无法将位图绘制为叠加层

Question

我正在尝试在我的活动上绘制位图，显示按钮作为叠加层就像魅力，但我不能处理位图。我应该在代码中更改什么？ 我试图通过onCreate（）中的mView.invalidate（）手动调用onDraw，但它不起作用。 提前谢谢！

package com.example.btserver

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.PixelFormat;
import android.graphics.Rect;
import android.os.IBinder;
import android.view.Gravity;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.view.ViewGroup;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.Toast;

public class HUD extends Service implements OnTouchListener {
    Button mButton;
    OverlayView mView;
    @Override
    public IBinder
    onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mView = new OverlayView(this);
        mButton = new Button(this);
        mButton.setText("Overlay button");
        mButton.setOnTouchListener(new OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {   
                setText();
                return true;
            }
        });

        WindowManager.LayoutParams params = new WindowManager.LayoutParams(

                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY,//TYPE_SYSTEM_ALERT,//TYPE_SYSTEM_OVERLAY,
                WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE |
                        WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN, //will cover status bar as well!!!
                PixelFormat.TRANSLUCENT);


        params.gravity = Gravity.LEFT | Gravity.TOP;
        params.y = 100;
        params.x = 100;
        params.setTitle("Load Average");
        WindowManager wm = (WindowManager) getSystemService(WINDOW_SERVICE);
        wm.addView(mButton, params);
        wm.addView(mView, params);

    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if(mButton != null)
        {
            ((WindowManager) getSystemService(WINDOW_SERVICE)).removeView(mButton);
            mButton = null;
        }
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        Toast.makeText(this,"Overlay button event", Toast.LENGTH_SHORT).show();
        return false;
    }


    public void setText() {
        Toast.makeText(this,"Overlay button event", Toast.LENGTH_SHORT).show();
    }

}

class OverlayView extends ViewGroup {
    private Paint mLoadPaint;

    Bitmap cursor;
    public int x = 0,y = 0;

    public void Update(int nx, int ny) {
        x = nx; y = ny;
    }

    public OverlayView(Context context) {
        super(context);
        cursor = BitmapFactory.decodeResource(context.getResources(), R.drawable.cursor);

        mLoadPaint = new Paint();
        mLoadPaint.setAntiAlias(true);
        mLoadPaint.setTextSize(10);
        mLoadPaint.setARGB(255, 255, 0, 0);

    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawBitmap(cursor, 0,0, mLoadPaint);
    }

    @Override
    protected void onLayout(boolean arg0, int arg1, int arg2, int arg3, int arg4) {
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        return true;
    }
}