android:设计一个在屏幕上移动小球的程序。

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<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"

    tools:context="com.example.yanhsama.ex4_4.MainActivity">
    <com.example.yanhsama.ex4_4.TestView
        android:id="@+id/testview1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />
</RelativeLayout>

MainActivity.java

package com.example.yanhsama.ex4_4;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;

public class MainActivity extends AppCompatActivity {
    TestView tView = null;
    @Override
    public 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 {
        public boolean onTouch(View v, MotionEvent event) {
            int x1,y1;
            x1 = (int) event.getX();
            y1 = (int) event.getY();
            if (event.getAction() == MotionEvent.ACTION_MOVE) {
                tView.getXY(x1,y1);
              tView.invalidate();
              return true;
            }
           else if (event.getAction() == MotionEvent.ACTION_DOWN) {
                tView.getXY(x1,y1);
                y1 = (int) event.getY();
                tView.invalidate();
                return true;
            }
            return tView.onTouchEvent(event);
        }

    }
}

TextView.java

package com.example.yanhsama.ex4_4;
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); /*设置背景为青色*/
        Paint paint = new Paint();
        paint.setAntiAlias(true); /*去锯齿*/
        paint.setColor(Color.BLACK); /*设置paint的颜色*/
        canvas.drawCircle(x, y, 30, paint); /*画一个实心圆*/
        paint.setColor(Color.WHITE);/*画一个实心圆上的小白点*/
        canvas.drawCircle(x - 9, y - 9, 6, paint);
    }
}

效果显示