实现跟随手指移动的小球

创建一个类

package com.bwie.liaixuan2017112;

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 Administrator on 2017/11/2.
*/

class MyView extends View {

Paint paint;
float a=50f;
float b=50f;
public MyView(Context context) {
super(context);
}

public MyView(Context context, AttributeSet attrs) {
super(context, attrs);
//创建画笔
paint=new Paint();
paint.setColor(Color.RED);
paint.setAntiAlias(true);
paint.setTextSize(2);
paint.setStyle(Paint.Style.FILL_AND_STROKE);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawCircle(a,b,30,paint);
}

@Override
public boolean onTouchEvent(MotionEvent event) {
a=event.getX();
b=event.getY();
invalidate();
return true;
}
}
其他都一样
mian.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
>

<com.bwie.liaixuan2017112.MyView
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

</LinearLayout>