Android开发中项目实现一个画板功能

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:paddingBottom="@dimen/activity_vertical_margin"
 android:paddingLeft="@dimen/activity_horizontal_margin"
 android:paddingRight="@dimen/activity_horizontal_margin"
 android:paddingTop="@dimen/activity_vertical_margin"
 tools:context="com.tomes.paint.MainActivity" >

 <ImageView 
  android:id="@ id/iv_drawingBoard"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:src="@drawable/bg"/>

</RelativeLayout>

public void init() {
 Bitmap bitmap = BitmapFactory.decodeResource(getResources(),
   R.drawable.bg);
 copyBitmap = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), bitmap.getConfig());
 paint = new Paint();
 canvas = new Canvas(copyBitmap);
 Matrix matrix=new Matrix();
 canvas.drawBitmap(bitmap, matrix, paint);

 imageView = (ImageView) findViewById(R.id.iv_drawingBoard);

 imageView.setImageBitmap(copyBitmap);
 
 imageView.setOnTouchListener(new OnTouchListener() {

  @SuppressLint("ClickableViewAccessibility")
  @Override
  public boolean onTouch(View v, MotionEvent event) {
   int action = event.getAction();
   switch (action) {
   case MotionEvent.ACTION_DOWN:
     startX=event.getX();
     startY=event.getY();
    break;
   case MotionEvent.ACTION_MOVE:
    float currentX=event.getX();
    float currentY=event.getY();
    canvas.drawLine(startX, startY, currentX, currentY, paint);
    imageView.setImageBitmap(copyBitmap);
    startX=currentX;
    startY=currentY;
    
    break;
   case MotionEvent.ACTION_UP:

    break;

   }
   return true;
  }
 });

}