒钟的时间并完全锁定 UI 线程，所以我正在尝试解决该问题。

我正在尝试通过一个 AsyncTask 来修复它，它将抓取图像并在准备就绪时将其绘制在 map 上。我正在尝试将 Canvas 传递给 AsyncTask，以便在准备就绪时可以进行绘制，但不会进行绘制，并且我注意到在绘制时 Canvas 大小为 0x0。

在我尝试将其放入 AsyncTask 之前，所有绘图代码都有效，只是速度很慢。

这一切都在我的自定义叠加层中:

public class MapOverlaySevereWeather extends com.google.android.maps.Overlay
{
    private GeoPoint lastTopLeft = new GeoPoint(0, 0);
    private GeoPoint lastBotRight = new GeoPoint(0, 0);
    private Bitmap mapImage;
    private Canvas thisCanvas;
    private MapView mMapView;

    @Override
    public void draw(Canvas canvas, MapView mapView, boolean shadow)
    {
        super.draw(canvas, mapView, shadow);

        if( shadow || MapOverlayHandler.isInMotion() )
        { return; }

        mMapView = mapView;
        thisCanvas = canvas;
        Rect curShown = canvas.getClipBounds();
        GeoPoint topLeft = mapView.getProjection().fromPixels(0,0);
        GeoPoint bottomRight = mapView.getProjection().fromPixels(curShown.right, curShown.bottom);

        if( !topLeft.equals(lastTopLeft) || !bottomRight.equals(lastBotRight) )
        {
            int sizeX = mapView.getWidth();//curShown.right - curShown.left;
            int sizeY = mapView.getHeight();////curShown.bottom - curShown.top;
            float minLat = (float)bottomRight.getLatitudeE6() / 1E6f;
            float minLon = (float)topLeft.getLongitudeE6() / 1E6f;
            float maxLat = (float)topLeft.getLatitudeE6() / 1E6f;
            float maxLon = (float)bottomRight.getLongitudeE6() / 1E6f;
            String fileUrl = "url that gets image based off lat long size";

            new SevereWeatherAsync().execute(new AsyncOverlayData(fileUrl, canvas, curShown));
        }

        lastTopLeft = topLeft;
        lastBotRight = bottomRight;
        return;
    }

    private class SevereWeatherAsync extends AsyncTask<AsyncOverlayData, Void, AsyncOverlayData>
    {
        @Override
        protected void onPostExecute(AsyncOverlayData result)
        {
            super.onPostExecute(result);
            Log.w("Severe","Drawing on " + thisCanvas.getHeight() + " x " + thisCanvas.getWidth());
            Paint paint = new Paint();
            paint.setAlpha(100);        
            thisCanvas.drawBitmap(mapImage, null, result.getCurRect(), paint);
            mMapView.invalidate();
        }

        @Override
        protected AsyncOverlayData doInBackground(AsyncOverlayData... params)
        {
            Log.w("Severe","getting image");
            URL imageFileURL = null;       
            try
            {
                imageFileURL = new URL(params[0].getURL());
                HttpURLConnection conn = (HttpURLConnection) imageFileURL.openConnection();
                conn.setDoInput(true);
                conn.connect();
                InputStream is = conn.getInputStream();
                mapImage = BitmapFactory.decodeStream(is);
            }
            catch(Exception e)
            { return null; }        

            return params[0];
        }
    }

}

Answer 1

我怀疑发生的事情是您传递给 AsyncTask 的 Canvas 在下载图像时无效。

我认为更好的解决方案是将 result.getImage() 返回的位图保存到一个实例变量中，然后重绘叠加层本身。

所以你会得到类似..

public class MyOverlay extends Overlay {
    private Bitmap mBitmap;
    private MapView mMapView;

    public void draw(Canvas canvas, MapView mapView, boolean shadow) {
        // .. your drawing code
        if(mBitmap == null) {
            // Better download your image!
            new SevereWeatherAsync().execute(new AsyncOverlayData(fileUrl, void, curShown));
        } else {
            //Draw the bitmap!
            canvas.drawBitmap(mBitmap, ...);
        }
    } 

    private class SevereWeatherAsync extends AsyncTask<> {
        @Override
        protected AsyncOverlayData doInBackground(AsyncOverlayData... params) {
            //You background work..
        }

        @Override
        protected void onPostExecute(AsyncOverlayData result) {
            mBitmap = result.getImage();

            //Now we can redraw the overlay!
            mMapView.invalidate();
        }
    }       

}