在屏幕上移动的小球

代码

MainActivity.java

package com.example.me.myapplication;

import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;

import android.support.v7.app.AppCompatActivity;


public class MainActivity extends AppCompatActivity{
    TestView tView = null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tView = (TestView)findViewById(R.id.testview1);
        tView.setOnTouchListener(new mOnTouch());
    }
    class mOnTouch implements View.OnTouchListener{
        @Override
        public boolean onTouch(View v, MotionEvent event){
            int x1,y1;
            x1 = (int)event.getX();
            y1 = (int)event.getY();
            if(event.getAction() == MotionEvent.ACTION_DOWN){
                tView.getXY(x1, y1);
                tView.invalidate();
                return true;
            }else if(event.getAction() == MotionEvent.ACTION_MOVE){
                tView.getXY(x1, y1);
                tView.invalidate();
                return true;
            }
            return tView.onTouchEvent(event);
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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"
    tools:context=".MainActivity">

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
        <com.example.me.myapplication.TestView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/testview1"/>
    </RelativeLayout>

</android.support.constraint.ConstraintLayout>

TestView.java

package com.example.me.myapplication;

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

class TestView extends View {
    int x = 150, y = 50;
    public TestView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    void getXY(int _x,int _y){
        x = _x;
        y = _y;
    }
    @Override
    protected void onDraw(Canvas canvas){
        super.onDraw(canvas);
        canvas.drawColor(Color.CYAN);
        Paint paint = new Paint();
        paint.setAntiAlias(true);
        paint.setColor(Color.BLACK);
        canvas.drawCircle(x, y, 30, paint);
        paint.setColor(Color.WHITE);
        canvas.drawCircle(x-9, y-9, 6, paint);
    }
}

执行结果

通过触摸屏事件获取 x 和 y 的坐标，再使用 onDraw 函数重新画圆，invalidate 刷新画面。