在Android Linear Layout上使画布透明

Question

我尝试进行浮动活动，然后单击打开透明画布进行绘制。这个内容正在绘制，但我想让它透明。

我试过的是：

final DrawingView drawingView = (DrawingView) findViewById(R.id.drawing);

我的DrawingView类：

import java.util.ArrayList; 
import android.content.Context;
import android.graphics.*;
import android.graphics.Point;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.hardware.Camera.Size;
import android.util.AttributeSet;
import android.util.Log;
import android.util.Pair;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;

public class DrawingView extends View implements OnTouchListener {
    private Canvas m_Canvas;

    private Path m_Path;

    private Paint m_Paint;

    ArrayList<Pair<Path, Paint>> paths = new ArrayList<Pair<Path, Paint>>();

    ArrayList<Pair<Path, Paint>> undonePaths = new ArrayList<Pair<Path, Paint>>();

    private float mX, mY;

    private static final float TOUCH_TOLERANCE = 4;

    public static boolean isEraserActive = false; 

    public DrawingView(Context context, AttributeSet attr) {
        super(context);
        setFocusable(true);
        setFocusableInTouchMode(true);
         this.setBackgroundColor(Color.TRANSPARENT);                

        this.setOnTouchListener(this);

        onCanvasInitialization();
    }

    public void onCanvasInitialization() {
        m_Paint = new Paint();
        m_Paint.setAntiAlias(true);
        m_Paint.setDither(true);
        m_Paint.setColor(Color.parseColor("#000000")); 
        m_Paint.setStyle(Paint.Style.STROKE);
        m_Paint.setStrokeJoin(Paint.Join.ROUND);
        m_Paint.setStrokeCap(Paint.Cap.ROUND);
        m_Paint.setStrokeWidth(30);

        m_Canvas = new Canvas();
        m_Canvas.drawColor(Color.argb(0, 255, 255, 255));

        m_Path = new Path();
        Paint newPaint = new Paint(m_Paint);
        paths.add(new Pair<Path, Paint>(m_Path, newPaint));

    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
    }

    public boolean onTouch(View arg0, MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            touch_start(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_MOVE:
            touch_move(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            touch_up();
            invalidate();
            break;
        }
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        for (Pair<Path, Paint> p : paths) {
            canvas.drawPath(p.first, p.second);
        }
    }

    private void touch_start(float x, float y) {

        if (isEraserActive) {
            m_Paint.setColor(Color.WHITE);
            m_Paint.setStrokeWidth(6);
            Paint newPaint = new Paint(m_Paint); // Clones the mPaint object
            paths.add(new Pair<Path, Paint>(m_Path, newPaint));

        } else { 
            m_Paint.setColor(Color.BLACK);
            m_Paint.setStrokeWidth(30);
            Paint newPaint = new Paint(m_Paint); // Clones the mPaint object
            paths.add(new Pair<Path, Paint>(m_Path, newPaint));

        }


        m_Path.reset();
        m_Path.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            m_Path.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        m_Path.lineTo(mX, mY);

        // commit the path to our offscreen
        m_Canvas.drawPath(m_Path, m_Paint);

        // kill this so we don't double draw
        m_Path = new Path();
        Paint newPaint = new Paint(m_Paint); // Clones the mPaint object
        paths.add(new Pair<Path, Paint>(m_Path, newPaint));
    }

我的活动主XML：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawing_question"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginLeft="0dp"
    android:layout_marginRight="0dp"
    android:orientation="horizontal"
    android:visibility="visible" >

                <com.swap.handdrawing.DrawingView
                    android:id="@+id/drawing"
                    android:layout_width="wrap_content"
                    android:layout_height="fill_parent"
                    android:layout_marginBottom="0dp"
                    android:layout_marginRight="0dp"
                    android:background="@null" >

                </com.swap.handdrawing.DrawingView>

</LinearLayout>

＆安培;我怎么能这样做：