Android studio实现画板功能

public class MainActivity extends AppCompatActivity{

 Paint paint;
 Canvas canvas;
 ImageView imageview;
 Bitmap bitmap,newbitmap;
 TextView tv_stroke;
 int startX, startY, endX, endY;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_my_paint_tools);
  LinearLayout ll_layout = findViewById(R.id.ll_layout);
  RadioGroup rg_color = findViewById(R.id.rg_color);

for (int i = 0;i<rg_color.getChildCount();i++){
   RadioButton rb = (RadioButton) rg_color.getChildAt(i);
   rb.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() { 
    @Override
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
     if (buttonView.isChecked()){
      paint.setColor(buttonView.getTextColors().getDefaultColor()); 
      paint.setStrokeWidth(5);
     }
    }
   });
  }

 imageview.setOnTouchListener(new View.OnTouchListener() {
   @Override
   public boolean onTouch(View v, MotionEvent event) {
    switch(event.getAction()){
     case MotionEvent.ACTION_DOWN:
      Log.i("MyPaintToolsActivity","ACTION_DOWN");

      
      startX = (int) (event.getX()/1.4);
      startY = (int) (event.getY()/1.4);
      break;
     case MotionEvent.ACTION_MOVE:
      Log.i("MyPaintToolsActivity","ACTION_MOVE");

      
      endX = (int) (event.getX()/1.4);
      endY = (int) (event.getY()/1.4);

      canvas.drawLine(startX,startY,endX,endY,paint);
      startX = (int) (event.getX()/1.4);
      startY = (int) (event.getY()/1.4);
      imageview.setImageBitmap(bitmap);
      break;
     case MotionEvent.ACTION_UP:
      Log.i("MyPaintToolsActivity","ACTION_UP");
      break;
    }
    imageview.invalidate();
    return true;
   }
  });

 Button btn_clear = findViewById(R.id.btn_clear);
  btn_clear.setOnClickListener(new View.OnClickListener() {
   @Override
   public void onClick(View v) {
    canvas.drawColor(0,PorterDuff.Mode.CLEAR);
    bitmap = Bitmap.createBitmap(888,1200,Bitmap.Config.ARGB_8888); 
    canvas = new Canvas(bitmap); 
    canvas.drawColor(Color.argb(100,0,0,0)); 
    paint = new Paint();
    paint.setStrokeWidth(5);
    paint.setAntiAlias(true);
    paint.setColor(Color.RED);
    canvas.drawBitmap(bitmap,new Matrix(),paint); 
    imageview.setImageBitmap(bitmap);   

   }
  });

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 android:id="@+id/ll_layout">
<!-- tools:context=".MyPaintToolsActivity">-->

 <ImageView

  android:id="@+id/imageview"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:layout_weight="1" />
 <RadioGroup
  android:background="#747373"
  android:layout_width="match_parent"
  android:orientation="horizontal"
  android:id="@+id/rg_color"
  android:layout_height="wrap_content">

  <RadioButton
   android:id="@+id/rb_red"
   android:layout_width="wrap_content"
   android:layout_height="43dp"
   android:layout_weight="1"
   android:text="红色"
   android:textColor="#FF0000"
   android:textSize="18sp" />

  <RadioButton
   android:id="@+id/rb_green"
   android:layout_width="wrap_content"
   android:layout_height="30dp"
   android:layout_weight="1"
   android:text="黑色"
   android:textColor="#000000"
   android:textSize="18sp" />

  <RadioButton
   android:id="@+id/rb_blue"
   android:layout_width="wrap_content"
   android:layout_height="30dp"
   android:layout_weight="1"
   android:text="白色"
   android:textColor="#FFFFFF"
   android:textSize="18sp" />

 </RadioGroup>
 <LinearLayout
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:gravity="center"
  android:orientation="horizontal">
  <Button
   android:id="@+id/btn_clear"
   android:layout_width="wrap_content"
   android:layout_weight="1"
   android:layout_height="wrap_content"
   android:background="#000000"
   android:textColor="#FFFFFF"
   android:textSize="18sp"
   android:text="清除"/>
  <Button
   android:id="@+id/btn_eraser"
   android:layout_width="wrap_content"
   android:layout_weight="1"
   android:layout_height="wrap_content"
   android:textColor="#FFFFFF"
   android:textSize="18sp"
   android:background="#000000"
   android:text="擦除"/>

 </LinearLayout>

</LinearLayout>