Android开发：2d图形透明？

Question

触摸时是否可以绘制图像并具有透明的BG，所以我看到我的家园用这个：

android:theme="@android:style/Theme.Translucent.NoTitleBar"

到目前为止，这是我的项目：

    import java.util.ArrayList;



import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Bitmap.Config;
import android.os.Bundle;
import android.view.Display;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.Window;
import android.view.WindowManager;
import android.os.Bundle;


public class main extends Activity {
  private Panel mG;


 @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
       // setContentView(new Panel(this));
        setContentView(R.layout.main);

        mG = new Panel(this);
        setContentView(mG);
    }

    class Panel extends SurfaceView implements SurfaceHolder.Callback {
        private TutorialThread _thread;
        private ArrayList<GraphicObject> _graphics = new ArrayList<GraphicObject>();

        public Panel(Context context) {
            super(context);
            getHolder().addCallback(this);
            _thread = new TutorialThread(getHolder(), this);
            setFocusable(true);
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            synchronized (_thread.getSurfaceHolder()) {
                if (event.getAction() == MotionEvent.ACTION_DOWN) {
                    GraphicObject graphic = new GraphicObject(BitmapFactory.decodeResource(getResources(), R.drawable.myicon));
                    graphic.getCoordinates().setX((int) event.getX() - graphic.getGraphic().getWidth() / 2);
                    graphic.getCoordinates().setY((int) event.getY() - graphic.getGraphic().getHeight() / 2);
                    _graphics.add(graphic);
                }
                return true;
            }
        }

       @Override
        public void onDraw(Canvas canvas) {
         Display display = ((WindowManager)getSystemService(WINDOW_SERVICE)).getDefaultDisplay();
         int height = display.getHeight(); // not exactly correct
            int width = display.getWidth(); // not exactly correct

         //Bitmap bitmap = Bitmap.createBitmap(width, height, Config.ALPHA_8);//Config.RGB_565);



            //canvas.drawColor(Color.Transparent);
            Bitmap bitmap;
          GraphicObject.Coordinates coords;
           for (GraphicObject graphic : _graphics) {
              bitmap = graphic.getGraphic();
                coords = graphic.getCoordinates();
               canvas.drawBitmap(bitmap, coords.getX(), coords.getY(), null);
           }
        }

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
            // TODO Auto-generated method stub
        }

        @Override
        public void surfaceCreated(SurfaceHolder holder) {
            _thread.setRunning(true);
            _thread.start();
        }

        @Override
        public void surfaceDestroyed(SurfaceHolder holder) {
            // simply copied from sample application LunarLander:
            // we have to tell thread to shut down & wait for it to finish, or else
            // it might touch the Surface after we return and explode
            boolean retry = true;
            _thread.setRunning(false);
            while (retry) {
                try {
                    _thread.join();
                    retry = false;
                } catch (InterruptedException e) {
                    // we will try it again and again...
                }
            }
        }
    }

    class TutorialThread extends Thread {
        private SurfaceHolder _surfaceHolder;
        private Panel _panel;
        private boolean _run = false;

        public TutorialThread(SurfaceHolder surfaceHolder, Panel panel) {
            _surfaceHolder = surfaceHolder;
            _panel = panel;
        }

        public void setRunning(boolean run) {
            _run = run;
        }

        public SurfaceHolder getSurfaceHolder() {
            return _surfaceHolder;
        }

        @Override
        public void run() {
            Canvas c;
            while (_run) {
                c = null;
                try {
                    c = _surfaceHolder.lockCanvas(null);
                    synchronized (_surfaceHolder) {
                        _panel.onDraw(c);
                    }
                } finally {
                    // do this in a finally so that if an exception is thrown
                    // during the above, we don't leave the Surface in an
                    // inconsistent state
                    if (c != null) {
                        _surfaceHolder.unlockCanvasAndPost(c);
                    }
                }
            }
        }
    }

    class GraphicObject {
        /**
         * Contains the coordinates of the graphic.
         */
        public class Coordinates {
            private int _x = 100;
            private int _y = 0;

            public int getX() {
                return _x + _bitmap.getWidth() / 2;
            }

            public void setX(int value) {
                _x = value - _bitmap.getWidth() / 2;
            }

            public int getY() {
                return _y + _bitmap.getHeight() / 2;
            }

            public void setY(int value) {
                _y = value - _bitmap.getHeight() / 2;
            }

            public String toString() {
                return "Coordinates: (" + _x + "/" + _y + ")";
            }
        }

        private Bitmap _bitmap;
        private Coordinates _coordinates;

        public GraphicObject(Bitmap bitmap) {
            _bitmap = bitmap;
            _coordinates = new Coordinates();
        }

        public Bitmap getGraphic() {
            return _bitmap;
        }

        public Coordinates getCoordinates() {
            return _coordinates;
        }
    }
}

然而背景是黑色的，我没有看到我的主屏幕（或我的背景活动）

我做错了什么？

//你的朋友！

Answer 1

很高兴看到有人在阅读我的教程：）

我不确定是否可以通过你的背景看到主屏幕，但也许你看看这个：How do I create a transparent Activity on Android?