public class CustomView extends View {
 //创建point对象 参数为x坐标和y坐标
 private PointF point = new PointF(100, 100);
 public CustomView(Context context) {
  super(context);
 }
 public CustomView(Context context, @Nullable AttributeSet attrs) {
  super(context, attrs);
 }
 public CustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
  super(context, attrs, defStyleAttr);
 }
 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  //参数为圆的横坐标 ,纵坐标,半径,创建
  canvas.drawCircle(point.x,point.y, 50, new Paint());
 }
}

<ydtx.bwie.com.xiangmu_project02.CustomView
  android:layout_width="match_parent"
  android:layout_height="match_parent" />

@Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
 }

public class CustomView extends View {
 //创建point对象 参数为x坐标和y坐标
 private PointF point = new PointF(100, 100);
 public CustomView(Context context) {
  super(context);
 }
 public CustomView(Context context, @Nullable AttributeSet attrs) {
  super(context, attrs);
 }
 public CustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
  super(context, attrs, defStyleAttr);
 }
 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  //参数为圆的横坐标 ,纵坐标,半径,创建 
  canvas.drawCircle(point.x,point.y, 50, new Paint());
 }
 //触摸事件
 @Override
 public boolean onTouchEvent(MotionEvent event) {
  //获得触摸事件
  switch (event.getAction()) {
   case MotionEvent.ACTION_DOWN:
    break;
   //ACTION_MOVE不要设置break，否则圆形不会跟随手指活动 只会手指松开屏幕的时候圆形直接到了屏幕停止的位置
   case MotionEvent.ACTION_MOVE:
   case MotionEvent.ACTION_UP:
    //获取手指触摸位置的x坐标
    point.x = event.getX();
    //获取手指触摸位置的y坐标
    point.y = event.getY();
    //启动
    postInvalidate();
    break;
  }
  return true;
 }
}