ViewTest.java

package com.toby.personal.datatypeconverttest;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

/**
 * Created by TT on 2017/1/15.
 */

public class ViewTest extends View {

    public float currentX = 200;
    public float currentY = 200;

    Paint paint = new Paint();

    public ViewTest(Context context) {
        super(context);
    }

    public ViewTest(Context context, AttributeSet set) {
        super(context, set);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        paint.setColor(Color.RED);
        paint.setAntiAlias(true);
        canvas.drawCircle(currentX, currentY, 180, paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        currentX = event.getX();
        currentY = event.getY();

        invalidate();

        return true;
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.toby.personal.datatypeconverttest.MainActivity">

    <com.toby.personal.datatypeconverttest.ViewTest
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

MainActivity.java

package com.toby.personal.datatypeconverttest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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